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{{Short description|Substance composed of multiple chemically bonded elements}}
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| footer            = Pure [[Properties of water|water]] (H<sub>2</sub>O) is an example of a compound. The [[ball-and-stick model]] of the molecule shows the spatial association of two parts [[hydrogen]] (white) and one part [[oxygen]] (red)
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A '''chemical compound''' is a [[chemical substance]] composed of many identical [[molecule]]s (or [[molecular entity|molecular entities]]) containing [[atom]]s from more than one [[chemical element]] held together by [[chemical bond]]s. A [[homonuclear molecule|molecule consisting of atoms of only one element]] is therefore not a compound. A compound can be transformed into a different substance by a [[chemical reaction]], which may involve interactions with other substances. In this process, bonds between atoms may be broken or new bonds formed or both.

There are four major types of compounds, distinguished by how the constituent atoms are bonded together. [[Molecular compound]]s are held together by [[covalent bond]]s; [[ionic compound]]s are held together by [[ionic bond]]s; [[intermetallic compounds]] are held together by [[metallic bond]]s; [[coordination complex]]es are held together by [[coordinate covalent bond]]s. [[Non-stoichiometric compound]]s form a disputed marginal case.

A [[chemical formula]] specifies the number of atoms of each element in a compound molecule, using the standard [[chemical symbol]]s with numerical [[subscript]]s. Many chemical compounds have a unique [[CAS number]] identifier assigned by the [[Chemical Abstracts Service]]. Globally, more than 350,000 chemical compounds (including mixtures of chemicals) have been registered for production and use.<ref>{{Cite journal|last1=Wang|first1=Zhanyun|last2=Walker|first2=Glen W.|last3=Muir|first3=Derek C. G.|last4=Nagatani-Yoshida|first4=Kakuko|date=2020-01-22|title=Toward a Global Understanding of Chemical Pollution: A First Comprehensive Analysis of National and Regional Chemical Inventories|journal=[[Environmental Science & Technology]]|volume=54|issue=5|pages=2575–2584|doi=10.1021/acs.est.9b06379|pmid=31968937|bibcode=2020EnST...54.2575W|doi-access=free|hdl=20.500.11850/405322|hdl-access=free}}</ref>

== History of the concept ==

=== Robert Boyle ===
[[File:Portret_van_Robert_Boyle,_RP-P-OB-4578_(cropped).jpg|alt=Portrait of Robert Boyle, c. 1740|thumb|192x192px|Robert Boyle]]
[[File:Sceptical_chymist_1661_Boyle_Title_page_AQ18_(3).jpg|thumb|330x330px|Title page of ''The Sceptical Chymist'']]
The term "compound"—with a meaning similar to the modern—has been used at least since 1661 when [[Robert Boyle|Robert Boyle's]] ''[[The Sceptical Chymist]]'' was published. In this book, Boyle variously used the terms "compound",{{Sfn|Boyle|1661|p=41}} "compounded body",{{Sfn|Boyle|1661|p=13}} "perfectly mixt body",{{Sfn|Boyle|1661|p=29}} and "concrete".{{Sfn|Boyle|1661|p=42}} "Perfectly mixt bodies" included for example gold,{{Sfn|Boyle|1661|p=29}} lead,{{Sfn|Boyle|1661|p=29}} mercury,{{Sfn|Boyle|1661|p=41}} and wine.{{Sfn|Boyle|1661|p=145}} While the distinction between compound and [[mixture]] is not so clear, the distinction between [[Chemical element|element]] and compound is a central theme. {{Blockquote|text=Quicksilver ... with ''Aqua fortis'' will be brought into a ... white Powder ... with Sulphur it will compose a blood-red and volatile Cinaber. And yet out of all these exotick Compounds, we may recover the very same running Mercury.{{Sfn|Boyle|1661|p=40–41}}}}

==== Corpuscles of elements and compounds ====
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=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}}}}

=== Isaac Watts ===
[[File:Portrait_of_Isaac_Watts,_D.D..jpg|thumb|216x216px|Portrait of Isaac Watts by John Shury, {{circa|1830}}]]
In his ''[[Isaac Watts#Logic and science|Logick]]'', published in 1724, the English minister and logician [[Isaac Watts]] gave an early definition of chemical element, and contrasted element with chemical compound in clear, modern terms.<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=Among Substances, some are called Simple, some are Compound ... Simple Substances ... are usually called Elements, of which all other Bodies are compounded: 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 : This seems to come nearer the Truth ; tho' they are not all agreed ... 

Compound Substances are made up of two or more simple Substances ... So a Needle is simple Body, being made only of Steel; but a Sword or a Knife is a compound because its ... Handle is made of Materials different from the Blade.|author=|title=}}

== Definitions ==
Any substance consisting of two or more different types of [[atom]]s ([[chemical element]]s) in a fixed [[stoichiometric]] proportion can be termed a ''chemical compound''; the concept is most readily understood when considering pure [[chemical substance]]s.<ref name="Whitten">{{Citation | last1 = Whitten  | first1 = Kenneth W.  | last2 = Davis  | first2 = Raymond E.  | last3 = Peck  | first3 = M. Larry  | title = General Chemistry  | place = Fort Worth, TX  | publisher = Saunders College Publishing/Harcourt College Publishers  | year = 2000  | edition = 6th  | isbn = 978-0-03-072373-5}}</ref>{{rp|15}}<ref name="Brown p.6">{{Citation  | last1 = Brown  | first1 = Theodore L.  | last2 = LeMay  | first2 = H. Eugene  | last3 = Bursten  | first3 = Bruce E.  | last4 = Murphy  | first4 = Catherine J.  | last5 = Woodward  | first5 = Patrick  | title = Chemistry: The Central Science  | place = Frenchs Forest, NSW  | publisher = Pearson/Prentice Hall  | year = 2013  | edition = 3rd  | pages = 5–6  | url = https://books.google.com/books?id=zSziBAAAQBAJ&pg=PA6  | isbn = 9781442559462  | access-date = 2020-12-08  | archive-date = 2021-05-31  | archive-url = https://web.archive.org/web/20210531151453/https://books.google.com/books?id=zSziBAAAQBAJ&pg=PA6  | url-status = live  }}</ref><ref name="Hill p.6">{{Citation  | last1 = Hill  | first1 = John W.  | last2 = Petrucci  | first2 = Ralph H.  | last3 = McCreary  | first3 = Terry W.  | last4 = Perry  | first4 = Scott S.  | title = General Chemistry  | place = Upper Saddle River, NJ  | publisher = Pearson/Prentice Hall  | year = 2005  | edition = 4th  | page = 6  | url = http://www.pearsonhighered.com/educator/academic/product/0,3110,0131402838,00.html  | isbn = 978-0-13-140283-6  | url-status = live  | archive-url = https://web.archive.org/web/20090322043924/http://www.pearsonhighered.com/educator/academic/product/0,3110,0131402838,00.html  | archive-date = 2009-03-22  }}</ref> It follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, via [[chemical reaction]], into compounds or substances each having fewer atoms.<ref name="Wilbraham p.36">{{Citation  | last1 = Wilbraham  | first1 = Antony  | last2 = Matta  | first2 = Michael  | last3 = Staley  | first3 = Dennis  | last4 = Waterman  | first4 = Edward  | title = Chemistry  | place = Upper Saddle River, NJ  | publisher = Pearson/Prentice Hall  | year = 2002  | edition = 1st  | page = [https://archive.org/details/prenticehallchem0000wilb/page/36 36]  | isbn = 978-0-13-251210-7  | url-access = registration  | url = https://archive.org/details/prenticehallchem0000wilb/page/36  }}</ref> A [[chemical formula]] is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using [[chemical symbol]]s for the chemical elements, and [[subscript]]s to indicate the number of atoms involved. For example, [[water]] is composed of two [[hydrogen atom]]s bonded to one [[oxygen]] atom: the chemical formula is H<sub>2</sub>O. In the case of [[non-stoichiometric compound]]s, the proportions may be reproducible with regard to their preparation, and give fixed proportions of their component elements, but proportions that are not integral [e.g., for [[palladium hydride]], PdH<sub>x</sub> (0.02 < x < 0.58)].<ref name=PdH>{{cite journal|doi=10.1007/BF02667685|title=The H-Pd (hydrogen-palladium) System|year=1994|last1=Manchester|first1=F. D.|last2=San-Martin|first2=A.|last3=Pitre|first3=J. M.|journal=Journal of Phase Equilibria|volume=15|pages=62–83|s2cid=95343702}} [https://archive.today/20080229180236/http://www.msm.cam.ac.uk/mmc/people/jw476/pdh.html Phase diagram for Palladium-Hydrogen System]</ref>

Chemical compounds have a unique and defined [[chemical structure]] held together in a defined spatial arrangement by [[chemical bond]]s. Chemical compounds can be [[molecule|molecular]] compounds held together by [[covalent bond]]s, [[salt (chemistry)|salts]] held together by [[ionic bond]]s, [[intermetallic compounds]] held together by [[metallic bond]]s, or the subset of [[Coordination complex|chemical complexes]] that are held together by [[coordinate covalent bond]]s.<ref name="ChemPrinciples">{{cite book |last1=Atkins |first1=Peter |author-link1=Peter Atkins |last2=Jones |first2=Loretta |date=2004 |title=Chemical Principles: The Quest for Insight |isbn=978-0-7167-5701-6 |publisher=W.H. Freeman |url-access=registration |url=https://archive.org/details/chemicalprincipl00pete }}</ref> Pure [[chemical element]]s are generally not considered chemical compounds, failing the two or more atom requirement, though they often consist of molecules composed of multiple atoms (such as in the [[diatomic molecule]] H<sub>2</sub>, or the [[polyatomic molecule]] S<sub>8</sub>, etc.).<ref name="ChemPrinciples"/> Many [[chemistry|chemical]] compounds have a unique numerical identifier assigned by the [[Chemical Abstracts Service]] (CAS): its [[CAS number]].

There is varying and sometimes inconsistent nomenclature differentiating substances, which include truly non-stoichiometric examples, from chemical compounds, which require the fixed ratios. Many solid chemical substances—for example many [[silicate mineral]]s—are chemical substances, but do not have simple formulae reflecting chemically bonding of elements to one another in fixed ratios; even so, these [[Crystal structure|crystalline]] substances are often called "[[non-stoichiometric compound]]s".  It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is often due to either the presence of foreign elements trapped within the crystal structure of an otherwise known true ''chemical compound'', or due to perturbations in structure relative to the known compound that arise because of an excess of deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of the [[crust (geology)|crust]] and [[mantle (geology)|mantle]] of the Earth.  Other compounds regarded as chemically identical may have varying amounts of heavy or light [[isotope]]s of the constituent elements, which changes the ratio of elements by mass slightly.

== Types ==

=== Molecules ===
{{Main|Molecule}}
A molecule is an [[electrically neutral]] group of two or more atoms held together by chemical bonds.<ref name="iupac">{{GoldBookRef| title=Molecule|file=M04002|accessdate=23 February 2016}}</ref><ref>{{cite book| author= Ebbin, Darrell D.| title= General Chemistry |edition=3rd| date= 1990| publisher= [[Houghton Mifflin Co.]]| location= Boston| isbn= 978-0-395-43302-7}}</ref><ref>{{cite book| author= Brown, T.L. |author2=Kenneth C. Kemp |author3=Theodore L. Brown |author4=Harold Eugene LeMay |author5=Bruce Edward Bursten |title= Chemistry – the Central Science | url= https://archive.org/details/studentlectureno00theo | url-access= registration |edition=9th| date= 2003| publisher= [[Prentice Hall]]| location= New Jersey| isbn= 978-0-13-066997-1}}</ref> A molecule may be [[homonuclear]], that is, it consists of atoms of one chemical element, as with two atoms in the [[oxygen]] molecule (O<sub>2</sub>); or it may be [[heteronuclear]], a chemical compound composed of more than one element, as with [[water (molecule)|water]] (two hydrogen atoms and one oxygen atom; H<sub>2</sub>O). A molecule is the smallest unit of a substance that still carries all the physical and chemical properties of that substance.<ref>{{Cite web |date=2011-02-02 |title=Definition of molecule - NCI Dictionary of Cancer Terms |url=https://www.cancer.gov/publications/dictionaries/cancer-terms/def/molecule |access-date=2022-08-26 |website=NCI |language=en}}</ref>

=== Ionic compounds ===
{{Main|Ionic compound}}
An ionic compound is a chemical compound composed of [[ion]]s held together by [[Coulomb's law|electrostatic forces]] termed [[ionic bonding]]. The compound is neutral overall, but consists of positively charged ions called [[cation]]s and negatively charged ions called [[anion]]s. These can be [[simple ion]]s such as the [[sodium]] (Na<sup>+</sup>) and [[chloride]] (Cl<sup>−</sup>) in [[sodium chloride]], or [[Polyatomic ion|polyatomic]] species such as the [[ammonium]] ({{chem|NH|4|+}}) and [[carbonate]] ({{chem|CO|3|2−}}) ions in [[ammonium carbonate]]. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a [[Crystal structure|crystalline structure]].

Ionic compounds containing basic ions [[hydroxide]] (OH<sup>−</sup>) or [[oxide]] (O<sup>2−</sup>) are classified as bases. Ionic compounds without these ions are also known as [[salt (chemistry)|salts]] and can be formed by [[Acid–base reaction#Arrhenius theory|acid–base reactions]]. Ionic compounds can also be produced from their constituent ions by [[evaporation]] of their [[solvent]], [[precipitation (chemistry)|precipitation]], [[freezing]], a [[solid-state reaction route|solid-state reaction]], or the [[electron transfer]] reaction of [[Reactivity series|reactive]] metals with reactive non-metals, such as [[halogen]] gases.

Ionic compounds typically have high [[Melting point|melting]] and [[boiling point]]s, and are [[Hardness|hard]] and [[Brittleness|brittle]]. As solids they are almost always [[insulator (electricity)|electrically insulating]], but when [[melting|melted]] or [[Dissolution (chemistry)|dissolved]] they become highly [[electrical resistivity and conductivity|conductive]], because the ions are mobilized.

=== Intermetallic compounds ===
{{Main|Intermetallic compound}}
An intermetallic compound is a type of [[metallic bonding|metallic]] [[alloy]] that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties.<ref name=":0">{{Cite book|last1=Askeland|first1=Donald R.|last2=Wright|first2=Wendelin J.|author2-link=Wendelin Wright|url=https://www.worldcat.org/oclc/903959750|title=The Science and Engineering of Materials |publisher=Cengage Learning |isbn=978-1-305-07676-1|edition=Seventh|location=Boston, MA|pages=387–389|chapter=11-2 Intermetallic Compounds|date=January 2015|oclc=903959750|access-date=2020-11-10|archive-date=2021-05-31|archive-url=https://web.archive.org/web/20210531151448/https://www.worldcat.org/title/science-and-engineering-of-materials/oclc/903959750|url-status=live}}</ref><ref>{{Cite book|last=Panel On Intermetallic Alloy Development, Commission On Engineering And Technical Systems|url=https://www.worldcat.org/oclc/906692179|title=Intermetallic alloy development : a program evaluation|date=1997|publisher=National Academies Press|isbn=0-309-52438-5|pages=10|oclc=906692179|access-date=2020-11-10|archive-date=2021-05-31|archive-url=https://web.archive.org/web/20210531151435/https://www.worldcat.org/title/intermetallic-alloy-development-a-program-evaluation/oclc/906692179|url-status=live}}</ref><ref>{{Cite book|last=Soboyejo, W. O.|url=http://worldcat.org/oclc/300921090|title=Mechanical properties of engineered materials|date=2003|publisher=Marcel Dekker|isbn=0-8247-8900-8|chapter=1.4.3 Intermetallics|oclc=300921090|access-date=2020-11-10|archive-date=2021-05-31|archive-url=https://web.archive.org/web/20210531151500/https://www.worldcat.org/title/mechanical-properties-of-engineered-materials/oclc/300921090|url-status=live}}</ref> They can be classified as stoichiometric or nonstoichiometric intermetallic compounds.<ref name=":0" />

=== Complexes ===
{{Main|Coordination complex}}
A coordination complex consists of a central atom or ion, which is usually [[metal]]lic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''[[ligand]]s'' or complexing agents.<ref>{{cite book |title= Introduction to Coordination Chemistry |first= Geoffrey A. |last= Lawrance |year= 2010 |publisher= Wiley |isbn= 9780470687123 |doi= 10.1002/9780470687123}}</ref><ref>{{GoldBookRef | title = complex | file = C01203}}</ref><ref>{{GoldBookRef | file = C01330 | title = coordination entity}}</ref>  Many metal-containing compounds, especially those of [[transition metal]]s, are coordination complexes.<ref>{{Greenwood&Earnshaw2nd}}</ref> A coordination complex whose centre is a metal atom is called a metal complex of d block element.

== Bonding and forces ==
Compounds are held together through a variety of different types of bonding and forces. The differences in the types of bonds in compounds differ based on the types of elements present in the compound.

[[London dispersion forces]] are the weakest force of all [[intermolecular forces]]. They are temporary attractive forces that form when the [[electrons]] in two adjacent atoms are positioned so that they create a temporary [[dipole]]. Additionally, London dispersion forces are responsible for condensing  [[chemical polarity|non polar]] substances to liquids, and to further freeze to a solid state dependent on how low the temperature of the environment is.<ref>{{Cite web|url=https://www.chem.purdue.edu/gchelp/liquids/disperse.html|title=London Dispersion Forces|website=www.chem.purdue.edu|access-date=2017-09-13|url-status=live|archive-url=https://web.archive.org/web/20170113112106/http://www.chem.purdue.edu/gchelp/liquids/disperse.html|archive-date=2017-01-13}}</ref>

A [[covalent bond]], also known as a molecular bond, involves the sharing of electrons between two atoms. Primarily, this type of bond occurs between elements that fall close to each other on the [[periodic table of elements]], yet it is observed between some metals and nonmetals. This is due to the mechanism of this type of bond. Elements that fall close to each other on the periodic table tend to have similar [[electronegativity|electronegativities]], which means they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons so both elements have a more stable [[octet rule|octet]].

[[Ionic bonding]] occurs when [[valence electrons]] are completely transferred between elements. Opposite to covalent bonding, this chemical bond creates two oppositely charged ions. The metals in ionic bonding usually lose their valence electrons, becoming a positively charged [[cation]]. The nonmetal will gain the electrons from the metal, making the nonmetal a negatively charged [[anion]]. As outlined, ionic bonds occur between an electron donor, usually a metal, and an electron acceptor, which tends to be a nonmetal.<ref>{{Cite news|url=https://chem.libretexts.org/Core/Organic_Chemistry/Fundamentals/Ionic_and_Covalent_Bonds|title=Ionic and Covalent Bonds|date=2013-10-02|work=Chemistry LibreTexts|access-date=2017-09-13|language=en-US|url-status=live|archive-url=https://web.archive.org/web/20170913183643/https://chem.libretexts.org/Core/Organic_Chemistry/Fundamentals/Ionic_and_Covalent_Bonds|archive-date=2017-09-13}}</ref>

[[Hydrogen bonding]] occurs when a [[hydrogen atom]] bonded to an electronegative atom forms an [[electrostatic]] connection with another electronegative atom through interacting dipoles or charges.<ref>{{GoldBookRef |title=hydrogen bond |file=H02899}}</ref><ref>{{Cite web|url=https://www.chem.purdue.edu/gchelp/liquids/hbond.html|title=Hydrogen Bonding|website=www.chem.purdue.edu|access-date=2017-10-28|url-status=live|archive-url=https://web.archive.org/web/20110808201000/http://www.chem.purdue.edu/gchelp/liquids/hbond.html|archive-date=2011-08-08}}</ref><ref>{{Cite web|url=https://www.chemguide.co.uk/atoms/bonding/hbond.html|title=intermolecular bonding – hydrogen bonds|website=www.chemguide.co.uk|access-date=2017-10-28|url-status=live|archive-url=https://web.archive.org/web/20161219123038/http://chemguide.co.uk/atoms/bonding/hbond.html|archive-date=2016-12-19}}</ref>

== Reactions ==
{{Main|Chemical reaction}}
A compound can be converted to a different chemical composition by interaction with a second chemical compound via a [[chemical reaction]]. In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms. Schematically, this reaction could be described as {{nowrap|AB + CD → AD + CB}}, where A, B, C, and D are each unique atoms; and AB, AD, CD, and CB are each unique compounds.

== See also ==
* [[Chemical structure]]
* [[IUPAC nomenclature]]
* [[Dictionary of chemical formulas]]
* [[List of compounds]]

== References ==
{{reflist}}

== Sources ==

* {{Cite book |last=Boyle |first=R. |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 |year=1661}}

== Further reading ==
{{wiktionarypar|chemical compound}}
{{Commons category|Chemical compounds}}
* {{Citation|author=Robert Siegfried|title=From elements to atoms: a history of chemical composition|date=1 October 2002|publisher=American Philosophical Society|isbn=978-0-87169-924-4}}

{{Branches of chemistry}}
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[[Category:Chemistry]]
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