View source for Chemical element

{{Short description|Chemical substance not composed of simpler ones}}
{{pp-vandalism|small=yes}}
{{Use Oxford spelling|date=July 2025}}
{{Use dmy dates|date=August 2021}}
[[File:32-column periodic table.png|thumb|upright=1.33|The chemical elements ordered in the [[periodic table]], as shown in the 32-column format]]
{{Sidebar periodic table}}
A '''chemical element''' is a [[Chemical species|species]] of [[atom]] defined by its number of [[proton]]s. The number of protons is called the [[atomic number]] of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its [[atomic nucleus|nucleus]]. Atoms of the same element can have different numbers of [[neutron]]s in their nuclei, known as [[isotope]]s of the element. Atoms of one element can be transformed into atoms of a different element in [[nuclear reaction]]s, which change an atom's atomic number. Almost all [[baryonic matter]] in the universe is composed of elements (among rare exceptions are [[neutron star]]s).

The term "chemical element" is also widely used to mean a pure [[chemical substance]] consisting of a single element. For example, oxygen gas consists only of atoms of oxygen.

Historically, the term "chemical element" meant a substance that cannot be broken down into constituent substances by chemical reactions, and for most practical purposes this definition still has validity. There was some controversy in the 1920s over whether isotopes deserved to be recognised as separate elements if they could be separated by chemical means.<ref>{{cite journal | first=Helge | last=Kragh | year=2000 | title=Conceptual Changes in Chemistry: The Notion of a Chemical Element, ca. 1900-1925 | journal=  Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics| volume=31 | issue=4 | page=435 | doi=10.1016/S1355-2198(00)00025-3 | bibcode=2000SHPMP..31..435K }}</ref> By November 2016, the [[International Union of Pure and Applied Chemistry]] (IUPAC) recognized a total of 118 elements. The first 94 occur naturally on [[Earth]], and the remaining 24 are [[synthetic element]]s produced in nuclear reactions. Save for unstable radioactive elements (radioelements) which [[radioactive decay|decay]] quickly, nearly all elements are available industrially in varying amounts. The [[timeline of chemical element discoveries|discovery and synthesis of further new elements]] is an ongoing area of scientific study.

The history of the discovery and use of elements began with early [[society|human societies]] that discovered native minerals like [[carbon]], [[sulfur]], [[copper]] and gold (though the modern concept of an element was not yet understood). Attempts to classify materials such as these resulted in the concepts of [[classical element]]s, [[alchemy]], and similar theories throughout history. Much of the modern understanding of elements developed from the work of [[Dmitri Mendeleev]], a Russian chemist who published the first recognizable [[periodic table]] in 1869. This table organizes the elements by increasing atomic number into rows ("[[period (periodic table)|periods]]") in which the columns ("[[group (periodic table)|groups]]") share recurring ("periodic") [[physical property|physical]] and [[chemical property|chemical properties]]. The periodic table summarizes various properties of the elements, allowing chemists to derive relationships between them and to make predictions about elements not yet discovered, and potential new compounds.

== Description ==
The term "(chemical) element" is used in two different but closely related meanings:<ref name="iupac">{{cite journal |last1=Chemistry (IUPAC) |first1=The International Union of Pure and Applied |title=IUPAC – chemical element (C01022) |url=https://goldbook.iupac.org/terms/view/C01022 |website=goldbook.iupac.org|doi=10.1351/goldbook.C01022 |doi-access=free |url-access=subscription }}</ref> it can mean a chemical substance consisting of a single kind of atom (a [[free element]]), or it can mean that kind of atom as a component of various chemical substances. For example, water (H{{sub|2}}O) consists of the elements hydrogen (H) and oxygen (O) even though it does not contain the chemical substances (di)hydrogen (H{{sub|2}}) and (di)oxygen (O{{sub|2}}), as H{{sub|2}}O molecules are different from H{{sub|2}} and O{{sub|2}} molecules. For the meaning "chemical substance consisting of a single kind of atom", the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is widely used. For example, French distinguishes {{lang|fr|élément chimique}} (kind of atoms) and {{lang|fr|corps simple}} (chemical substance consisting of one kind of atom); Russian distinguishes {{lang|ru|химический элемент}} and {{lang|ru|простое вещество}}.
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[[List of chemical elements|Chemical elements can be organized]] by name, [[chemical symbol]], and also by properties (as atoms or as substances). The properties of chemical elements as kinds of atom include the atomic number, atomic weight, isotopes, abundance in nature, [[Molar ionization energies of the elements|ionization energy]], [[electron affinity]], [[oxidation state]]s, and [[electronegativity]].<ref>According to Russian-language sources, these are properties of '''chemical elements''' properly said, i.e. of kinds of atom. See: {{cite book | last = Врублевский | first=А. И. | title = Химия: базовый школьный курс | place = Минск |lang=ru| publisher = Юнипресс | year = 2009 | pages = 11–12 | isbn = 978-9-8550-7813-6 }}</ref> The radioactive nuclides can be [[list of nuclides|arranged]] by length of half-life.<ref>{{cite book | title=CRC Handbook of Tables for Applied Engineering Science | first1=Ray E. | last1=Bolz | first2=George L. | last2=Tuve | edition=2nd | publisher=CRC Press | year=2019 | isbn=978-1-351-83842-9 | pages=425–426 | url=https://books.google.com/books?id=lDkPEAAAQBAJ&pg=PA425 }}</ref> As substances, the properties of chemical elements include their density, melting point, boiling point, [[electrical conductance]], [[thermal conductivity and resistivity|thermal conductivity]].<ref>According to Russian-language sources, these are properties of substances, including '''simple substances''', i.e. substances consisting of a single element. They may depend on temperature and pressure and also depend on an [[allotrope]]. See: {{cite book | last = Врублевский | first=А. И. | title = Химия: базовый школьный курс | place = Минск |lang=ru| publisher = Юнипресс | year = 2009 | pages = 11–12 | isbn = 978-9-8550-7813-6 }}</ref>

One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the periodic table, which groups together elements with similar chemical properties (and usually also similar electronic structures).<ref name=Schwerdtfeger_et_al_2020/> Chemical elements can be categorised by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially via human-made nuclear reactions.<ref name=Aguirre_2024/>

=== Occurrence ===
{{Main|Abundance of elements in Earth's crust}}

The lightest elements are [[hydrogen]] and [[helium]], both created by [[Big Bang nucleosynthesis]] in the [[Chronology of the universe|first 20 minutes of the universe]]<ref>See the timeline on p.10 in {{cite journal | year=2006 | title=Evidence for Dark Matter | url=https://gaitskell.brown.edu/physics/talks/0408_SLAC_SummerSchool/Gaitskell_DMEvidence_v16.pdf | journal=Physical Review C | volume=74 | issue=4 | article-number=044602 | doi=10.1103/PhysRevC.74.044602 | bibcode=2006PhRvC..74d4602O | last1=Oganessian | first1=Yu. Ts. | last2=Utyonkov | first2=V. | last3=Lobanov | first3=Yu. | last4=Abdullin | first4=F. | last5=Polyakov | first5=A. | last6=Sagaidak | first6=R. | last7=Shirokovsky | first7=I. | last8=Tsyganov | first8=Yu. | doi-access=free | access-date=8 October 2007 | archive-date=13 February 2021 | archive-url=https://web.archive.org/web/20210213212406/http://gaitskell.brown.edu/physics/talks/0408_SLAC_SummerSchool/Gaitskell_DMEvidence_v16.pdf | url-status=live}}</ref> in a [[ratio]] of around 3:1 by mass (or 12:1 by number of atoms),<ref>{{cite web | url=http://pdgusers.lbl.gov/~pslii/uabackup/big_bang/elementabundancies/2300400.html |title=The Universe Adventure Hydrogen and Helium | publisher=Lawrence Berkeley National Laboratory U.S. Department of Energy | year=2005 | url-status=dead | archive-url=https://web.archive.org/web/20130921054844/http://pdgusers.lbl.gov/~pslii/uabackup/big_bang/elementabundancies/2300400.html | archive-date=21 September 2013 }}</ref><ref>{{cite web | url=http://www.astro.soton.ac.uk/~pac/PH112/notes/notes/node181.html | title=Formation of the light elements | author=astro.soton.ac.uk | publisher=University of Southampton | date=3 January 2001 | url-status=dead | archive-url=https://web.archive.org/web/20130921054428/http://www.astro.soton.ac.uk/~pac/PH112/notes/notes/node181.html | archive-date=21 September 2013 }}</ref> along with tiny traces of the next two elements, [[lithium]] and [[beryllium]]. Almost all other elements found in nature were made by various natural methods of [[nucleosynthesis]].<ref>{{cite web | url=http://www.foothill.edu/attach/938/Nucleosynthesis.pdf | title=How Stars Make Energy and New Elements | publisher=Foothill College | date=18 October 2006 | access-date=17 February 2013 | archive-date=11 August 2020 | archive-url=https://web.archive.org/web/20200811064522/https://foothill.edu/attach/938/Nucleosynthesis.pdf | url-status=live }}</ref> On Earth, small amounts of new atoms are naturally produced in [[nucleogenic]] reactions,<ref>{{cite journal | title=Production of nucleogenic neon in the Earth from natural radioactive decay | first1=Igor | last1=Yatsevich | first2=Masahiko | last2=Honda | journal=Journal of Geophysical Research: Solid Earth | volume=102 | issue=B5 | date=10 May 1997 | pages=10291–10298 | doi=10.1029/97JB00395 | bibcode=1997JGR...10210291Y }}</ref> or in [[cosmogenic]] processes, such as [[cosmic ray spallation]].<ref>{{cite journal | title=An updated simulation of particle fluxes and cosmogenic nuclide production in the Earth's atmosphere | last1=Masarik | first1=J. | last2=Beer | first2=J. | journal=Journal of Geophysical Research: Atmospheres | volume=114 | issue=D11 | id=D11103 | date=June 2009 | article-number=2008JD010557 | doi=10.1029/2008JD010557 | bibcode=2009JGRD..11411103M }}</ref> New atoms are also naturally produced on Earth as [[Radiogenic nuclide|radiogenic]] [[Decay product|daughter isotopes]] of ongoing [[radioactive decay]] processes such as [[alpha decay]], [[beta decay]], [[spontaneous fission]], [[cluster decay]], and other rarer modes of decay.<ref>{{cite book | chapter=Natural Radioactivity of the Crust and Mantle | last=van Schmus | first=W. R. | title=Global earth physics a handbook of physical constants | editor-first=Thomas J. | editor-last=Ahrens | editor-link=Thomas J. Ahrens | series=AGU reference shelf Series | volume=1 | isbn=0-87590-851-9 | location=Washington, DC | publisher=American Geophysical Union | page=283 | year=1995 | bibcode=1995geph.conf..283V | chapter-url=https://books.google.com/books?id=aqjU_NHyre4C&pg=PA283 }}</ref>

There are now 118 known elements. "Known" here means observed well enough, even from just a few decay products, to have been differentiated from other elements.<ref>{{cite journal | last=Sanderson | first=K. | date=17 October 2006 | title=Heaviest element made – again | journal=News@nature | url=http://www.nature.com/news/2006/061016/full/061016-4.html | doi=10.1038/news061016-4 | s2cid=121148847 | access-date=8 March 2007 | archive-date=16 May 2020 | archive-url=https://web.archive.org/web/20200516072856/https://www.nature.com/news/2006/061016/full/061016-4.html | url-status=live | url-access=subscription }}</ref><ref name="Schewe">{{cite journal | last3=Castelvecchi | first3=D. | last1=Schewe | first1=P. | last2=Stein | first2=B. | date=16 October 2006 | issue=797 | title=Elements 116 and 118 Are Discovered | url=http://www.aip.org/pnu/2006/797.html | journal=Physics News Update | publisher=American Institute of Physics | access-date=19 October 2006 | url-status=dead | archive-url=https://web.archive.org/web/20120101144201/http://www.aip.org/pnu/2006/797.html |archive-date=1 January 2012}}</ref> Most recently, the synthesis of element 118 (since named [[oganesson]]) was reported in October 2006, and the synthesis of element 117 ([[tennessine]]) was reported in April 2010.<ref>{{cite news | last=Glanz | first=J. | date=6 April 2010 | title=Scientists Discover Heavy New Element | url=https://www.nytimes.com/2010/04/07/science/07element.html | url-access=subscription | newspaper=The New York Times | access-date=15 February 2017 | archive-date=19 June 2017 | archive-url=https://web.archive.org/web/20170619122834/http://www.nytimes.com/2010/04/07/science/07element.html?hp | url-status=live }}</ref><ref>{{cite journal | last1=Oganessian | first1=Yu. Ts. | date=April 2010 | title=Synthesis of a New Element with Atomic Number Z=117 | bibcode-access=free | publisher=Physical Review Journals | journal=Physical Review Letters | volume=104 | article-number=142502 | doi=10.1103/PhysRevLett.104.142502 | last2=Abdullin| first2=F. Sh. | last3=Bailey | first3=P. D. | last4=Benker | first4=D. E. | last5=Bennett | first5=M. E. | last6=Dmitriev | first6=S. N. | last7=Ezold | first7=J. G. | last8=Hamilton | first8=J. H. | last9=Henderson | first9=R. A. | last10=Itkis | first10=M. G. | last11=Lobanov | first11=Yu. V. | last12=Mezentsev | first12=A. N. | last13=Moody | first13=K. J. | last14=Nelson | first14=S. L. | last15=Polyakov | first15=A. N. | last16=Porter | first16=C. E. | last17=Ramayya | first17=A. V. | last18=Riley | first18=F. D. | last19=Roberto | first19=J. B. | last20=Ryabinin | first20=M. A. | last21=Rykaczewski | first21=K. P. | last22=Sagaidak | first22=R. N. | last23=Shaughnessy | first23=D. A. | last24=Shirokovsky | first24=I. V. | last25=Stoyer | first25=M. A. | last26=Subbotin | first26=V. G. | last27=Sudowe | first27=R. | last28=Sukhov | first28=A. M. | last29=Tsyganov | first29=Yu. S. | last30=Utyonkov | first30=V. K. | pmid=20481935 | issue=14 | bibcode=2010PhRvL.104n2502O | display-authors=29 | doi-access=free}}</ref> Of these 118 elements, the first 94 elements have been detected directly on Earth as [[primordial nuclide]]s present from the formation of the [[Solar System]], or as naturally occurring fission or transmutation products of uranium and thorium.<ref>{{cite book | chapter=Crust and Lithospheric Structure – Seismic Structure of Mid-Ocean Ridges | title=Deep Earth Seismology | volume=1 | display-editors=1 | editor1-first=Gerald | editor1-last=Schubert | editor2-first=Barbara | editor2-last=Romanowicz | editor3-first=Amad | editor3-last=Dziewonski | series=Treatise on Geophysics | edition=2nd | publisher=Elsevier | year=2015 | isbn=978-0-444-53803-1 | pages=451–453 | chapter-url=https://books.google.com/books?id=Rg6dBAAAQBAJ&pg=RA7-PA451 }}</ref><ref name=Aguirre_2024>{{cite book | title=Periodic Table of the Universe: Symphony of Matter | first=Jaime | last=Aguirre | publisher=Cambridge Scholars Publishing | year=2024 | isbn=978-1-0364-1669-0 | pages=24–25 | url=https://books.google.com/books?id=YJA8EQAAQBAJ&pg=PA24 }}</ref> Six of these occur in extreme trace amounts: [[technetium]], atomic number 43; [[promethium]], number 61; [[astatine]], number 85; [[francium]], number 87; [[neptunium]], number 93; and [[plutonium]], number 94.<ref>{{cite journal | title=Completion and extension of the periodic table of elements beyond uranium | last=Guillaumont | first=Robert | journal=Comptes Rendus Physique | volume=20 | issue=7–8 | pages=617–630 | date=November 2019 | doi=10.1016/j.crhy.2018.12.006 | bibcode=2019CRPhy..20..617G | doi-access=free }}</ref> These 94 elements have been detected in the universe at large, in the spectra of stars, as well as [[neutron star]] mergers and supernovae, where short-lived radioactive elements are newly being made.<ref>{{cite book | title=Atomic Astrophysics and Spectroscopy | first1=Anil K. | last1=Pradhan | first2=Sultana N. | last2=Nahar | publisher=Cambridge University Press | year=2011 | isbn=978-1-139-49497-7 | page=231 | url=https://books.google.com/books?id=5948JMEGzm8C&pg=PA231 }}</ref><ref>{{cite journal | title=Neutron Star Mergers and Nucleosynthesis of Heavy Elements | last1=Thielemann | first1=F. -K. | last2=Eichler | first2=M. | last3=Panov | first3=I. V. | last4=Wehmeyer | first4=B. | journal=Annual Review of Nuclear and Particle Science | volume=67 | pages=253–274 | date=October 2017 | doi=10.1146/annurev-nucl-101916-123246 | arxiv=1710.02142 | bibcode=2017ARNPS..67..253T }}</ref>

Two or more atoms can combine to form [[molecule]]s. Some elements form [[Homonuclear molecule|molecules of atoms of said element only]]: e.g. atoms of hydrogen (H) form [[Diatomic molecule|diatomic molecules]] (H{{sub|2}}). [[Chemical compound]]s are substances made of atoms of different elements; they can have molecular or non-molecular structure. [[Mixture]]s are materials containing different chemical substances; that means (in case of molecular substances) that they contain different types of molecules. When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by [[chemical bond]]s.<ref name=Möller_2019>{{cite book | title=Fundamentals and Processes | first=Detlev | last=Möller | edition=3rd | publisher=Walter de Gruyter GmbH & Co KG | year=2019 | isbn=978-3-11-056126-5 | pages=180–181 | url=https://books.google.com/books?id=MOaTDwAAQBAJ&pg=PA180 }}</ref> Less than twenty elements, including the [[Group 11 element|gold]], [[Platinum group|platinum]], [[iron group]] metals, can sometimes be found uncombined as relatively pure [[native element mineral]]s.<ref>{{cite book | title=Earth Materials | first1=Kevin | last1=Hefferan | first2=John | last2=O'Brien | edition=2nd | publisher=John Wiley & Sons | year=2022 | isbn=978-1-119-51217-2 | pages=154–155 | url=https://books.google.com/books?id=fdF6EAAAQBAJ&pg=PA154 }}</ref> Nearly all other naturally occurring elements exist in the [[Earth]] as compounds or mixtures.<ref name=Möller_2019/> [[Atmosphere of Earth|Air]] is mostly a mixture of molecular [[nitrogen]] and [[oxygen]], though it does contain compounds including [[carbon dioxide]] and [[water]], as well as atomic [[argon]], a [[noble gas]] which is [[chemically inert]] and therefore does not undergo chemical reactions.<ref name=Allens-2002>{{Cite book | url=https://link.springer.com/10.1007/978-1-4612-1186-0 | title=Allen's Astrophysical Quantities | date=2002 | edition=4th | chapter=11. Earth | publisher=Springer New York | isbn=978-1-4612-7037-9 | editor-last=Cox | editor-first=Arthur N. | location=New York, NY | language=en | doi=10.1007/978-1-4612-1186-0}}</ref>{{rp|258}}

== Atomic nucleus properties ==
The standard model of an atom is of a dense nucleus of charged [[proton]]s and electrically-neutral [[neutron]]s, surrounded by an electrically-bound cloud of low mass, negatively charged [[electron]]s. Despite the force of mutual repulsion between the protons, the nucleus is held together by the short-ranged [[strong nuclear force]] between the particles. The [[neutron–proton ratio]] determines the stability of a nucleus, as a proper balance of neutrons counteracts the mutual repulsion of the protons.<ref>{{cite book | title=Handbook of Radioactivity Analysis | editor-first=Michael F. | editor-last=L'Annunziata | publisher=Academic Press | year=2012 | page=25 | isbn=978-0-12-384873-4 | url=https://books.google.com/books?id=mAfH8KEH3j0C&pg=PA25 }}</ref>

=== Nuclide ===
{{main|nuclide|atomic number|Isotope|Stable isotope ratio|List of nuclides}}
[[File:Isotopes and half-life.svg|right|thumb|[[Nuclide chart]] displaying of the number of protons (Z) versus neutrons (N) for each isotope, with the color showing resulting half life]]
A nuclide, or nuclear species, is a class of atoms characterized by their number of [[proton]]s, ''Z'', their number of [[neutron]]s, ''N'', and their nuclear [[energy state]].<ref>{{cite book | author=IUPAC | editor1-first=A. D. | editor1-last=McNaught | editor2-first=A. | editor2-last=Wilkinson | year=1997 | chapter=Nuclide | chapter-url=http://goldbook.iupac.org/terms/view/N04257 | title=Compendium of Chemical Terminology | publisher=[[Blackwell Scientific Publications]] | doi=10.1351/goldbook.N04257 | isbn=978-0-632-01765-2 | title-link=Compendium of Chemical Terminology | author-link=IUPAC }}</ref> Atomic nuclei other than {{nuclide|H|1}}, a lone proton, consist of protons and neutrons bound together by the [[residual strong force]], overcoming electrical repulsion between protons. For that reason, neutrons are required to bind protons together; as the number of protons increases, so does the [[neutron–proton ratio]] necessary for stability.<ref>{{cite book | chapter=Anthropogenic Radioactivity | first=Jerzy W. | last=Mietelski | title=Radionuclides in the Environment | series=EIC Books | editor-first=David A. | editor-last=Atwood | publisher=John Wiley & Sons | year=2013 | isbn=978-1-118-63269-7 | page=20 | chapter-url=https://books.google.com/books?id=R5ATOuAFRlYC&pg=PA20 }}</ref> For example, although light elements up through oxygen have stable nuclides with the same number of neutrons as protons, lead requires about 3 neutrons for 2 protons.<ref>{{cite book | title=Descriptive Inorganic Chemistry | edition=Third | first1=Geoff | last1=Rayner-Canham | first2=Tina | last2=Overton | publisher=Macmillan | year=2003 | isbn=978-0-7167-4620-1 | page=22 | url=https://books.google.com/books?id=LLfcBEkQamoC&pg=PA22 }}</ref>

The [[atomic number]] of an element is equal to the number of protons in each atom, and defines the element.<ref>{{cite web | url=http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/atomicmassnumber.htm | title=Atomic Number and Mass Numbers | publisher=ndt-ed.org | access-date=17 February 2013 | archive-url=https://web.archive.org/web/20140212155836/http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/atomicmassnumber.htm | archive-date=12 February 2014 | url-status=dead }}</ref> For example, all carbon atoms contain 6 protons in their [[atomic nucleus]]; so the atomic number of carbon is 6.<ref>{{cite web | url=http://periodic.lanl.gov/6.shtml | title=Periodic Table of Elements: LANL Carbon | author=periodic.lanl.gov | publisher=[[Los Alamos National Laboratory]] | access-date=17 February 2013 | archive-date=25 January 2021 | archive-url=https://web.archive.org/web/20210125032252/https://periodic.lanl.gov/6.shtml | url-status=live }}</ref> The number of protons in the nucleus determines its [[electric charge]], which in turn determines the number of bound [[electron]]s of an atom in its [[ionization|non-ionized]] state. The electrons occupy [[atomic orbital]]s that determine the atom's [[chemical property|chemical properties]].<ref name=SADR/>

[[Isotope]]s are atoms of the same element (that is, with the same number of [[proton]]s in their nucleus), but having ''different'' numbers of [[neutron]]s.<ref>{{cite web | url=http://faculty.piercecollege.edu/yamadak/classes/Atomic%20mass.pdf | title=Atomic mass, isotopes, and mass number. | first=Katsuya | last=Yamada | publisher=[[Los Angeles Pierce College]] | url-status=dead | archive-url=https://web.archive.org/web/20140111131537/http://faculty.piercecollege.edu/yamadak/classes/Atomic%20mass.pdf | archive-date=11 January 2014 }}</ref> Thus, for example, there are three main isotopes of carbon. All carbon atoms have 6 protons, but they can have either 6, 7, or 8 neutrons. Since the mass numbers of these are 12, 13 and 14 respectively, said three isotopes are known as [[carbon-12]], [[carbon-13]], and [[carbon-14]] ({{sup|12}}C, {{sup|13}}C, and {{sup|14}}C). Natural carbon is a [[mixture]] of {{sup|12}}C (about 98.9%), {{sup|13}}C (about 1.1%) and about 1 atom per trillion of {{sup|14}}C. The number of neutrons in a nucleus usually has very little effect on an element's chemical properties.<ref name=SADR>{{cite book | title=Scientific American Science Desk Reference | publisher=Turner Publishing Company | year=2008 | isbn=978-0-470-35376-9 | url=https://books.google.com/books?id=LHzuEAAAQBAJ&pg=RA2-PT21 }}</ref> An exception is hydrogen, for which the [[kinetic isotope effect]] is significant.<ref>{{cite book | chapter=Isotope effects, isotope separation, and isotope fractionation | first=Gabor | last=Jancso | title=Radiochemistry and Nuclear Chemistry | volume=1 | series=Encyclopedia of life support systems | editor-first=Sándor | editor-last=Nagy | publisher=EOLSS Publications | year=2009 | isbn=978-1-84826-126-6 | pages=115–131 | chapter-url=https://books.google.com/books?id=VbO2CwAAQBAJ&pg=PA128 }}</ref> Thus, all carbon isotopes have nearly identical chemical properties because they all have six electrons, even though they may have 6 to 8 neutrons. That is why atomic number, rather than [[mass number]] or [[atomic weight]], is considered the identifying characteristic of an element.<ref name=SADR/>

===Stability===
{{main|Stable nuclide|Radionuclide|Radiogenic nuclide}}
[[File:Periodic Table by Number of Stable Isotopes.PNG|right|thumb|upright=1.4|Periodic table by number of stable isotopes, ranging from zero (red) to 7+ (blue)]]

All elements have radioactive isotopes (radioisotopes),{{cn|date=November 2025}} but many of these radioisotopes are not found in nature due to a low half life. Radioisotopes typically decay into other elements via [[alpha decay]], [[beta decay]], or [[inverse beta decay]]; some isotopes of the heaviest elements also undergo [[spontaneous fission]]. Isotopes that are not radioactive, are termed "stable" isotopes. Isotopes with even numbers of protons, even numbers of neutrons, or both, tend to be more stable as like particle can pair up with like.<ref>{{cite book | title=The Physics of Radiology and Imaging | first1=K. | last1=Thayalan 7 | first2=Ramamoorthy | last2=Ravichandran | publisher=JP Medical Ltd | year=2014 | isbn=978-93-5152-171-6 | url=https://books.google.com/books?id=JLrTAwAAQBAJ&pg=PA274 }}</ref><ref name=Choppin_et_al_2013/> This [[pairing effect]] allows the identical particles to align with opposite [[Spin (physics)|spins]], increasing the binding energy.<ref>{{cite book | title=Rare Isotope Beams: Concepts and Techniques | display-authors=1 | first1=Alok | last1=Chakrabarti | first2=Vaishali | last2=Naik | first3=Siddhartha | last3=Dechoudhury | publisher=CRC Press | year=2021 | isbn=978-1-4987-8879-3 | page=4 | url=https://books.google.com/books?id=Zh8sEAAAQBAJ&pg=PA4 }}</ref>

Most (54 of 94) naturally occurring elements have more than one stable isotope. Only 26 elements are [[Monoisotopic element|monoisotopic]], having exactly one stable isotope; these have an odd atomic number of protons, with the exception of [[beryllium-9]] which has an odd number of neutrons.<ref>{{cite book | chapter=The Beryllium Bond | display-authors=1 | first1=M. Merced | last1=Montero-Campillo | first2=Otilia | last2=Mó | first3=Manuel | last3=Yáñez | first4=Ibon | last4=Alkorta | first5=José | last5=Elguero | title=Computational Chemistry | volume=73 | series=Advances in Inorganic Chemistry | editor1-first=Rudi | editor1-last=van Eldik | editor2-first=Ralph | editor2-last=Puchta | publisher=Academic Press | year=2019 | isbn=978-0-12-815729-9 | page=75 | chapter-url=https://books.google.com/books?id=kQOJDwAAQBAJ&pg=PA75 }}</ref><ref>[[Tantalum-180|<sup>180m</sup>Ta]] is metastable, so technically tantalum has two observationally stable isotopes</ref> The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes for a single element is 10 (for [[tin]], element 50).<ref name=Choppin_et_al_2013>{{cite book | title=Radiochemistry and Nuclear Chemistry | display-authors=1 | first1=Gregory | last1=Choppin | first2=Jan-Olov | last2=Liljenzin | first3=Jan | last3=Rydberg | first4=Christian | last4=Ekberg | edition=4th | publisher=Academic Press | year=2013 | pages=65–66 | isbn=978-0-12-397868-4 | url=https://books.google.com/books?id=CN88gBPtiucC&pg=PA66  | doi=10.1016/bs.adioch.2018.10.003 }}</ref>

Elements with atomic numbers 1 through 82 each have at least one [[stable isotope]] (except for [[technetium]], element 43 and [[promethium]], element 61, which have no stable isotopes). However, observationally stable isotopes of some elements (such as [[tungsten]]<ref>{{cite journal | display-authors=1 | last1=Danevich | first1=F. A. | last2=Georgadze | first2=A. Sh. | last3=Kobychev | first3=V. V. | last4=Nagorny | first4=S. S. | last5=Nikolaiko | first5=A. S. | last6=Ponkratenko | first6=O. A. | last7=Tretyak | first7=V. I. | last8=Zdesenko | first8=S. Yu. | last9=Zdesenko | first9=Yu. G. | last10=Bizzeti | first10=P. G. | last11=Fazzini | first11=T. F. | last12=Maurenzig | first12=P. R. | title=α activity of natural tungsten isotopes | journal=Physical Review C | volume=67 | issue=1 | article-number=014310 | date=2003 | arxiv=nucl-ex/0211013 | doi=10.1103/PhysRevC.67.014310 | bibcode=2003PhRvC..67a4310D | s2cid=6733875}}</ref> and [[lead]]) are predicted to be slightly radioactive with very long half-lives:{{NUBASE2016|ref}} for example, the half-lives predicted for the observationally stable lead isotopes range from 10{{sup|35}} to 10{{sup|189}} years.<ref>{{cite journal | display-authors=3 | last1=Beeman | first1=J. W. | last2=Bellini | first2=F. | last3=Cardani | first3=L. | last4=Casali | first4=N. | title=New experimental limits on the ''α'' decays of lead isotopes | year=2013 | journal=[[European Physical Journal A]] | volume=49 | issue=50 | pages=50 | doi=10.1140/epja/i2013-13050-7 | arxiv=1212.2422 | bibcode=2013EPJA...49...50B | s2cid=119280082 }}</ref> Isotopes are [[observationally stable]] when they are theoretically unstable but no radioactive decay has yet been observed. Out of the over 250 nuclides that are called stable,<ref>{{cite book | title=Materials Handbook: A Concise Desktop Reference | first=François | last=Cardarelli | edition=3rd | publisher=Springer | year=2018 | isbn=978-3-319-38925-7 | url=https://books.google.com/books?id=koRjDwAAQBAJ&pg=PA1887 }}</ref> only 90 are considered theoretically stable, meaning they lack a known decay mode.<ref>{{cite book | title=Radiation: Fundamentals, Applications, Risks, and Safety | first=Ilya | last=Obodovskiy | page=260 | publisher=Elsevier | year=2019 | isbn=978-0-444-63986-8 | url=https://books.google.com/books?id=xmOMDwAAQBAJ&pg=PA260 }}</ref>

[[File:Table of nuclides (mul).svg|left|thumb|upright=1.3|A graph of isotope stability, with the [[Magic number (physics)|magic numbers]] that make a nucleus more stable]]
Elements with atomic numbers 83 through 94 are [[Radionuclide|unstable]] enough that radioactive decay of all isotopes can be detected.<ref>{{cite book | title=Radiation: Fundamentals, Applications, Risks, and Safety | first=Ilya | last=Obodovskiy | publisher=Elsevier | year=2019 | isbn=978-0-444-63986-8 | page=259 | url=https://books.google.com/books?id=xmOMDwAAQBAJ&pg=PA259 }}</ref> Some of these elements, notably [[bismuth]] (atomic number 83), [[thorium]] (atomic number 90), and [[uranium]] (atomic number 92), have one or more isotopes with half-lives long enough to survive from before the [[Solar System]] formed.<ref>{{cite book | title=Radioecology: Sources and Consequences of Ionising Radiation in the Environment | series=Cambridge Environmental Chemistry Series | first=R. J. | last=Pentreath | publisher=Cambridge University Press | year=2021 | isbn=978-1-107-09602-8 | page=78 | url=https://books.google.com/books?id=qwg3EAAAQBAJ&pg=PA78 }}</ref> The remaining longest-lived isotopes have [[Half-life|half lives]] too short for them to have been present at the beginning of the Solar System, and are therefore "transient elements". Of these 11 transient elements, five ([[polonium]], [[radon]], [[radium]], [[actinium]], and [[protactinium]]) are relatively common [[decay product]]s of [[thorium]] and [[uranium]].<ref>{{cite book | title=Nuclear Engineering Fundamentals: A Practical Perspective | first=Robert E. | last=Masterson | publisher=CRC Press | year=2017 | isbn=978-1-4822-2150-3 | pages=437–440 | url=https://books.google.com/books?id=kDoPEAAAQBAJ&pg=PA438 }}</ref> The remaining six transient elements (technetium, promethium, astatine, [[francium]], [[neptunium]], and [[plutonium]]) occur only rarely,<ref>{{cite book | title=The Periodic Table: Nature's Building Blocks: An Introduction to the Naturally Occurring Elements, Their Origins and Their Uses | first1=J. Theo | last1=Kloprogge | first2=Concepcion P. | last2=Ponce | first3=Tom | last3=Loomis | publisher=Elsevier | year=2020 | isbn=978-0-12-821538-8 | url=https://books.google.com/books?id=hGa8DwAAQBAJ&pg=PA48 }}</ref> as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

The remaining 24 heaviest elements ([[Transplutonium element|those beyond plutonium]], element 94) are radioactive, with [[half-life|half-lives]] so short that they are not found on Earth and must be [[synthetic element|synthesized]].<ref>{{cite book | title=Synthetic Transuranium Elements | series=Understanding the atom | first=Earl K. | last=Hyde | publisher=U.S. Atomic Energy Commission, Division of Technical Information | year=1964 | pages=14–19 | url=https://books.google.com/books?id=Rtj8VoBmZBAC&pg=PA14 }}</ref> Five have been discovered in the spectrum of [[Przybylski's star]], from element 95 ([[americium]]) to 99 ([[einsteinium]]). These are thought to be [[neutron capture]] products of uranium and thorium.<ref>{{cite conference | title=On the radioactive shells in peculiar main sequence stars: the phenomenon of Przybylski's star | display-authors=1 | display-editors=1 | last1=Gopka | first1=Vera F. | last2=Yushchenko | first2=Alexander V. | last3=Shavrina | first3=Angelina V. | last4=Mkrtichian | first4=David E. | last5=Hatzes | first5=Artie P. | last6=Andrievsky | first6=Sergey M. | last7=Chernysheva | first7=Larissa V. | conference=The A-Star Puzzle, held in Poprad, Slovakia, July 8-13, 2004. IAU Symposium, No. 224 | editor1-first=J. | editor1-last=Zverko | editor2-first=J. | editor2-last=Ziznovsky | editor3-first=S. J. | editor3-last=Adelman | editor4-first=W. W. | editor4-last=Weiss | location=Cambridge, UK | publisher=Cambridge University Press | pages=734–742 | date=December 2004 | doi=10.1017/S174392130500966X | bibcode=2004IAUS..224..734G }}</ref> All 24 heavier elements are radioactive, with short half-lives; if any of these elements were present when the Earth formed, they are certain to have completely decayed, and if present in novae, are in quantities too small to have been noted. Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, though traces of technetium have since been found in nature (and also the element may have been discovered naturally in 1925).<ref>{{citation-attribution | 1={{cite web|url=http://www.epa.gov/radiation/radionuclides/technetium.html | title=Technetium-99 | publisher=United States Environmental Protection Agency Radiation Protection | access-date=26 February 2013 | archive-date=1 September 2015 | archive-url=https://web.archive.org/web/20150901222619/http://www.epa.gov/radiation/radionuclides/technetium.html | url-status=dead }} }}</ref> This pattern of artificial production and later natural discovery has been repeated with several other radioactive naturally occurring rare elements.<ref>{{cite web | url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/text/text_stel_6.html | work=Cosmic Evolution - From Big Bang to Humankind | first=Eric J. | last=Chaisson | title=Origins of Heavy Elements | publisher=Harvard–Smithsonian Center for Astrophysics | access-date=26 February 2013 | archive-date=25 September 2020 | archive-url=https://web.archive.org/web/20200925165732/https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/text/text_stel_6.html | url-status=live }}</ref>

The lightest radioactive isotope is [[tritium]], which undergoes [[Beta decay]] with a half-life of 12.3&nbsp;years.<ref>{{cite book | title=Nuclear Power Explained | series=Springer Praxis Books | first=Dirk | last=Eidemüller | publisher=Springer Nature | year=2021 | isbn=978-3-030-72670-6 | pages=81–83 | url=https://books.google.com/books?id=1QY8EAAAQBAJ&pg=PA81 }}</ref> At 2{{e|19}} years, over 10{{sup|9}} times the estimated age of the universe, [[bismuth-209]] has the longest known alpha decay half-life of any nuclide, and is almost always considered on par with the 80 stable elements.<ref name="Dume2003">{{cite news | title=Bismuth breaks half-life record for alpha decay | last=Dumé | first=B. | date=23 April 2003 | work=Physicsworld.com | publisher=Institute of Physics | location=Bristol, England | url=http://physicsworld.com/cws/article/news/2003/apr/23/bismuth-breaks-half-life-record-for-alpha-decay | access-date=14 July 2015 | archive-date=13 December 2017 | archive-url=https://web.archive.org/web/20171213214524/http://physicsworld.com/cws/article/news/2003/apr/23/bismuth-breaks-half-life-record-for-alpha-decay | url-status=live}}</ref><ref name="Marcillac2003">{{cite journal | last1=de Marcillac | first1=P. | last2=Coron | first2=N. | last3=Dambier | first3=G. | last4=Leblanc | first4=J. | last5=Moalic | first5=J-P | year=2003 | title=Experimental detection of alpha-particles from the radioactive decay of natural bismuth | journal=Nature | volume=422 | pages=876–878 | doi=10.1038/nature01541 | pmid=12712201 | issue=6934 | bibcode=2003Natur.422..876D | s2cid=4415582}}</ref> The isotope [[tellurium-128]] transmutes through double beta decay with a half life of 2.25{{e|24}} years, over 100,000 longer than bismuth-209.<ref>{{cite journal | title=Status and Perspectives on Rare Decay Searches in Tellurium Isotopes | last1=Campani | first1=Alice | last2=Dompè | first2=Valentina | last3=Fantini | first3=Guido | journal=Universe | volume=7 | issue=7 | article-number=212 | date=June 2021 | doi=10.3390/universe7070212 | doi-access=free | bibcode=2017nuco.confb0702M | hdl=11573/1557551 | hdl-access=free }}</ref> The primary source of radiation exposure from isotope decays in the human body come from [[carbon-14]] and [[potassium-40]] intake, which produce an annual effective dose of {{Val|0.17|ul=mSv}}.<ref>{{cite book | title=The Physics of Energy | first1=Robert L. | last1=Jaffe | first2=Washington | last2=Taylor | publisher=Cambridge University Press | year=2018 | isbn=978-1-107-01665-1 | page=393 | url=https://books.google.com/books?id=drZDDwAAQBAJ&pg=PA393 }}</ref>
{{clear|left}}

=== Isotopic mass and atomic mass ===
{{main|Atomic mass|Relative atomic mass}}

The [[mass number]] of an element, ''A'', is the number of [[nucleon]]s (protons and neutrons) in the atomic nucleus. Different isotopes of a given element are distinguished by their mass number, which is written as a superscript on the left hand side of the chemical symbol (e.g., {{sup|238}}U). The mass number is always an integer and has units of "nucleons".<ref name=Becker_2008>{{cite book | title=Inorganic Mass Spectrometry: Principles and Applications | first=Sabine | last=Becker | publisher=John Wiley & Sons | year=2008 | isbn=978-0-470-51720-8 | page=1–3 | url=https://books.google.com/books?id=z-YqpI3otNcC&pg=PA1 }}</ref> Thus, [[Isotopes of magnesium|magnesium-24]] (24 is the mass number) is an atom with 24 nucleons (12 protons and 12 neutrons).

[[File:Binding energy curve - common isotopes.svg|right|thumb|upright=1.2|Nuclear binding energy versus mass number for common nuclides, illustrating how the binding energy peaks with isotopes of iron and nickel.<ref name=Fewell_1995>{{cite journal | title=The atomic nuclide with the highest mean binding energy | last=Fewell | first=M. P. | journal=American Journal of Physics | volume=63 | issue=7 | pages=653–658 | date=July 1995 | doi=10.1119/1.17828 | bibcode=1995AmJPh..63..653F }}</ref>]]
Whereas the mass number simply counts the total number of neutrons and protons and is thus an integer, the [[atomic mass]] of a particular isotope (or "nuclide") of the element is the mass of a single atom of that isotope, and is typically expressed in [[Dalton (unit)|dalton]]s (symbol: Da), aka universal atomic mass units (symbol: u). Its [[relative atomic mass]] is a dimensionless number equal to the atomic mass divided by the [[atomic mass constant]], which equals 1&nbsp;Da.<ref name=IUPAC/> In general, the mass number of a given nuclide differs in value slightly from its relative atomic mass. This [[mass deficit]] is primarily due to the [[nuclear binding energy]].<ref name=Becker_2008/> For example, the atomic mass of chlorine-35 to five significant digits is 34.969&nbsp;Da and that of chlorine-37 is 36.966&nbsp;Da.<ref name=Johll_2008>{{cite book | title=Investigating Chemistry: A Forensic Science Perspective | first=Matthew | last=Johll | publisher=Macmillan | year=2008 | isbn=978-1-4292-0989-2 | page=69 | url=https://books.google.com/books?id=AJt8tADB_wcC&pg=PA69 }}</ref> However, the relative atomic mass of each isotope is quite close to its mass number (always within 1%). The only isotope whose atomic mass is exactly a [[natural number]] is {{sup|12}}C, which has a mass of 12&nbsp;Da; because the dalton is defined as 1/12 of the mass of a free neutral carbon-12 atom in the ground state.<ref>{{GoldBookRef | title=atomic mass constant | file=A00497}}</ref>

During the [[nuclear fusion]] of lower mass atoms such as hydrogen, the net change in mass deficit is released as energy, as determined by the [[mass–energy equivalence]] relationship. This process of fusing hydrogen atoms into helium is what drives the energy output of the Sun. Over time, the result is an increasing concentration of helium at the [[stellar core]]. During the [[stellar evolution|evolution of stars]] much more massive than the Sun, increasingly massive nuclei are then formed through a type of fusion called the [[alpha process]], until [[iron-52]] is reached.<ref name=Diem_2025/> The binding energy of a nucleus reaches its peak value for isotopes of iron and nickel.<ref name=Fewell_1995/> Hence, beyond that point, further fusion results in a lower binding energy, so energy is absorbed rather than released. As a result, an inert iron core forms that does not contribute to the star's energy output.<ref name=Diem_2025>{{cite book | title=Understanding Essential Chemistry | first=Max | last=Diem | publisher=John Wiley & Sons | year=2025 | isbn=978-1-394-32119-3 | pages=205–206 | url=https://books.google.com/books?id=A9ZZEQAAQBAJ&pg=PA205 }}</ref>

In the [[nuclear fission]] process, the resulting particles have a higher net binding energy. This change in the net mass deficit again results in a release of energy. Hence, highly radioactive elements such as [[uranium-235]] can be useful sources of energy production.<ref>{{cite book | title=Introduction to Nuclear Reactor Physics | first=Robert E. | last=Masterson | publisher=CRC Press | year=2017 | isbn=978-1-4987-5149-0 | page=269 | url=https://books.google.com/books?id=ZjwPEAAAQBAJ&pg=PA269 }}</ref>

The [[standard atomic weight]] (commonly called "atomic weight") of an element is the ''average'' of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance, relative to the atomic mass unit.<ref name=IUPAC>{{cite journal | display-authors=1 | last1=Prohaska | first1=Thomas | last2=Irrgeher | first2=Johanna | last3=Benefield | first3=Jacqueline | last4=Böhlke | first4=John K. | last5=Chesson | first5=Lesley A. | last6=Coplen | first6=Tyler B. | last7=Ding | first7=Tiping | last8=Dunn | first8=Philip J. H. | last9=Gröning | first9=Manfred | last10=Holden | first10=Norman E. | last11=Meijer | first11=Harro A. J. | last12=Moossen | first12=Heiko | last13=Possolo | first13=Antonio | last14=Takahashi | first14=Yoshio | last15=Vogl | first15=Jochen | last16=Walczyk | first16=Thomas | last17=Wang | first17=Jun | last18=Wieser | first18=Michael E. | last19=Yoneda | first19=Shigekazu | last20=Zhu | first20=Xiang-Kun | last21=Meija | first21=Juris | title=Standard atomic weights of the elements 2021 | series=IUPAC Technical Report | journal=Pure and Applied Chemistry | volume=94 | issue=5 | year=2022 | pages=573–600 | doi=10.1515/pac-2019-0603 }}</ref> This number may be a fraction that is ''not'' close to a whole number. For example, the relative atomic mass of chlorine is 35.453&nbsp;u, which differs greatly from a whole number as it is an average of about 76% chlorine-35 and 24% chlorine-37.<ref name=Johll_2008/> Whenever a relative atomic mass value differs by more than ~1% from a whole number, it is due to this averaging effect, as significant amounts of more than one isotope are naturally present in a sample of that element.

=== Chemically pure and isotopically pure ===
Chemists and nuclear scientists have different definitions of a ''pure element''. In chemistry, a pure element means a substance whose atoms all (or in practice almost all) have the same atomic number, or number of [[proton]]s. Nuclear scientists, however, define a pure element as one that consists of only one isotope.<ref name=ENS>{{cite news |url=http://www.euronuclear.org/info/encyclopedia/p/pure-element.htm |title=Pure element |publisher=[[European Nuclear Society]] |access-date=13 August 2013 |archive-url=https://web.archive.org/web/20170613073021/http://www.euronuclear.org/info/encyclopedia/p/pure-element.htm |archive-date=13 June 2017 |url-status=dead }}</ref>

For example, a [[copper]] wire is 99.99% chemically pure if 99.99% of its atoms are copper, with 29 protons each. However it is not isotopically pure since natural copper consists of two stable isotopes, 69% {{sup|63}}Cu and 31% {{sup|65}}Cu, with different numbers of neutrons.<ref>{{cite book | title=Copper and Copper Alloys | series=ASM specialty handbook | editor-first=Joseph R. | editor-last=Davis | publisher=ASM International | year=2001 | page=450 | isbn=978-0-87170-726-0 | url=https://books.google.com/books?id=sxkPJzmkhnUC&pg=PA450 }}</ref> (See [[Isotopes of copper]].) However, pure gold would be both chemically and isotopically pure, since ordinary gold consists only of one isotope, {{sup|197}}Au.<ref name=ENS/>

== Chemical and physical properties ==
[[Quantum mechanics]] causes the bound electrons to be organized into a set of layered [[Electron shell|shells]]. Each shell can only contain a fixed number of electrons occupying paired [[Atomic orbital|orbitals]]. The [[electron configuration]]s of these shells mediate the interaction with neighboring atoms and determine the chemical properties of atoms. The shell configuration determines the structure of the periodic table.

=== Shells ===
{{main|Electron shell|Electron configuration}}

[[File:Atomic orbitals spdf m-eigenstates and superpositions.png|right|thumb|upright=1.1|Atomic single-electron orbitals for the lowest four quantum numbers]]
Electrons that are confined to an atom are only allowed to assume certain discrete [[energy level]]s. This restriction, known as [[Quantization (physics)|quantization]], is a fundamental facet of the [[quantum mechanics]] theory that predicts the [[Wave–particle duality|wave-like behavior]] of particles and energy at the smallest scale.  For atoms, these energy levels are represented by [[electron subshell]]s, where the wave form of an electron is held in a type of [[standing wave]] with a specific wavelength. Each whole number of wavelengths yields one or more [[atomic orbital]]s, which describe each the electron's [[charge distribution]] at that energy level. Every orbital can hold a pair of electrons. The arrangement of electrons in an atom's orbitals is called the [[electron configuration]].

These orbitals and their sub-shells are grouped together in [[Electron shell|shells]], with each shell having a [[principal quantum number]] that indicates the energy level. Each shell can only have a fixed number of electrons, which is given by the formula <math>2n^2</math>, where <math>n</math> is 1, 2, 3, 4, and so on. Hence, the count of electrons each shell can contain are 2, 8, 18, 32, and so forth. A shell is represented by a row on the periodic table.<ref>{{cite book | title=Materials Science and Engineering: An Introduction | first1=William D. | last1=Callister, Jr. | first2=David G. | last2=Rethwisch | edition=10th | publisher=John Wiley & Sons | year=2020 | isbn=978-1-119-72177-2 | pages=22–25 | url=https://books.google.com/books?id=dmoTEQAAQBAJ&pg=RA1-PA22 }}</ref>

The simplest atom is ordinary hydrogen, which consists of one proton and one electron. In its minimum energy [[ground state]], the electron occupies the first shell, designated ''K''. This shell has one subshell designated 1s. The second element, helium, has two protons, two electrons, and usually two neutrons. The second electron occupies the same orbital as the first, completing the shell with [[Electron pair|spin-paired electrons]]. For lithium there are three electrons, so one needs to occupy an orbital in the second shell, designated ''L''. Per the [[Aufbau principle]], the third electron occupies the next lowest available energy subshell, which is 2s. This process continues, with successive electrons being placed in the next available lowest energy orbital.<ref>{{cite book | title=Chemistry: A Project of the American Chemical Society | editor-first=Jerry A. | editor-last=Bell | publisher=Macmillan | year=2005 | isbn=978-0-7167-3126-9 | pages=264–267 | url=https://books.google.com/books?id=hg6xMliSqlgC&pg=PA264 }}</ref>

=== Periodic trends ===
{{main|Periodic trends|Periodic table}}
[[File:Periodic-Table-Metals.jpg|right|thumb|upright=1.2|Metals (blue) and metalloids (green) in the periodic table]]
Several terms are commonly used to characterise the general physical and chemical properties of the chemical elements. A first distinction is between [[metal]]s, which readily conduct [[electricity]], [[nonmetal]]s, which do not, and a small group, the ''[[metalloid]]s'', having intermediate properties and often behaving as [[semiconductor]]s.<ref name=Olmstead_Williams_1997>{{cite book | title=Chemistry: The Molecular Science | first1=John | last1=Olmsted | first2=Gregory M. | last2=Williams | publisher=Jones & Bartlett Learning | year=1997 | isbn=978-0-8151-8450-8 | pages=10–12 | url=https://books.google.com/books?id=1vnk6J8knKkC&pg=PA10 }}</ref>

Besides being different by physical properties as substances, metallic and nonmetallic elements have different chemical properties as kinds of atoms: metallic elements form simple [[cation]]s, whereas nonmetallic elements (and weak metals) form simple [[anion]]s and [[oxoanion]]s.<ref>{{cite book | title=Descriptive Inorganic Chemistry | first1=Geoff | last1=Rayner-Canham | first2=Tina | last2=Overton | publisher=Macmillan | year=2009 | isbn=978-1-4292-2434-5 | page=195 | url=https://books.google.com/books?id=poO9RHq-IoIC&pg=PA195 }}</ref> An exception is noble gases, which do not form ''simple'' anions, whereas oxoanions are known for xenon.<ref>{{cite journal | title=Noble Gas Anions: An Overview of Strategies and Bonding Motifs | first1=Stefano | last1=Borocci | first2=Felice | last2=Grandinetti | first3=Nico | last3=Sanna | first4=Costantino | last4=Zazza | journal=Chemistry, an Asian Journal | volume=19 | issue=14 | date=July 15, 2024 | article-number=e202400191 | doi=10.1002/asia.202400191 | doi-access=free }}</ref> However, some metallic elements in higher oxidation states form oxoanions as well. Atoms of nonmetallic elements also form compounds with [[covalent bond]]s.{{cn|date=September 2025}}

Distinguishing terms are used for certain sets of the more broadly viewed metals and nonmetals. These sets include: [[actinide]]s, [[alkali metal]]s, [[alkaline earth metal]]s, [[halogen]]s, [[lanthanide]]s, [[transition metal]]s, [[post-transition metal]]s, [[metalloid]]s, [[reactive nonmetal]]s, and [[noble gas]]es. In this system, the alkali metals, alkaline earth metals, and transition metals, as well as the lanthanides and the actinides, are special groups of the metals viewed in a broader sense. Similarly, the reactive nonmetals and the noble gases are nonmetals viewed in the broader sense.<ref name=Olmstead_Williams_1997/> In some presentations, the halogens are not distinguished, with [[astatine]] identified as a metalloid and the others identified as nonmetals.

A more refined classification is often shown in coloured presentations of the periodic table. The properties of the elements can be summarized in this form, which powerfully and elegantly organizes the elements by physical and chemical properties. Each row forms a [[Period (periodic table)|period]] of elements that have the same number of [[electron shell]]s. There are 18 numbered [[Group (periodic table)|groups]], each forming its own column of elements whose chemical properties are dominated by the orbital location of the outermost electron. A [[Block (periodic table)|block]] is set of elements sharing [[atomic orbitals]] that their [[valence electrons]] or vacancies occupy.<ref>{{cite book | title=Chemical Structure and Reactivity: An Integrated Approach | first1=James | last1=Keeler | first2=Peter | last2=Wothers | publisher=OUP Oxford | year=2013 | isbn=978-0-19-9604135 | page=259 | url=https://books.google.com/books?id=2RgbAgAAQBAJ&pg=PA259 }}</ref>

Use of the periodic table is now ubiquitous in chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior. The table has also found wide application in [[physics]],<ref name=Schwerdtfeger_et_al_2020>{{cite journal | display-authors=1 | last1=Schwerdtfeger | first1=P. | last2=Smits | first2=O. R. | last3=Pyykkö | first3=P. | title=The periodic table and the physics that drives it | journal=Nature Reviews Chemistry | pages=359–380 | year=2020 | volume=4 | issue=7 | doi=10.1038/s41570-020-0195-y | pmid=37127952 | hdl=10138/323849 | hdl-access=free }}</ref> [[geology]],<ref>{{cite journal | title=The Periodic Table of the Elements as a Basis of Geochemistry | first=S. I. | last=Tomkeieff | journal=Science Progress | volume=46 | issue=181 | date=January 1958 | pages=46–62 | publisher=Sage Publications, Ltd. | jstor=43424627 }}</ref> [[biology]],<ref>{{cite book | last1=Börner | first1=A. | last2=Zeidler | first2=J. | year=2023 | chapter=The Periodic Table of Elements and Basic Consequences for the Structure of Natural Substances and the Course of Biochemical Processes | title=The Chemistry of Biology | publisher=Springer | location=Berlin, Heidelberg | isbn=978-3-662-66521-3 | doi=10.1007/978-3-662-66521-3_1 }}</ref> [[materials science]], [[engineering]], [[agriculture]], [[medicine]],<ref>{{cite journal | title=Exploration of the medical periodic table: towards new targets | last1=Nicolas | first1=P. E. Barry | first2=Peter J. | last2=Sadler | journal=Chemical Communications | date=2013 | volume=49 | issue=45 | pages=5106–5131 | pmid=23636600 | doi=10.1039/c3cc41143e | doi-access=free }}</ref> [[nutrition]],<ref name=Spallholz_et_al_2018/> [[environmental health]], and [[astronomy]].<ref>{{cite book | last=Trimble | first=V. | year=2021 | chapter=Astronomy Meets the Periodic Table, Or, How Much Is There of What, and Why? | display-editors=1 | editor1-last=Giunta | editor1-first=C. J. | editor2-last=Mainz | editor2-first=V. V. | editor3-last=Girolami | editor3-first=G. S. | title=150 Years of the Periodic Table. Perspectives on the History of Chemistry | pages=387–407 | publisher=Springer | doi=10.1007/978-3-030-67910-1_15 | isbn=978-3-030-67909-5 }}</ref> Its principles are especially important in [[chemical engineering]].

=== Phase transition ===
[[File:Phase diagram of hydrogen.svg|right|thumb|upright=1.1|[[Phase diagram]] of hydrogen, showing the element state for different combinations of temperature and pressure. Theoretically, at a sufficiently high pressure (such as the interior of Jupiter), [[Metallic hydrogen|hydrogen becomes metallic]].<ref>{{cite journal | title=Metallic Hydrogen | display-authors=1 | last1=Silvera | first1=Isaac | last2=Zaghoo | first2=Mohamed | last3=Salamat | first3=Ashkan | year=2018 | journal=Journal of Physics: Condensed Matter | volume=30 | issue=25 | article-number=254003 | doi=10.1088/1361-648X/aac401 | pmid=29749966 | bibcode=2018JPCM...30y4003S }}</ref>]]
{{main|State of matter|Phase transition}}
A commonly used basic distinction among the elements is their [[state of matter]] (phase), whether [[solid]], [[liquid]], or [[gas]], at [[standard temperature and pressure]] (STP). Most elements are solids at STP, while several are gases. Only [[bromine]] and [[mercury (element)|mercury]] are liquid at 0 degrees Celsius (32 degrees Fahrenheit) and 1 atmosphere pressure;<ref>{{cite book | title=General Chemistry: Atoms First | first1=Donald A. | last1=McQuarrie | first2=Stanley | last2=Gill | date=15 June 2011 | edition=revised | publisher=MIT Press=2011 | isbn=978-1-891389-60-3 | pages=44–45 | url=https://books.google.com/books?id=B4ppEQAAQBAJ&pg=PA44 }}</ref> [[caesium]] and [[gallium]] are solid at that temperature, but melt at {{cvt|28.4|C|F}} and {{cvt|29.8|C|F}}, respectively.<ref>{{cite book | chapter=Thermal Properties of Liquid Metal | first=Gui Lin | last=Liu | title=Handbook of Liquid Metals | editor1-first=Jing | editor1-last=Liu | editor2-first=Wei | editor2-last=Rao | publisher=Springer Nature | year=2024 | isbn=978-981-97-1614-2 | pages=187–189 | chapter-url=https://books.google.com/books?id=ODQtEQAAQBAJ&pg=PA188 }}</ref>

[[Melting point|Melting]] and [[boiling point]]s, typically expressed in degrees [[Celsius]] at a pressure of one atmosphere, are commonly used in characterizing the various elements. Those elements with boiling points above {{Val|2000|u=C|fmt=commas}} are termed [[refractory]],<ref>{{cite book | title=Inorganic Chemistry: From Periodic Classification to Crystals | series=Analytical and inorganic chemistry series | first=Robert | last=Valls | publisher=John Wiley & Sons | year=2018 | isbn=978-1-78630-254-0 | page=58 | url=https://books.google.com/books?id=GY9FDwAAQBAJ&pg=PA58 }}</ref> while those easily vaporized are [[Volatility (chemistry)|volatiles]]. The element with the widest range between melting and boiling points is gallium, which boils at {{cvt|2204|C|F}}.<ref>{{cite book | chapter=Gallium | display-authors=1 | first1=Nora K. | last1=Foley | first2=Brian W. | last2=Jaskula | first3=Bryn E. | last3=Kimball | first4=Ruth F. | last4=Schulte | title=Critical Mineral Resources of the United States: Economic and Environmental Geology and Prospects for Future Supply | series=U.S. Geological Survey professional paper | volume=1802 | display-editors=1 | editor1-first=K. J. | editor1-last=Schulz | editor2-first=John H. | editor2-last=DeYoung | editor3-first=Robert R. | editor3-last=Seal | editor4-first=Dwight C. | editor4-last=Bradley | publisher=Government Printing Office | year=2017 | page=H2 | isbn=978-1-4113-3991-0 | chapter-url=https://books.google.com/books?id=F_khT5-GL7sC&pg=SL8-PA2 }}</ref> While known for most elements, either or both of these measurements is still undetermined for some of the radioactive elements available in only tiny quantities.<ref>For example: {{cite journal | title=Copernicium: A Relativistic Noble Liquid | display-authors=1 | last1=Mewes | first1=Jan-Michael | last2=Smits | first2=Odile R. | last3=Kresse | first3=Georg | last4=Schwerdtfeger | first4=Peter | journal=Angewandte Chemie | volume=131 | issue=50 | pages=18132–18136 | date=December 2019 | doi=10.1002/ange.201906966 | bibcode=2019AngCh.13118132M | doi-access=free }}</ref> Since helium remains a liquid even at [[absolute zero]] at atmospheric pressure, it has only a boiling point, and not a melting point, in conventional presentations.<ref>{{cite book | chapter=Properties of liquid helium | last=Kent | first=A. | year=1993 | title=Experimental low-temperature physics | series=Macmillan Physical Science Series | pages=52–75 | publisher=Palgrave | location=London | isbn=978-0-333-51951-6 | doi=10.1007/978-1-349-22736-5_3 }}</ref>

=== Allotropes ===
{{Main|Allotropy}}
Atoms of the same element may bond to each other chemically in more than one way, allowing the pure element to exist in multiple [[chemical structure]]s ([[Molecular geometry|spatial arrangements of atoms]]) which differ in their properties. The ability of an element to [[Crystal polymorphism|polymorph]] in one of many structural forms is known as 'allotropy'.<ref name=Campbell_2012>{{cite book | title=Phase Diagrams: Understanding the Basics | series=EngineeringPro collection | editor-first=Flake C. | editor-last=Campbell | publisher=ASM International | pages=36–37 | year=2012 | isbn=978-1-61503-986-9 | url=https://books.google.com/books?id=ton3r8vBtbgC&pg=PA36 }}</ref> Non-metallic elements known for polymorphism include carbon, sulfur, phosphorus, oxygen, and nitrogen.<ref>{{cite book | chapter=Thermodynamics of Polymorphs and Solvates | first=Gerald | last=Coquerel | title=Polymorphism in the Pharmaceutical Industry: Solid Form and Drug Development | editor1-first=Rolf | editor1-last=Hilfiker | editor2-first=Markus | editor2-last=von Raumer | publisher=John Wiley & Sons | year=2019 | page=93 | isbn=978-3-527-34040-8 | chapter-url=https://books.google.com/books?id=ynl7DwAAQBAJ&pg=PA93 }}</ref>

[[File:AllotropiCarbonisNaturali.png|right|thumb|upright=1.2|Three of the eight carbon allotropes<ref name=George_2021/>]]
For example, carbon can be found as [[diamond]], which has a tetrahedral structure around each carbon atom; [[graphite]], which has layers of carbon atoms with a hexagonal structure stacked on top of each other; [[graphene]], which is a single layer of graphite that is very strong; [[fullerene]]s, which have nearly spherical shapes; and [[carbon nanotube]]s, which are tubes with a hexagonal structure (even these may differ from each other in electrical properties).<ref name=George_2021>{{cite book | chapter=Utilization of carbon allotropes with special reference to carbon nanotubes and graphene for the high performance of natural rubber | display-authors=1 | first1=Neena | last1=George | first2=Ajalesh B. | last2=Nair | first3=Neethumol | last3=Varghese | first4=Rani | last4=Joseph | title=Chemistry, Manufacture and Applications of Natural Rubber | series=Woodhead Publishing in Materials | editor1-first=Shinzo | editor1-last=Kohjiya | editor2-first=Yuko | editor2-last=Ikeda | edition=2 | publisher=Woodhead Publishing | year=2021 | isbn=978-0-12-818844-6 | pages=203–208 | chapter-url=https://books.google.com/books?id=5nDhDwAAQBAJ&pg=PA204 | doi=10.1016/B978-0-12-818843-9.00014-X }}</ref>

The [[Standard state|reference state]] of an element is defined by convention, usually as the [[Chemical stability|thermodynamically most stable]] allotrope and physical state at a pressure of 1 [[bar (unit)|bar]] and a given temperature (typically {{Val|298.15|ul=K}}). However, for phosphorus, the reference state is [[white phosphorus]] even though it is not the most stable allotrope, and the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes. In [[thermochemistry]], an element is defined to have an [[Standard enthalpy of formation|enthalpy of formation]] of zero in its reference state.<ref>{{cite book | title=Practical Chemical Thermodynamics for Geoscientists | first1=Bruce | last1=Fegley | first2=Rose | last2=Osborne | publisher=Academic Press | year=2013 | isbn=978-0-12-251100-4 | pages=139–142 | url=https://books.google.com/books?id=Z8VNZNLNsTcC&pg=PA141 }}</ref>

=== Crystal structures ===
{{Main|Crystal structure}}

Under conditions of stable equilibrium, solid elements are structured into a crystalline form, with each element having its own structure.<ref name=Campbell_2012/> These belong to the seven families of [[crystal structure]]s: [[cubic crystal system|cubic]] (including [[cubic crystal system|body-centered]] and face-centered), [[Triclinic crystal system|triclinic]], [[Hexagonal crystal system|hexagonal]], [[Monoclinic crystal system|monoclinic]], [[orthorhombic crystal system|orthorhombic]], [[Trigonal crystal system|rhombohedral]], and [[Tetragonal crystal system|tetragonal]].<ref>{{cite book | title=Solid State Chemistry and its Applications | first=Anthony R. | last=West | edition=2nd | publisher=John Wiley & Sons | year=2022 | page=9 | isbn=978-1-118-69557-9 | url=https://books.google.com/books?id=BwhpEAAAQBAJ&pg=RA1-PA9 }}</ref> Over 30 elements crystallize in the cubic form.
40% of the elements form [[Close-packing of equal spheres|close-packed]] crystals: either face-centered cubic or hexagonal close-packed.<ref>{{cite book | title=Basic Elements of Crystallography | first1=Nevill Gonzalez | last1=Szwacki | first2=Teresa | last2=Szwacka | publisher=Pan Stanford Publishing | year=2010 | isbn=978-981-4241-59-5 | pages=67–102 | url=https://books.google.com/books?id=DjXs_-BlgHYC&pg=PA67 }}</ref> For some of the synthetically produced transuranic elements, available samples have been too small to determine crystal structures.

Under the high pressure conditions found within a planetary interior, elements can appear in new crystalline forms, forming allotropes.<ref>{{cite journal | title=High-pressure phases of group-IV, III V, and II VI compounds | display-authors=1 | last1=Mujica | first1=A. | last2=Rubio | first2=Angel | last3=Muñoz | first3=A. | last4=Needs | first4=R. J. | journal=Reviews of Modern Physics | volume=75 | issue=3 | pages=863–912 | date=July 2003 | doi=10.1103/RevModPhys.75.863 | bibcode=2003RvMP...75..863M | hdl=10261/98248 | hdl-access=free }}</ref> For example, seven dense classes of silicon crystals can appear at pressures from {{Val|1|u=MPa}} to {{Val|100|u=GPa}}, under room temperature conditions.<ref>{{cite journal | title=Exploring silicon allotropy and chemistry by high pressure - high temperature conditions | display-authors=1 | last1=Kurakevych | first1=O. O. | last2=Le Godec | first2=Y. | last3=Strobel | first3=T. A. | last4=Kim | first4=D. Y. | last5=Crichton | first5=W. A. | last6=Guignard | first6=J. | journal=Journal of Physics: Conference Series | volume=950 | issue=4 | article-number=042049 | date=October 2017 | doi=10.1088/1742-6596/950/4/042049 | bibcode=2017JPhCS.950d2049K | doi-access=free }}</ref> In the extreme conditions found inside a carbon-rich [[white dwarf]], diamond-like amorphous glass may form.<ref>{{cite journal | title=A 'diamond-like star' in the lab. Diamond-like glass | display-authors=1 | last1=Shumilova | first1=T. G. | last2=Tkachev | first2=S. N. | last3=Isaenko | first3=S. I. | last4=Shevchuk | first4=S. S. | last5=Rappenglück | first5=M. A. | last6=Kazakov | first6=V. A. | journal=Carbon | volume=100 | pages=703–709 | date=April 2016 | doi=10.1016/j.carbon.2016.01.068 | bibcode=2016Carbo.100..703S | osti=1702292 }}</ref>

=== Mass densities ===
{{Main|Mass density|Densities of the elements (data page)}}

The [[density]] at selected [[standard temperature and pressure]] (STP) is often used in characterizing the elements. The mass density of an element depends on the mass of the atomic nucleus and the separation beween the atoms created by the bound electrons. Density is given in kilograms per cubic meter (kg/m{{sup|3}}), but may also be expressed in grams per cubic centimetre (g/cm{{sup|3}}). Since several elements are gases at commonly encountered temperatures, their densities are usually stated for their gaseous forms; when liquefied or solidified, the gaseous elements have densities similar to those of the other elements. The general trend is for densities to increase as the atomic number rises. Lower density elements are the noble gases and the alkali metals. Higher densities are found in the middle of the rows of the periodic elements, as they can form more [[covalent bond]]s, drawing the atoms closer together.<ref>{{cite book | title=Understanding the Properties of Matter | first=Michael | last=de Podesta | edition=2 | publisher=CRC Press | year=2020 | isbn=978-1-4822-6782-2 | pages=176–182 | url=https://books.google.com/books?id=5joPEAAAQBAJ&pg=PA176 }}</ref>

[[File:Empirical atomic radius trends.svg|right|thumb|upright=1.1|Variation in atomic radius by atomic number, listing hydrogen and the alkali elements]]
Atoms do not have a fixed [[Atomic radius|radius]], but rather their dimension is determined by the charge distribution of their electron cloud. The measured size depends on the interaction of this cloud with the instrument used to measure it. Hence, various methods will give similar, but slightly different results.<ref>{{cite book | title=Atoms, Molecules and Photons: An Introduction to Atomic-, Molecular- and Quantum Physics | series=Graduate Texts in Physics | first=Wolfgang | last=Demtröder | edition=2nd | publisher=Springer Science & Business Media | year=2010 | isbn=978-3-642-10298-1 | pages=29–32 | url=https://books.google.com/books?id=vbc5mA7OEuYC&pg=PA29 }}</ref> Along each row of the periodic table, the radius tends to decrease from the alkali metal column to the noble gases. This is caused by the increasing attraction of the nuclear charge overcoming the mutual repulsion of the electrons as a shell is filled. A smaller radius means the atom is less chemically reactive, as the valence electrons are drawn closer to the nucleus.<ref>{{cite book | title=A Primer on Quantum Chemistry | first=S. M. | last=Blinder | publisher=John Wiley & Sons | year=2023 | isbn=978-1-394-19116-1 | pages=135–137 | url=https://books.google.com/books?id=JGTiEAAAQBAJ&pg=PA135 }}</ref> Depending on the type of chemical bond, the atomic radius determines the atomic separation in a crystal, and hence the mass density.<ref>{{cite book | title=Basic Elements of Crystallography | first1=Nevill Gonzalez | last1=Szwacki | first2=Teresa | last2=Szwacka | edition=2nd | publisher=CRC Press | year=2016 | isbn=978-981-4613-58-3 | page=102 | url=https://books.google.com/books?id=6abjCwAAQBAJ&pg=PA102 }}</ref>

When an element has allotropes with different densities, one representative allotrope is typically selected in summary presentations,{{cn|date=September 2025}} while densities for each allotrope can be stated where more detail is provided. For example, the three familiar [[allotropes of carbon]] ([[amorphous carbon]], [[graphite]], and [[diamond]]) have densities of 1.8–2.1, 2.267, and 3.515&nbsp;g/cm{{sup|3}}, respectively.<ref>{{cite book | chapter=Amorphous and Non-Crystalline Carbons | first=J. | last=Robertson | title=Graphite and Precursors | editor-first=Pierre | editor-last=Delhaes | publisher=Gordon and Breach Science Publishers | year=2000 | isbn=978-1-4822-9692-1 | page=250 | chapter-url=https://books.google.com/books?id=oU1ZDwAAQBAJ&pg=PA250 }}</ref>

== Nomenclature and symbols ==
{{anchor|Nomenclature}}

The various chemical elements are formally identified by their unique atomic numbers, their accepted names, and their [[chemical symbol]]s.

=== Atomic numbers ===
The known elements have atomic numbers from 1 to 118, conventionally presented as [[Arabic numerals]]. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest (as in a periodic table), sets of elements are sometimes specified by such notation as "through", "beyond", or "from ... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities), as in "lighter than carbon" or "heavier than lead", though their atomic weights of the elements do not always increase [[monotonic function|monotonically]] with their atomic numbers. For example, argon with an atomic number of 18 has an atomic weight of 39.95, while potassium with an atomic number of 19 has an atomic weight of 39.098.<ref name=IUPAC/>

=== Element names ===
{{main|Naming of chemical elements}}
[[File:Chemical Element Place Names World & Solar System Map.png|right|thumb|upright=2|Map illustrating the place names of 41 of the chemical elements on Earth and (inset) in the Solar System]]
The naming of various substances now known as elements precedes the [[atomic theory]] of matter, as names were given locally by various cultures to various minerals, metals, compounds, alloys, mixtures, and other materials, though at the time it was not known which chemicals were elements and which compounds. As they were identified as elements, the existing names for anciently known elements (e.g., gold, mercury, iron) were kept in most countries.

National differences emerged over the element names either for convenience, linguistic niceties, or nationalism.<ref name=Childs_2012/> For example, German speakers use "Wasserstoff" (water stuff) for "hydrogen", "Sauerstoff" (acid stuff) for "oxygen", and "Stickstoff" (smothering stuff) for "nitrogen"; English and some other languages use "sodium" for "natrium", and "potassium" for "kalium"; and the French, Italians, Greeks, Portuguese and Poles prefer "azote/azot/azoto" (from roots meaning "no life") for "nitrogen".

In the past, the name for new elements was traditionally decided by their discoverers.<ref name=Childs_2012>{{cite book | chapter=From hydrogen to meiternium: naming the chemical elements | first=P. E. | last=Childs | title=Chemical Nomenclature | editor-first=K. J. | editor-last=Thurlow | publisher=Springer Science & Business Media | year=2012 | pages=41–54 | isbn=978-94-011-4958-7 | chapter-url=https://books.google.com/books?id=0ODsCAAAQBAJ&pg=PA41 }}</ref> This changed in 1947, when a conference of the [[International Union of Pure and Applied Chemistry]] (IUPAC) decided that the names and symbols of new elements would be determined by the IUPAC. The discoverer of a new element had the right to suggest a name, but for purposes of international communication and trade, the [[Table of chemical elements|official names of the chemical elements]] both ancient and more recently recognised are decided by the IUPAC.<ref name=Koppenol_et_al_2016>{{cite journal | title=How to name new chemical elements (IUPAC Recommendations 2016) | display-authors=1 | first1=Willem H. | last1=Koppenol | first2=John | last2=Corish | first3=Javier | last3=García-Martínez | first4=Juris | last4=Meija | first5=Jan | last5=Reedijk | date=April 21, 2016 | journal=Pure and Applied Chemistry | volume=88 | issue=4 | pages=401–405 | doi=10.1515/pac-2015-0802 | hdl=10045/55935 | hdl-access=free }}</ref>

The IUPAC organization has decided on a style of international English language as a ''[[Lingua franca]]'', drawing on traditional English names even when an element's chemical symbol is based on a Latin or other traditional word. For example, adopting "gold" rather than "aurum" as the name for the 79th element (Au). IUPAC prefers the British spellings "[[aluminium]]" and "caesium" over the U.S. spellings "aluminum" and "cesium", and the U.S. "sulfur" over British "sulphur".<ref>{{cite book | title=Nature's Building Blocks: An A-Z Guide to the Elements | first=John | last=Emsley | edition=2nd | publisher=Oxford University Press | year=2011 | isbn=978-0-19-257046-8 | url=https://books.google.com/books?id=dGZaDwAAQBAJ&pg=PR9 }}</ref> However, elements that are practical to sell in bulk in many countries often still have locally used national names, and countries whose national language does not use the [[Latin alphabet]] are likely to use the IUPAC element names.

New elements have been named for their properties, after a mineral from which it was extracted, the location of its discovery, a mythical subject, an astronomical object, or a prominent scientist.<ref name=Koppenol_et_al_2016/><ref name=Childs_2012/> According to IUPAC, element names are not proper nouns; therefore, the full name of an element is not capitalised in English, even if derived from a [[proper noun]], as in [[californium]] and [[einsteinium]]. Isotope names are also uncapitalised if written out, ''e.g.,'' [[carbon-12]] or [[uranium-235]]. Chemical element ''symbols'' (such as Cf for californium and Es for einsteinium), are always capitalised.

In the second half of the 20th century, physics laboratories became able to produce elements with half-lives too short for an appreciable amount of them to exist at any time. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This practice can lead to the [[element naming controversy|controversial question of which research group actually discovered an element]], a question that delayed the naming of elements with atomic number of 104 and higher for a considerable amount of time.<ref>{{cite journal | first=Linda | last=Rothstein | title=The Transfermium Wars | journal=Bulletin of the Atomic Scientists | date=1995 | volume=51 | issue=1 | pages=5–6 | issn=0096-3402 | url=https://books.google.com/books?id=twwAAAAAMBAJ&pg=PA5 | publisher=Educational Foundation for Nuclear Science, Inc | doi=10.1080/00963402.1995.11658020 }}</ref>

Precursors of such controversies involved the nationalistic namings of elements in the late 19th century. For example, ''[[lutetium]]'' was named after Paris, France. The Germans were reluctant to relinquish naming rights to the French, often calling it ''cassiopeium''.<ref name=Holden_2001>{{cite conference | title=History of the Origin of the Chemical Elements and their Discoverers | first=Norman E. | last=Holden | conference=41st IUPAC General Assembly in Brisbane, Australia, June 29th – July 8th, 2001 | publisher=Brookhaven National Lab | location=Upton, NY (United States) | url=https://www-nds.iaea.org/workshops/smr1939/Documents/origindc.pdf | access-date=2025-09-26 }}</ref> Similarly, the British discoverer of ''[[niobium]]'' originally named it ''columbium'', in reference to the [[New World]].<ref name=Childs_2012/> It was used extensively as such by American publications before the international standardisation (in 1950).<ref name=Holden_2001/>

=== Chemical symbols ===
{{For|lists of current chemical symbols, symbols not currently used, and other symbols that may look like chemical symbols|Chemical symbol}}

==== Specific elements ====
[[File:Dalton's Element List.jpg|right|thumb|John Dalton's list of atomic weights & symbols, 1808]]
Before chemistry became a [[science]], [[alchemy|alchemists]] designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures;<ref>{{cite book | title=Alchemy and Alchemists | series=Dover classics of science and mathematics | first=Charles John Samuel | last=Thompson | edition=reprint | publisher=Courier Corporation | year=2002 | isbn=978-0-486-42110-0 | pages=9–11 | url=https://books.google.com/books?id=zaBAMNBJHIIC&pg=PA9 }}</ref> there was no concept of atoms combining to form [[molecule]]s. With his advances in the atomic theory of matter, [[John Dalton]] devised his own simpler symbols, based on circles, to depict atoms and molecules.<ref name=Perkins_2005/>

The current system of chemical notation was invented by [[Jöns Jacob Berzelius]] in 1814. In this system, chemical symbols are not mere abbreviations—though each consists of letters of the [[Latin alphabet]]. They are intended as universal symbols for people of all languages and alphabets.<ref name=Perkins_2005>{{cite journal | title=A History of Molecular Representation. Part One: 1800 to the 1960s | last=Perkins | first=J. A. | year=2005 | journal=Journal of Biocommunications | volume=31 | issue=1 | page=1 | url=http://jbiocommunication.org/issues/31-1/features3.html }}</ref>

Since Latin was the common language of science at Berzelius' time, his symbols were abbreviations based on the [[Latin]] names of elements (they may be Classical Latin names of elements known since antiquity or [[Neo-Latin]] coinages for later elements). The symbols are not followed by a period (full stop) as with abbreviations. In most cases, Latin names of elements as used by Berzelius have the same roots as the modern English name. For example, [[hydrogen]] has the symbol "H" from Neo-Latin {{lang|la|hydrogenium}}, which has the same Greek roots as English ''hydrogen''. However, in eleven cases Latin (as used by Berzelius) and English names of elements have different roots. Eight of them are the seven [[metals of antiquity]] and a metalloid also known since antiquity: "Fe" (Latin {{lang|la|ferrum}}) for [[iron]], "Hg" (Latin {{lang|la|hydrargyrum}}) for [[Mercury (element)|mercury]], "Sn" (Latin {{lang|la|stannum}}) for [[tin]], "Au" (Latin {{lang|la|aurum}}) for gold, "Ag" (Latin {{lang|la|argentum}}) for [[silver]], "Pb" (Latin {{lang|la|plumbum}}) for [[lead]], "Cu" (Latin {{lang|la|cuprum}}) for [[copper]], and "Sb" (Latin {{lang|la|stibium}}) for [[antimony]]. The three other mismatches between Neo-Latin (as used by Berzelius) and English names are "Na" (Neo-Latin {{lang|la|natrium}}) for [[sodium]], "K" (Neo-Latin {{lang|la|kalium}}) for [[potassium]], and "W" (Neo-Latin {{lang|la|wolframium}}) for [[tungsten]].<ref>{{cite journal | last=Meija | first=J. | year=2014 | title=Symbols of the Elements | journal=Chemistry International | volume=36 | issue=1 | pages=20–21 | series=NOTeS | doi=10.1515/ci.2014.36.1.20 }}</ref> These mismatches came from different suggestings of naming the elements in the [[Modern era]]. Initially Berzelius had suggested "So" and "Po" for sodium and potassium, but he changed the symbols to "Na" and "K" later in the same year.

Elements discovered after 1814 were also assigned unique chemical symbols, based on the name of the element. The use of Latin as the universal language of science was fading, but chemical names of newly discovered elements came to be borrowed from language to language with little or no modification. Symbols of elements discovered after 1814 match their names in English, French (ignoring the [[acute accent]] on ⟨é⟩), and German (though German often allows alternate spellings with ⟨k⟩ or ⟨z⟩ instead of ⟨c⟩: e.g., the name of [[calcium]] may be spelled {{lang|de|Calcium}} or {{lang|de|Kalzium}} in German, but its symbol is always "Ca"). Other languages sometimes modify element name spellings: Spanish {{lang|es|iterbio}} (ytterbium), Italian {{lang|it|afnio}} (hafnium), Swedish {{lang|sv|moskovium}} (moscovium); but those modifications do not affect chemical symbols: Yb, Hf, Mc.

Chemical symbols are understood internationally when element names might require translation. There have been some differences in the past. For example, Germans in the past have used "J" (for the name {{lang|de|Jod}}) for iodine, but now use "I" and {{lang|de|Iod}}.

The first letter of a chemical symbol is always capitalised, and the subsequent letters, if any, are always lowercase; see the preceding examples.

==== General chemical symbols ====
There are also symbols in chemical equations for groups of elements, for example in comparative formulas. These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, "'''X'''" indicates a variable group (usually a halogen) in a class of compounds, while "'''R'''" is a [[Radical (chemistry)|radical]], meaning a compound structure such as a hydrocarbon chain. The letter "'''Q'''" is reserved for "heat" in a chemical reaction.<ref name=CRC95>{{cite book | title=CRC Handbook of Chemistry and Physics | editor-first=William M. | editor-last=Haynes | edition=95, revised | publisher=CRC Press | year=2014 | page=2–29 | isbn=978-1-4822-0868-9 | url=https://books.google.com/books?id=bNDMBQAAQBAJ&pg=SA2-PA29 }}</ref> "'''Y'''" is also often used as a general chemical symbol,{{cn|date=September 2025}} though it is also the symbol of [[yttrium]] and [[tyrosine]].<ref name=CRC95/> "'''Z'''" is also often used as a general variable group.{{cn|date=September 2025}} "'''E'''" is used in organic chemistry to denote an [[Polar effect|electron-withdrawing group]] or an [[electrophile]];{{cn|date=September 2025}} similarly "'''Nu'''" denotes a [[nucleophile]].<ref name=CRC95/> "'''L'''" is used to represent a general [[ligand]] in [[inorganic chemistry|inorganic]] and [[organometallic chemistry]].{{cn|date=September 2025}} "'''M'''" is often used in place of a general metal.{{cn|date=September 2025}}

At least two other, two-letter generic chemical symbols are also in informal use, "'''Ln'''" for any [[lanthanide]] and "'''An'''" for any [[actinide]].{{cn|date=September 2025}} "'''Rg'''" was formerly used for any [[rare gas]] element, but the group of rare gases has now been renamed [[noble gas]]es and "'''Rg'''" now refers to [[roentgenium]].{{cn|date=September 2025}}

==== Isotope symbols ====
Isotopes of an element are distinguished by mass number (total protons and neutrons), with this number combined with the element's symbol. IUPAC prefers that isotope symbols be written in superscript notation when practical, for example {{sup|12}}C and {{sup|235}}U.<ref name=Hanson_2019>{{cite book | title=Organic Chemistry of Isotopic Labelling | first=James R. | last=Hanson | publisher=Royal Society of Chemistry | year=2019 | isbn=978-1-78801-818-0 | url=https://books.google.com/books?id=rbarDwAAQBAJ&pg=PT29 }}</ref> However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.<ref>{{cite book | title=Physics for Diagnostic Radiology | series=Series in Medical Physics and Biomedical Engineering | first1=Philip Palin | last1=Dendy | first2=Brian | last2=Heaton | edition=3rd | publisher=CRC Press | year=2011 | isbn=978-1-4398-9692-1 | page=5 | url=https://books.google.com/books?id=TWbNBQAAQBAJ&pg=PA5 }}</ref>

As a special case, the three naturally occurring isotopes of hydrogen are often specified as '''H''' for {{sup|1}}H ([[protium]]), '''D''' for {{sup|2}}H ([[deuterium]]), and '''T''' for {{sup|3}}H ([[tritium]]).<ref name=Hanson_2019/> This convention is easier to use in chemical equations, replacing the need to write out the mass number each time. Thus, the formula for [[heavy water]] may be written D{{sub|2}}O instead of {{sup|2}}H{{sub|2}}O.

== Origin of the elements ==
{{main|Nucleosynthesis}}
[[File:Universe content pie chart.jpg|thumb|upright=1.2|Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy labeled "atoms" is composed of elements.<ref name=Valle_Romao_2015/>]]
Only about 4.6% of the total mass of the universe is made of ordinary matter, including atoms or [[ion]]s represented by elements, plus [[neutrino]]s and photons. The rest of the mass (63%) is an unknown [[dark matter]] that is not composed of atoms of elements because it contains no protons, neutrons, or electrons. The remaining non-matter part of the mass of the universe is composed of the even less well understood [[dark energy]].<ref name=Valle_Romao_2015>{{cite book | title=Neutrinos in High Energy and Astroparticle Physics | series=Physics textbook | display-authors=1 | first1=Jose W. F. | last1=Valle | first2=Jorge | last2=Romao | publisher=John Wiley & Sons | year=2015 | isbn=978-3-527-41197-9 | page=318 | url=https://books.google.com/books?id=VWKxBgAAQBAJ&pg=PA318 }}</ref>

The 94 naturally occurring elements were produced by at least four classes of astrophysical process. Most of the hydrogen, helium and a very small quantity of lithium were produced in the first few minutes of the [[Big Bang]]. This [[Big Bang nucleosynthesis]] happened only once; the other processes are ongoing.<ref name=Randich_Magrini_2021/> [[Nuclear fusion]] inside stars produces elements through stellar nucleosynthesis, including all elements from carbon to [[iron]] in atomic number. Elements higher in atomic number than iron, including heavy elements like uranium and plutonium, are produced by various forms of explosive nucleosynthesis in [[supernova]]e and [[neutron star merger]]s. The light elements [[lithium]], [[beryllium]] and [[boron]] are produced mostly through [[cosmic ray spallation]] (fragmentation induced by [[cosmic ray]]s) of carbon, nitrogen, and oxygen.<ref name=Johnson_2019>{{cite journal | title=Populating the periodic table: Nucleosynthesis of the elements | last=Johnson | first=Jennifer A. | journal=Science | volume=363 | issue=6426 | pages=474–478 | date=February 2019 | doi=10.1126/science.aau9540 | pmid=30705182 | bibcode=2019Sci...363..474J }}</ref>

In the early phases of the Big Bang, nucleosynthesis of hydrogen resulted in the production of hydrogen-1 (protium, {{sup|1}}H) and helium-4 ({{sup|4}}He), as well as a smaller amount of deuterium ({{sup|2}}H) and tiny amounts (on the order of 10{{sup|−10}}) of lithium and beryllium. Even smaller amounts of boron may have been produced in the Big Bang, since it has been observed in some very old stars, while carbon has not.<ref>{{cite news | last=Wilford | first=J. N. | date=January 14, 1992 | title=Hubble Observations Bring Some Surprises | url=https://query.nytimes.com/gst/fullpage.html?res=9E0CE5D91F3AF937A25752C0A964958260 | newspaper=[[The New York Times]] | access-date=15 February 2017 | archive-date=5 March 2008 | archive-url=https://web.archive.org/web/20080305211140/http://query.nytimes.com/gst/fullpage.html?res=9E0CE5D91F3AF937A25752C0A964958260 | url-status=live}}</ref> No elements heavier than boron were produced in the Big Bang. As a result, the primordial abundance of atoms (or ions) consisted of ~75% {{sup|1}}H, 25% {{sup|4}}He, and 0.01% deuterium, with only tiny traces of lithium, beryllium, and perhaps boron.<ref>{{cite web | last=Wright | first=E. L. | date=September 12, 2004 | title=Big Bang Nucleosynthesis | url=http://www.astro.ucla.edu/~wright/BBNS.html|publisher=[[UCLA]], Division of Astronomy | access-date=22 February 2007 | archive-date=13 January 2018 | archive-url=https://web.archive.org/web/20180113051655/http://astro.ucla.edu/~wright/BBNS.html | url-status=live }}</ref> Subsequent enrichment of [[galactic spheroid|galactic halos]] occurred due to stellar nucleosynthesis and [[supernova nucleosynthesis]].<ref name="synthesis">{{cite journal | year=1999 | title=Synthesis of the elements in stars: forty years of progress | url=http://www.cococubed.com/papers/wallerstein97.pdf | journal=[[Reviews of Modern Physics]] | volume=69 | issue=4 | pages=995–1084 | doi=10.1103/RevModPhys.69.995 | bibcode=1997RvMP...69..995W | last1=Wallerstein | first1=George | author1-link=George Wallerstein | last2=Iben | first2=Icko | last3=Parker | first3=Peter | last4=Boesgaard | first4=Ann | last5=Hale | first5=Gerald | last6=Champagne | first6=Arthur |last7=Barnes |first7=Charles | last8=Käppeler | first8=Franz | last9=Smith | first9=Verne | display-authors=8 | url-status=dead | archive-url=https://web.archive.org/web/20060928043229/http://www.cococubed.com/papers/wallerstein97.pdf | archive-date=28 September 2006 | hdl=2152/61093 | hdl-access=free }}</ref> However, the element abundance in [[intergalactic space]] can still closely resemble primordial conditions, unless it has been enriched by a [[Galactic superwind|galactic wind]] or some other means.<ref>{{cite journal | title=On the enrichment of the intergalactic medium by galactic winds | last1=Nath | first1=Biman B. | last2=Trentham | first2=Neil | journal=Monthly Notices of the Royal Astronomical Society | volume=291 | issue=3 | pages=505–516 | date=November 1997 | doi=10.1093/mnras/291.3.505 | doi-access=free | arxiv=astro-ph/9707177 | bibcode=1997MNRAS.291..505N }}</ref>

[[File:Nucleosynthesis periodic table.svg|thumb|upright=1.78|Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the [[alpha process]]. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the [[r-process]]. Certain large stars slowly produce other elements heavier than iron, in the [[s-process]]; these may then be blown into space in the off-gassing of [[planetary nebulae]]]]

On Earth (and elsewhere), trace amounts of various elements continue to be produced from other elements as products of [[nuclear transmutation]] processes. These include some produced by [[cosmic ray]]s or other nuclear reactions (see [[cosmogenic]] and [[nucleogenic]] nuclides), and others produced as [[decay product]]s of long-lived primordial nuclides.<ref name="Earnshaw1997">{{cite book | last1=Earnshaw | first1=A. | last2=Greenwood | first2=N. | year=1997 | title=Chemistry of the Elements | edition=2nd | publisher=[[Butterworth-Heinemann]] }}</ref> For example, trace (but detectable) amounts of [[carbon-14]] ({{sup|14}}C) are continually produced in the air by cosmic rays impacting nitrogen atoms,<ref>{{cite journal | title=A new model of cosmogenic production of radiocarbon {{sup|14}}C in the atmosphere | display-authors=1 | last1=Kovaltsov | first1=Gennady A. | last2=Mishev | first2=Alexander | last3=Usoskin | first3=Ilya G. | journal=Earth and Planetary Science Letters | volume=337 | pages=114–120 | date=July 2012 | doi=10.1016/j.epsl.2012.05.036 | arxiv=1206.6974 | bibcode=2012E&PSL.337..114K }}</ref> and argon-40 ({{sup|40}}Ar) is continually produced by the decay of primordially occurring but unstable potassium-40 ({{sup|40}}K).<ref>{{cite journal | title=Production of {{sup|40}}Ar by an overlooked mode of {{sup|40}}K decay with implications for K-Ar geochronology | display-authors=1 | last1=Carter | first1=Jack | last2=Ickert | first2=Ryan B. | last3=Mark | first3=Darren F. | last4=Tremblay | first4=Marissa M. | last5=Cresswell | first5=Alan J. | last6=Sanderson | first6=David C. W. | journal=Geochronology | volume=2 | issue=2 | pages=355–365 | date=November 2020 | doi=10.5194/gchron-2-355-2020 | doi-access=free | bibcode=2020GeChr...2..355C }}</ref>

Three primordially occurring but radioactive actinides, thorium, uranium, and plutonium, decay through a series of recurrently produced but unstable elements such as radium and [[radon]], which are transiently present in any sample of containing these metals.<ref>{{cite book | chapter=Natural Radiation and Environment | display-authors=1 | last1=Jha | first1=S. K. | last2=Patra | first2=A. C. | last3=Verma | first3=G. P. | last4=Iyer | first4=I. S. | last5=Aswal | first5=D. K. | year=2024 | title=Handbook on Radiation Environment | volume=1 | pages=27–72 | publisher=Springer | location=Singapore | doi=10.1007/978-981-97-2795-7_2 | isbn=978-981-97-2794-0 }}</ref> Three other radioactive elements, technetium, promethium, and neptunium, occur only incidentally in natural materials, produced as individual atoms by [[nuclear fission]] of the nuclei of various heavy elements or in other rare nuclear processes.<ref name=Pentreath_2021/><ref name=Attrep_jr_Kuroda_1968/>

Besides the 94 naturally occurring elements, several [[artificial element]]s have been produced by [[nuclear physics]] technology. By 2016, these experiments had produced all elements up to atomic number 118. {{Asof|2021}}, more than a thousand different isotopes have been created through [[nuclear transmutation]], of which 900 do not appear naturally.<ref>{{cite book | title=Chemistry: The Molecular Nature of Matter | first1=Neil D. | last1=Jespersen | first2=Alison | last2=Hyslop | date=2 November 2021 | edition=8th | publisher=John Wiley & Sons, 2021 | isbn=978-1-119-13028-4 | url=https://books.google.com/books?id=UghHEAAAQBAJ&pg=PA1011 }}</ref>

== Abundance ==
{{Main|Abundance of the chemical elements}}
[[File:SolarSystemAbundances.svg|thumb|upright=1.7|Plot of element abundances in the Solar System.<ref>{{cite journal | title=Solar System Abundances and Condensation Temperatures of the Elements | last=Lodders | first=Katharina | journal=The Astrophysical Journal | volume=591 | issue=2 | pages=1220–1247 | date=July 2003 | doi=10.1086/375492 | bibcode=2003ApJ...591.1220L }}</ref> The two general trends are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the [[Oddo–Harkins rule]]), and (2) a  decrease in abundance as elements become heavier.]]
The abundance of elements in the Solar System is in keeping with their origin [[Big Bang nucleosynthesis]] and a number of progenitor supernova stars. Very abundant hydrogen and helium are products of the Big Bang, but the next three elements are rare since they had little time to form in the Big Bang and are not made in stars. They are, however, produced in small quantities by the breakup of heavier elements in interstellar dust, as a result of impact by cosmic rays.<ref name=Randich_Magrini_2021>{{cite journal | title=Light Elements in the Universe | last1=Randich | first1=Sofia | last2=Magrini | first2=Laura | journal=Frontiers in Astronomy and Space Sciences | volume=8   | date=March 2021 | article-number=616201 | doi=10.3389/fspas.2021.616201 | doi-access=free | arxiv=2103.11000 | bibcode=2021FrASS...8....6R }}</ref>

Beginning with carbon, elements are produced in stars by buildup from alpha particles (helium nuclei), resulting in an alternatingly larger abundance of elements with even atomic numbers, as these are more stable. In general, such elements up to the [[iron peak]] are made in massive stars in the process of becoming [[supernova]]s.<ref name=Johnson_2019/> Iron-56 is particularly common, since it is the most stable nuclide that can easily be made from alpha particles, being a product of decay of radioactive nickel-56, ultimately made from 14 helium nuclei.<ref>{{cite journal | title=Star Formation, Supernovae, Iron, and α: Consistent Cosmic and Galactic Histories | last1=Maoz | first1=Dan | last2=Graur | first2=Or | journal=The Astrophysical Journal | volume=848 | issue=1 | at=id. 25 | date=October 2017 | doi=10.3847/1538-4357/aa8b6e | doi-access=free | arxiv=1703.04540 | bibcode=2017ApJ...848...25M }}</ref> Elements heavier than iron and up to bismuth are made in neutron capture processes in lower mass stars,<ref name=Johnson_2019/> and their abundance in the universe (and on Earth) generally decreases with their atomic number.

{|class="wikitable" style="font-size: 90%; float: left; margin: 0.5em; margin-left: 1em;"
|+ Top Milky Way abundances<ref name="croswell">{{cite book |last=Croswell |first=Ken |author-link=Ken Croswell |year=1996 |title=Alchemy of the Heavens |url=http://kencroswell.com/alchemy.html |publisher=Anchor |isbn=978-0-385-47214-2 |access-date=10 October 2007 |archive-date=13 May 2011 |archive-url=https://web.archive.org/web/20110513233910/http://www.kencroswell.com/alchemy.html |url-status=live }}</ref>
|-
!Element!!Parts per million<br />by mass
|-
|[[Hydrogen]]
|739,000
|-
|[[Helium]]
|240,000
|-
|[[Oxygen]]
|10,400
|-
|[[Carbon]]
|4,600
|-
|[[Neon]]
|1,340
|-
|[[Iron]]
|1,090
|-
|[[Nitrogen]]
|960
|-
|[[Silicon]]
|650
|-
|[[Magnesium]]
|580
|-
|[[Sulfur]]
|440
|-
|[[Potassium]]
|210
|-
|[[Nickel]]
|100
|}
Nearby galaxies that have evolved along similar lines to the Milky Way have a corresponding enrichment of elements heavier than hydrogen and helium. The more distant galaxies are being viewed as they appeared in the past, so their abundances of elements appear closer to the primordial mixture.<ref>{{cite book | chapter=Abundance evolution with cosmic time | last=Truran | first=J. W. | year=2008 | title=The Cosmic Circuit of Matter | pages=261–274 | series=Reviews in Modern Astronomy | volume=16 | editor-first=Reinhard E. | editor-last=Schielicke | publisher=John Wiley & Sons | isbn=978-3-527-61765-4 | chapter-url=https://books.google.com/books?id=GlNbSOf3oVMC&pg=PA261 }}</ref> However, as physical laws and processes appear common throughout the [[visible universe]],<ref>{{cite journal | title=Evidence for large-scale uniformity of physical laws | last1=Tubbs | first1=A. D. | last2=Wolfe | first2=A. M. | journal=Astrophysical Journal, Part 2 - Letters to the Editor | volume=236 | pages=L105–L108 | date=March 1980 | doi=10.1086/183208 | bibcode=1980ApJ...236L.105T }}</ref> scientists expect that these galaxies evolved elements in similar abundance.

The [[abundance of the chemical elements]] on Earth varies from air to crust to ocean, and in various types of life. The abundance of elements in Earth's crust differs from that in the Solar System (as seen in the Sun and massive planets like Jupiter) mainly in selective loss of the very lightest elements (hydrogen and helium) and also volatile neon, carbon (as hydrocarbons), nitrogen and sulfur, as a result of solar heating in the early formation of the Solar System.<ref name=Klein_Philpotts_2013>{{cite book | title=Earth Materials: Introduction to Mineralogy and Petrology | first1=Cornelis | last1=Klein | first2=Anthony R. | last2=Philpotts | publisher=Cambridge University Press | year=2013 | isbn=978-0-521-14521-3 | page=6 | url=https://books.google.com/books?id=V7nUnYKmrxgC&pg=PA6 }}</ref> Oxygen, the most abundant element by mass in the Earth as a whole,<ref>{{cite book | title=Introduction to Geophysics: Global Physical Fields and Processes in the Earth | series=Springer Textbooks in Earth Sciences, Geography and Environment | first=Christoph | last=Clauser | publisher=Springer Nature | year=2024 | page=28 | isbn=978-3-031-17867-2 | url=https://books.google.com/books?id=Z_8CEQAAQBAJ&pg=PA28 }}</ref> is retained on Earth by combination with silicon, magnesium, and iron.<ref name=Klein_Philpotts_2013/> Aluminium at 8% by mass is more common in the Earth's crust compared to the solar abundance, but it is only 2% by mass in the mantle, which has magnesium and iron in place of aluminium.{{cn|date=November 2025}} The surface abundance of iron is lower because it has migrated to the Earth's core during the process of [[planetary differentiation]].<ref>{{cite book | title=Principles of Environmental Chemistry | series=G - Reference, Information and Interdisciplinary Subjects Series | first=James E. | last=Girard | edition=2nd | publisher=Jones & Bartlett Learning | year=2010 | isbn=978-0-7637-5939-1 | url=https://books.google.com/books?id=NuQV-Prt9f0C&pg=PA9 }}</ref>

The [[composition of the human body]], by contrast, more closely follows the composition of [[seawater]]—save that the human body has additional stores of carbon and nitrogen necessary to form the [[protein]]s and [[nucleic acid]]s, together with [[phosphorus]] in the nucleic acids and energy transfer molecule [[adenosine triphosphate]] (ATP) that occurs in the cells of all living organisms. The bulk of all life forms on Earth consist of just six elements, described by the acronym [[CHNOPS]]: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.<ref name=Spallholz_et_al_2018>{{cite book | title=Nutrition: Chemistry and Biology | display-authors=1 | first1=Julian E. | last1=Spallholz | first2=Mallory | last2=Boylan | first3=Judy A. | last3=Driskell | edition=2, reprint | publisher=Routledge | year=2018 | isbn=978-1-351-42739-5 | pages=1–5 | url=https://books.google.com/books?id=qnx0DwAAQBAJ&pg=PA1 }}</ref> Certain kinds of [[organism]]s require particular additional elements, for example the [[magnesium]] in [[chlorophyll]] in green plants, the [[calcium]] in [[mollusc shell]]s, or the iron in the [[hemoglobin]] in [[vertebrate]]s' [[red blood cell]]s.<ref>{{cite book | title=Essential AS Biology | series=Essential Biology Series | display-authors=1 | first1=A. G. | last1=Toole | first2=Glenn | last2=Toole | first3=S. M. | last3=Toole | publisher=Nelson Thornes | year=2002 | isbn=978-0-7487-6505-8}}</ref>

{{clear}}

{{Periodic table (dietary elements)}}

== History ==
The concept of an "element" as an indivisible substance has developed through three major historical phases: Classical definitions (such as those of the ancient Greeks), chemical definitions, and atomic definitions.

=== Classical definitions ===
{{Main|Classical element#Hellenistic philosophy}}
[[Ancient philosophy]] posited a set of [[classical element]]s to explain observed patterns in [[nature]]. These ''elements'' originally referred to ''[[Earth (classical element)|earth]]'', ''[[Water (classical element)|water]]'', ''[[Air (classical element)|air]]'' and ''[[Fire (classical element)|fire]]'' rather than the chemical elements of modern science.<ref>{{cite book | chapter=Aristotle, Empedocles, and the Reception of the Four Elements Hypothesis | first=Timothy J. | last=Crowley | title=Brill's Companion to the Reception of Presocratic Natural Philosophy in Later Classical Thought | date=2021 | publisher=Brill | pages=352–376 | isbn=978-90-04-44335-8 | doi=10.1163/9789004443358_013 }}</ref>

The term 'elements' (''stoicheia'') was first used by Greek philosopher [[Plato]] around 360&nbsp;BCE in his dialogue [[Timaeus (dialogue)|Timaeus]], which includes a discussion of the composition of inorganic and organic bodies and is a speculative treatise on chemistry. Plato believed the elements introduced a century earlier by [[Empedocles]] were composed of small [[regular polyhedron|polyhedral]] [[Theory of Forms|forms]]: [[tetrahedron]] (fire), [[octahedron]] (air), [[icosahedron]] (water), and [[cube]] (earth).<ref>{{cite book | author=Plato | year=2008 | orig-year=c. 360 BC | title=Timaeus | url=https://books.google.com/books?id=xSjvowNydN8C&q=Plato%20timaeus&pg=PA45 | publisher=Forgotten Books | page=45 | isbn=978-1-60620-018-6 | access-date=9 November 2020 | archive-date=14 April 2021 | archive-url=https://web.archive.org/web/20210414140053/https://books.google.com/books?id=xSjvowNydN8C&q=Plato%20timaeus&pg=PA45 | url-status=live}}</ref><ref>{{cite web | last=Hillar | first=M. | year=2004 | title=The Problem of the Soul in Aristotle's De anima | url=http://www.socinian.org/aristotles_de_anima.html | publisher=[[NASA]]/[[WMAP]] | access-date=10 August 2006 | url-status=dead | archive-url=https://web.archive.org/web/20060909004214/http://www.socinian.org/aristotles_de_anima.html | archive-date=9 September 2006 }}</ref>

[[Aristotle]], {{circa|350 BCE}}, also used the term ''stoicheia'' and added a fifth element, [[Aether (classical element)|aether]], which formed the heavens. Aristotle defined an element as:

{{blockquote|Element – one of those bodies into which other bodies can decompose, and that itself is not capable of being divided into other.<ref>{{cite book | last=Partington | first=J. R. | year=1937 | title=A Short History of Chemistry | location=New York | publisher=Dover Publications | isbn=((978-0-486-65977-0)) | url-access=registration | url=https://archive.org/details/shorthistoryofch0000part_q6h4 }}</ref>}}

=== Chemical definitions ===

==== Robert Boyle ====
[[File:Portret van Robert Boyle, RP-P-OB-4578 (cropped).jpg|thumb|200x200px|Robert Boyle, {{circa|1740}}]]
[[File:Sceptical chymist 1661 Boyle Title page AQ18 (3).jpg|thumb|330x330px|Title page of ''The Sceptical Chymist,'' published in 1661]]

In 1661, in ''[[The Sceptical Chymist]]'', [[Robert Boyle]] proposed his theory of corpuscularism, which favoured the analysis of matter as constituted of irreducible units of matter (atoms). Boyle argued against deciding upon a pre-determined number of elements. This was in contrast with Aristotle's view of the four [[classical element]] and [[Paracelsus]]' view of three chemical [[Principle (chemistry)|principles]] of sulfur, mercury, and salt.

{{Blockquote|text=Much of what I am to deliver ... may be indifferently apply'd to the four Peripatetick Elements, and the three Chymical Principles ... the Chymical ''Hypothesis'' seeming to be much more countenanc'd by Experience then the other, it will be expedient to insist chiefly upon the disproving of that; especially since most of the Arguments that are imploy'd against it, may, by a little variation, be made ... at least as strongly against the less plausible, ''Aristotelian'' Doctrine.{{Sfn|Boyle|1661|p=36}}}}

Boyle stated his view in four propositions. In the first and second, he suggests that matter consists of particles, but that these particles may be difficult to separate. Boyle used the concept of "corpuscles"—or "atomes",{{Sfn|Boyle|1661|p=38}} as he also called them—to explain how a limited number of elements could combine into a vast number of compounds.

{{Blockquote|text='''Propos. I.''' ''... At the first Production of mixt Bodies, the Universal Matter whereof they ... consisted, was actually divided into little Particles.''{{Sfn|Boyle|1661|p=37}} ... The Generation ... and wasting of Bodies ... and ... the Chymical Resolutions of mixt Bodies, and ... Operations of ... Fires upon them ... manifest their consisting of parts very minute...  ''Epicurus'' ... as you well know, supposes ... all ... Bodies ... to be produc'd by ... Atomes, moving themselves to and fro ... in the ... Infinite ''Vacuum''.{{Sfn|Boyle|1661|pp=37–38}} ... '''Propos. II.''' ''... These minute Particles ... were ... associated into minute ... Clusters ... not easily dissipable into such Particles as compos'd them.''{{Sfn|Boyle|1661|pp=38–39}} ... If we assigne to the Corpuscles, whereof each Element consists, a peculiar size and shape ... such ... Corpuscles may be mingled in such various Proportions, and ... connected so many ... wayes, that an almost incredible number of ... Concretes may be compos'd of them.{{Sfn|Boyle|1661|p=42}}}}

Boyle explained that gold reacts with ''[[aqua regia]],'' and mercury with nitric acid, sulfuric acid, and sulfur to produce various "compounds", and that they could be recovered from those compounds, just as would be expected of elements. Yet, Boyle did not consider gold,{{Sfn|Boyle|1661|p=29}} mercury,{{Sfn|Boyle|1661|p=41}} or lead{{Sfn|Boyle|1661|p=29}} elements, but rather—together with wine{{Sfn|Boyle|1661|p=145}}—"perfectly mixt bodies".
 
{{Blockquote|text=Quicksilver ... with ''Aqua fortis'' will be brought into a ... white Powder ... with Sulphur it will compose a blood-red ... Cinaber. And yet out of all these exotick Compounds, we may recover the very same running Mercury.{{Sfn|Boyle|1661|pp=40–41}} ... '''Propos. III.''' ''... From most of such mixt Bodies ... there may by the Help of the Fire, be actually obtain'd a determinate number (whether Three, Four or Five, or fewer or more) of Substances ...''
The Chymists are wont to call the Ingredients of mixt Bodies, ''Principles'', as the ''Aristotelians'' name them ''Elements''. ... ''Principles'' ... as not being compounded of any more primary Bodies: and ''Elements'', in regard that all mix'd Bodies are compounded of them.{{Sfn|Boyle|1661|p=46}}}}

Even though Boyle is primarily regarded as the first modern chemist, ''The Sceptical Chymist'' still contains old ideas about the elements, which are alien to a contemporary viewpoint. For example, sulfur is not only the familiar yellow non-metal but also an inflammable "spirit".{{Sfn|Boyle|1661|p=145}}

==== Isaac Watts ====
[[File:Portrait of Isaac Watts, D.D..jpg|thumb|216x216px|Portrait of Isaac Watts by John Shury, {{circa|1830}}]]
In 1724, in his book ''[[Logick]]'', the English minister and logician [[Isaac Watts]] enumerated the elements then recognised by chemists. Watts' list of elements included two of Paracelsus' ''principles'' (sulfur and salt) and two classical elements (earth and water) as well as "spirit". Watts did, however, note a lack of consensus among chemists.<ref>{{Cite book |last=Watts |first=Isaac |url=https://archive.org/details/logickorrightuse00wattuoft/page/13/mode/1up |title=Logick: Or, the right use of reason in the enquiry after truth, with a variety of rules to guard against error in the affairs of religion and human life, as well as in the sciences |publisher=Printed for John Clark and Richard Hett |year=1726 |pages=13–15 |orig-date=1724}}</ref>
{{Blockquote|text=Elements are such Substances as cannot be resolved, or reduced, into two or more Substances of different Kinds. ... Followers of Aristotle made Fire, Air, Earth and Water to be the four Elements, of which all earthly Things were compounded; and they suppos'd the Heavens to be a Quintessence, or fifth sort of Body, distinct from all these : But, since experimental Philosophy ... have been better understood, this Doctrine has been abundantly refuted. The Chymists make Spirit, Salt, Sulphur, Water and Earth to be their five Elements, because they can reduce all terrestrial Things to these five :.. tho' they are not all agreed.}}

==== Tabulation ====
[[File:Mendeleev's 1869 periodic table.svg|upright=1.35|thumb|[[Mendeleev]]'s 1869 periodic table: ''An experiment on a system of elements. Based on their atomic weights and chemical similarities.'']]
The first modern list of elements was given in [[Antoine Lavoisier]]'s 1789 ''[[Traité Élémentaire de Chimie|Elements of Chemistry]]'', which contained 33 elements, including [[light]] and [[Caloric theory|caloric]].<ref>{{cite book|last=Lavoisier|first=A. L.|year=1790|title=Elements of chemistry translated by Robert Kerr|url=https://books.google.com/books?id=4BzAjCpEK4gC&pg=PA175|place=Edinburgh|pages=175–176|isbn=((978-0-415-17914-0))|access-date=24 August 2020|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414100525/https://books.google.com/books?id=4BzAjCpEK4gC&pg=PA175|url-status=live}}</ref><ref>{{Cite book |last=Lavoisier |first=Antoine |author-link=Antoine Lavoisier |url=https://www.gutenberg.org/cache/epub/30775/pg30775-images.html#Page_175 |title=Elements of chemistry: In a new systematic order, containing all the modern discoveries: Illustrated with thirteen copperplates |publisher=William Creech |year=1790 |pages=175–176 |language=en |translator-last=Kerr |translator-first=Robert |orig-date=1789}}</ref> By 1818, [[Jöns Jacob Berzelius]] had determined atomic weights for 45 of the 49 then-accepted elements.<ref>{{cite journal | title=Jöns Jacob Berzelius – A Father of Chemistry | first1=Robert A. | last1=Kyle | first2=David P. | last2=Steensma | journal=Mayo Clinic Proceedings | volume=93 | issue=5 | pages=e53–e54 | date=May 2018 | pmid=29728209 | doi=10.1016/j.mayocp.2017.07.020 }}</ref> Russian chemist [[Dmitri Mendeleev]] had 63 elements in his 1869 periodic table. Though earlier precursors to this presentation exist, its invention is generally credited to Mendeleev, who intended the table to illustrate recurring trends in the properties of the elements.<ref>{{cite journal | title=D. I. Mendeleev: reflecting on his death in 1907 | first=M. D. | last=Gordin | journal=Angewandte Chemie (International Edition in English) | year=2007 | volume=46 | issue=16 | pages=2758–2765 | pmid=17274095 | doi=10.1002/anie.200601976 | bibcode=2007ACIE...46.2758G }}</ref> The layout of the table has been refined and extended over time as new elements have been discovered and new theoretical models have been developed to explain chemical behavior.

[[File:DIMendeleevCab.jpg|thumb|left|upright|Dmitri Mendeleev, 1897]]
From Boyle until the early 20th century, an element was defined as a pure substance that cannot be decomposed into any simpler substance and cannot be transformed into other elements by chemical processes. Elements at the time were generally distinguished by their atomic weights, a property measurable with fair accuracy by available analytical techniques.

=== Atomic definitions ===
[[File:Henry Moseley.jpg|thumb|upright=1.0|[[Henry Moseley]]]]

The 1913 discovery by English physicist [[Henry Moseley]] that the nuclear charge is the physical basis for the atomic number, further refined when the nature of protons and [[neutron]]s became appreciated, eventually led to the current definition of an element based on atomic number (number of protons).<ref>{{cite journal | title=Henry Moseley, X-ray spectroscopy and the periodic table | first1=Russell G. | last1=Egdell | first2=Elizabeth | last2=Bruton | journal=Philosophical Transactions A: Mathematical, Physical and Engineering Sciences | date=September 18, 2020 | volume=378 | issue=2180 | pages=1–33 | pmid=32811359 | jstor=26933299 | doi=10.1098/rsta.2019.0302 | bibcode=2020RSPTA.37890302E }}</ref> The use of atomic numbers, rather than atomic weights, to distinguish elements has greater predictive value (since these numbers are integers) and also resolves some ambiguities in the chemistry-based view due to varying properties of isotopes and [[allotrope]]s within the same element. Currently, IUPAC defines an element to exist if it has isotopes with a lifetime longer than the 10{{sup|−14}} seconds it takes the nucleus to form an electronic cloud.<ref>[http://www.kernchemie.de/Transactinides/Transactinide-2/transactinide-2.html Transactinide-2] {{Webarchive|url=https://web.archive.org/web/20160303222840/http://www.kernchemie.de/Transactinides/Transactinide-2/transactinide-2.html |date=3 March 2016 }}. www.kernchemie.de</ref>

By 1914, eighty-seven elements were known, all naturally occurring (see [[Discovery of chemical elements]]).{{cn|date=September 2025}} The remaining naturally occurring elements were discovered or isolated in subsequent decades, and various additional elements have also been produced synthetically, with much of that work pioneered by [[Glenn T. Seaborg]].<ref>{{cite book | title=Chemists | series=Great Scientists | first=Dean | last=Miller | publisher=Cavendish Square Publishing, LLC | year=2014 | page=125 | isbn=978-1-62712-556-7 | url=https://books.google.com/books?id=i1xmDwAAQBAJ&pg=PA125 }}</ref> The final naturally-occurring radioactive element, [[francium]], was discovered in 1939 by [[Marguerite Perey]].<ref>{{cite journal | last1=Adloff | first1=J. P. | first2=George B. | last2=Kauffman | title=Triumph over Prejudice: The Election of Radiochemist Marguerite Perey (1909–1975) to the French Académie des Sciences  | journal=The Chemical Educator | volume=10 | pages=395–399 | year=2005 | url=https://www.perey.org/genealogy/MP%203.pdf | access-date=2025-09-27 }}</ref> In 1955, element 101 was discovered and named [[mendelevium]] in honor of D. I. Mendeleev, the first to arrange the elements periodically.<ref>{{cite web | title=The Discovery of the Transuranium Elements: Their History and a Presentation of the Different Methods Used in Their Discovery | last=Thompson | first=S. G. | date=March 31, 1959 | publisher=U. S. Atomic Energy Commission | doi=10.2172/4253971 | osti=4253971 | url=https://escholarship.org/uc/item/84k060kr | access-date=2025-09-27 }}</ref>

=== Discovery and recognition of various elements ===
{{For timeline|Discovery of chemical elements}}
[[File:Discovery of chemical elements.svg|right|thumb|upright=1.4|Periodic table color coded by date of discovery, with red showing elements known since antiquity]]
Eleven materials familiar to various prehistoric cultures are now known to be elements: [[antimony]], carbon, copper, [[gold]], iron, lead, mercury, silver, sulfur, [[tin]], and [[zinc]].<ref>{{cite journal | title=The discovery of the elements. I. Elements known to the ancient world | first=Mary Elvira | last=Weeks | author-link=Mary Elvira Weeks | journal=Journal of Chemical Education | volume=9 | issue=1 | page=4 | date=January 1932 | doi=10.1021/ed009p4 | bibcode=1932JChEd...9....4W }}</ref> Two additional materials now accepted as elements, [[arsenic]] and [[bismuth]], were recognised as distinct substances before 1500 AD.<ref>{{cite book | chapter=Metallurgy of the elements | first=I. J. | last=Polmear | editor-first=N. C. | editor-last=Norman | pages=39–65 | publisher=Springer Science & Business Media | year=1997 | title=Chemistry of Arsenic, Antimony and Bismuth | series=Blackie Academic and Professional | isbn=978-0-7514-0389-3 | chapter-url=https://books.google.com/books?id=vVhpurkfeN4C&pg=PA39 }}</ref> [[Platinum]] was known in Pre-Columbian America. The first recorded discovery of a new element was of [[phosphorus]] by the German alchemist [[Hennig Brand]] in 1669.<ref name=Heilbronner_Miller_2004>{{cite book | title=A Philatelic Ramble Through Chemistry | first1=Edgar | last1=Heilbronner | first2=Foil A. | last2=Miller | publisher=John Wiley & Sons | year=2004 | isbn=978-3-906390-31-4 | page=40 | url=https://books.google.com/books?id=PcoFlVkjnjoC&pg=PA40 }}</ref> In 1896 [[Henri Becquerel]] serendipitously discovered radioactivity from uranium.<ref>{{cite journal | title=The discovery of radioactivity | last1=Radvanyi | first1=Pierre | last2=Villain | first2=Jacques | journal=Comptes Rendus Physique | volume=18 | issue=9–10 | pages=544–550 | date=November 2017 | doi=10.1016/j.crhy.2017.10.008 | bibcode=2017CRPhy..18..544R }}</ref>

Most of the remaining naturally occurring elements were identified and characterised by 1900, including:<ref name=Holden_2001/>
* Such now-familiar [[Industry (manufacturing)|industrial]] materials as [[aluminium]], [[silicon]], [[chromium]], [[cobalt]], [[nickel]], magnesium, and tungsten
* Reactive metals such as [[lithium]], [[sodium]], potassium, and [[calcium]]
* The [[halogen]]s [[fluorine]], [[chlorine]], [[bromine]], and [[iodine]]
* Gases such as hydrogen, oxygen, nitrogen, helium, [[argon]], and [[neon]]
* Most of the [[rare-earth elements]], including [[cerium]], [[lanthanum]], [[gadolinium]], and [[neodymium]]
* The more common [[radioactive]] elements, including uranium, thorium, and [[radium]]

Elements isolated or produced since 1900 include:<ref name=Holden_2001/>
* The three remaining undiscovered stable elements: [[hafnium]], [[lutetium]], and [[rhenium]]
* [[Plutonium]], which was first produced synthetically in 1940 by [[Glenn T. Seaborg]], but is now also known from a few long-persisting natural occurrences
* The three incidentally occurring natural elements ([[neptunium]], promethium, and technetium), which were all first produced synthetically but later discovered in trace amounts in geological samples<ref name=Pentreath_2021>{{cite book | title=Radioecology: Sources and Consequences of Ionising Radiation in the Environment | series=Cambridge Environmental Chemistry Series | first=R. J. | last=Pentreath | publisher=Cambridge University Press | year=2021 | isbn=978-1-009-04033-4 | page=86 | url=https://books.google.com/books?id=avRVEAAAQBAJ&pg=PA86 }}</ref><ref name=Attrep_jr_Kuroda_1968>{{cite journal | title=Promethium in pitchblende | first1=Moses | last1=Attrep Jr. | first2=P. K. | last2=Kuroda | journal=Journal of Inorganic and Nuclear Chemistry | volume=30 | issue=3 | date=May 1968 | pages=699–703 | doi=10.1016/0022-1902(68)80427-0 }}</ref>
* Four scarce decay products of uranium or thorium (astatine, francium, [[actinium]], and [[protactinium]]),<ref>{{cite book | series=Nuclear Non-Proliferation in International Law | volume=IV | title=Human Perspectives on the Development and Use of Nuclear Energy | editor1-first=Jonathan L. | editor1-last=Black-Branch | editor2-first=Dieter | editor2-last=Fleck | publisher=Springer | year=2018 | isbn=978-94-6265-267-5 | url=https://books.google.com/books?id=_HWADwAAQBAJ&pg=PA231 }}</ref> and
* All known synthetic [[transuranic]] elements, beginning with [[americium]] and [[curium]]

=== Recently discovered elements ===
The first [[transuranium element]] (element with an atomic number greater than 92) discovered was [[neptunium]] in 1940. Since 1999, the [[IUPAC/IUPAP Joint Working Party]] has considered claims for the discovery of new elements. As of January 2016, all 118 elements have been confirmed by IUPAC as being discovered. The discovery of element 112 was acknowledged in 2009, and the name ''copernicium'' and the chemical symbol ''Cn'' were suggested for it.<ref>{{cite web|date=20 July 2009|title=IUPAC Announces Start of the Name Approval Process for the Element of Atomic Number 112|url=http://media.iupac.org/news/112_Naming_Process_20090720.pdf|publisher=IUPAC|access-date=27 August 2009|archive-date=13 March 2012|archive-url=https://web.archive.org/web/20120313014559/http://media.iupac.org/news/112_Naming_Process_20090720.pdf|url-status=live}}</ref> The name and symbol were officially endorsed by IUPAC on 19 February 2010.<ref>{{cite web|date=20 February 2010 |title= Element 112 is Named Copernicium |url=http://www.iupac.org/web/nt/2010-02-20_112_Copernicium |publisher=IUPAC |url-status=dead |archive-url=https://web.archive.org/web/20100224054826/http://www.iupac.org/web/nt/2010-02-20_112_Copernicium |archive-date=24 February 2010 }}</ref> The heaviest element that is believed to have been synthesised to date is element 118, [[oganesson]], on 9 October 2006, by the [[Joint Institute for Nuclear Research|Flerov Laboratory]] of Nuclear Reactions in [[Dubna]], Russia.{{r|Schewe}}<ref>{{cite journal|year=2006|title=Evidence for Dark Matter |journal=[[Physical Review C]]|volume=74|issue=4|article-number=044602|doi=10.1103/PhysRevC.74.044602|bibcode=2006PhRvC..74d4602O |bibcode-access=free |last1=Oganessian|first1=Yu. Ts.|last2=Utyonkov|first2=V.|last3=Lobanov|first3=Yu.|last4=Abdullin|first4=F.|last5=Polyakov|first5=A.|last6=Sagaidak|first6=R.|last7=Shirokovsky|first7=I.|last8=Tsyganov|first8=Yu.|last9=Voinov|first9=A.|display-authors=8|doi-access=free}}</ref> [[Tennessine]], element 117 was the latest element claimed to be discovered, in 2009.<ref name="jinr">{{cite web|last=Greiner |first=W. |title=Recommendations |url=http://www.jinr.ru/img_sections/PAC/NP/31/PAK_NP_31_recom_eng.pdf |work=31st meeting, PAC for Nuclear Physics |publisher=[[Joint Institute for Nuclear Research]] |url-status=dead |archive-url=https://web.archive.org/web/20100414173735/http://www.jinr.ru/img_sections/PAC/NP/31/PAK_NP_31_recom_eng.pdf |archive-date=14 April 2010 }}</ref> On 28 November 2016, scientists at the IUPAC officially recognised the names for the four newest elements, with atomic numbers 113, 115, 117, and 118.<ref name="IUPAC-20161130">{{cite news |author=Staff |title=IUPAC Announces the Names of the Elements 113, 115, 117, and 118 |url=https://iupac.org/iupac-announces-the-names-of-the-elements-113-115-117-and-118/ |date=30 November 2016 |work=IUPAC |access-date=1 December 2016 |archive-date=29 July 2018 |archive-url=https://web.archive.org/web/20180729210456/https://iupac.org/iupac-announces-the-names-of-the-elements-113-115-117-and-118/ |url-status=live }}</ref><ref name="NYT-20161201">{{cite news |last=St. Fleur |first=Nicholas |title=Four New Names Officially Added to the Periodic Table of Elements |url=https://www.nytimes.com/2016/12/01/science/periodic-table-new-elements.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2016/12/01/science/periodic-table-new-elements.html |archive-date=2022-01-01 |url-access=limited |date=1 December 2016 |work=[[The New York Times]] |access-date=1 December 2016 }}{{cbignore}}</ref>

{{Periodic table}}

== See also ==
{{div col|colwidth=22em}}
* [[Biological roles of the elements]]
* [[Chemical database]]
* [[Discovery of chemical elements]]
* [[Element collecting]]
* [[Exotic atom]]
* [[Fictional element]]
* [[Goldschmidt classification]]
* [[Island of stability]]
* [[List of chemical elements]]
* [[List of nuclides]]
* [[Densities of the elements (data page)|List of the elements' densities]]
* [[Mineral (nutrient)]]
* [[Periodic systems of small molecules]]
* [[Prices of chemical elements]]
* [[Systematic element name]]
* [[Table of nuclides]]
* [[Roles of chemical elements]]
{{div col end}}

== References ==
{{Reflist}}

== Bibliography ==
* {{Cite book |last=Boyle |first=R.|year=1661 |url=https://www.gutenberg.org/files/22914/22914-h/22914-h.htm |title=The Sceptical Chymist: or Chymico-Physical Doubts & Paradoxes, Touching the Spagyrist's Principles Commonly call'd Hypostatical; As they are wont to be Propos'd and Defended by the Generality of Alchymists. Whereunto is præmis'd Part of another Discourse relating to the same Subject |publisher=Printed by J. Cadwell for J. Crooke}}

== Further reading ==
{{Commons category|Chemical elements}}{{Wikiquote}}{{Wikijunior|The Elements}}{{Wikibooks|General Chemistry/Chemistries of Various Elements}}

* {{cite book|last=Ball|first=P.|year=2004|title=The Elements: A Very Short Introduction|publisher=Oxford University Press|isbn=978-0-19-284099-8}}

* {{cite book|last=Emsley|first=J.|year=2003|title=Nature's Building Blocks: An A–Z Guide to the Elements|publisher=Oxford University Press|isbn=978-0-19-850340-8|url-access=registration|url=https://archive.org/details/naturesbuildingb0000emsl}}
* {{cite book|last=Gray|first=T.|year=2009|title=The Elements: A Visual Exploration of Every Known Atom in the Universe|publisher=Black Dog & Leventhal Publishers Inc|isbn=978-1-57912-814-2|url-access=registration|url=https://archive.org/details/elementsvisualex0000gray}}
* {{cite book|last=Scerri|first=E.R.|year=2007|title=The Periodic Table, Its Story and Its Significance|url=https://archive.org/details/periodictableits0000scer|url-access=registration|publisher=Oxford University Press|isbn=978-0-19-530573-9}}
* {{cite book|last=Strathern|first=P.|year=2000|title=Mendeleyev's Dream: The Quest for the Elements|publisher=[[Hamish Hamilton Ltd]]|isbn=978-0-241-14065-9}}
* {{cite book|title=The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements|last=Kean|first=Sam|year=2011|publisher=Back Bay Books}}
* {{cite book|title=Compendium of Chemical Terminology |edition=2nd |editor1=A.D. McNaught |editor2=A. Wilkinson |publisher=Blackwell Scientific Publications |location=Oxford |date=1997 |isbn=978-0-9678550-9-7 |doi=10.1351/goldbook |url=https://repositorio.unal.edu.co/handle/unal/80198 }} XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins

== External links ==
* [http://periodicvideos.com/ Videos for each element] by the University of Nottingham
* [https://www.bbc.co.uk/programmes/p00546sz "Chemical Elements"], ''In Our Time'', BBC Radio 4 discussion with Paul Strathern, Mary Archer and [[John Murrell (chemist)|John Murrell]] (25 May 2000)

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