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{{short description|Type of material}}
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[[File:Iron electrolytic and 1cm3 cube.jpg|thumb|right|alt=refer to caption|[[Iron]], shown here as fragments and a 1&nbsp;cm<sup>3</sup> cube, is an example of a [[chemical element]] that is a metal.]]
[[File:Sauce boat.jpg|thumb|right|alt=A metal gravy boat|Metal in the form of a [[gravy boat]] made from stainless steel, an [[alloy]] largely composed of iron, chromium and nickel]]
{{sidebar periodic table|metalicity}}

A '''metal''' ({{etymology|grc|''{{Wikt-lang|grc|μέταλλον}}'' ({{grc-transl|μέταλλον}})|mine, quarry, metal}}) is a [[material]] that, when polished or fractured, shows a lustrous appearance, and conducts [[electrical resistivity and conductivity|electricity]] and [[thermal conductivity|heat]] relatively well. These properties are all associated with having electrons available at the [[Fermi level]], as against nonmetallic materials which do not.<ref name="Kittel-2018">{{Cite book |last=Kittel |first=Charles |title=Introduction to solid state physics |date=2018 |publisher=Wiley |others=Paul McEuen |isbn=978-1-119-45416-8 |edition=Global edition, [9th edition] |location=Hoboken, NJ}}</ref>{{Rp|location=Chpt 8 & 19}}<ref name="Ashcroft-1976">{{Cite book |last1=Ashcroft |first1=Neil W. |title=Solid state physics |last2=Mermin |first2=N. David |date=1976 |publisher=Holt, Rinehart and Winston |isbn=978-0-03-083993-1 |location=New York}}</ref>{{Rp|pages=Chpt 7 & 8}} Metals are typically [[ductile]] (can be drawn into a wire) and [[malleable]] (can be shaped via hammering or pressing).<ref name="Callister-1997">{{Cite book |last=Callister |first=William D. |title=Materials science and engineering: an introduction |date=1997 |publisher=John Wiley & Sons |isbn=978-0-471-13459-6 |edition=4th |location=New York}}</ref>

A metal may be a [[chemical element]] such as [[iron]]; an [[alloy]] such as [[stainless steel]]; or a molecular compound such as [[polythiazyl|polymeric sulfur nitride]].<ref>{{cite journal |last1=Chiang |first1=CK| date=1977 |title=Transport and optical properties of polythiazyl bromides: (SNBr<sub>0.4</sub>)x |journal=Solid State Communications |volume= 23|issue=9 |pages= 607–612 |doi=10.1016/0038-1098(77)90530-0|bibcode=1977SSCom..23..607C }}; {{cite book |last1=Greenwood |first1=NN |last2=Earnshaw |first2=A | date=1998 |title=Chemistry of the Elements |location=Oxford |publisher=Butterworth-Heinemann |page=727 |isbn=978-0-7506-3365-9}}; {{cite book |last1=Mutlu |first1=H |last2=Theato |first2=P |editor-last1=Zhang| editor-first1= X|editor-last2=Theato|editor-first2=P|title=Sulfur-Containing Polymers: From Synthesis to Functional Materials |location=Weinheim |publisher=Wiley-VCH |pages=191–234 (191)|date=2021|isbn=978-3-527-34670-7|chapter=Polymers with sulfur-nitrogen bonds}}</ref> The general science of metals is called [[metallurgy]], a subtopic of [[materials science]]; aspects of the electronic and thermal properties are also within the scope of [[condensed matter physics]] and [[solid-state chemistry]], it is a [[multidisciplinary]] topic. In colloquial use materials such as steel alloys are referred to as metals, while others such as polymers, wood or ceramics are [[nonmetallic material]]s.

A metal conducts electricity at a temperature of [[absolute zero]],<ref>{{cite book |title=Physics of Metal-Nonmetal Transitions |last=Yonezawa |first=F. |year=2017 |publisher=IOS Press |location=Amsterdam |isbn=978-1-61499-786-3 |page=257 |quote=Sir [[Nevill Mott]] (1905–1996) wrote a letter to a fellow physicist, [[Peter Edwards (chemist)|Prof. Peter P. Edwards]], in which he notes... I've thought a lot about 'What is a metal?' and I think one can only answer the question at ''T'' = 0 (the absolute zero of temperature). There a metal conducts and a nonmetal doesn't.}}</ref> which is a consequence of [[Delocalized electron|delocalized]] states at the Fermi energy.<ref name="Kittel-2018" /><ref name="Ashcroft-1976" /> Many elements and compounds become metallic under high pressures, for example, [[iodine]] gradually becomes a metal at a pressure of between 40 and 170 thousand times [[atmospheric pressure]]. [[Sodium]] becomes a nonmetal at pressure of just under two million times atmospheric pressure, and at even higher pressures it is expected to become a metal again.

When discussing the [[periodic table]] and some chemical properties, the term metal is often used to denote those elements which in pure form and at standard conditions are metals in the sense of electrical conduction mentioned above. The related term metallic may also be used for types of [[dopant]] atoms or alloying elements.

In [[astronomy]] metal refers to all chemical elements in a star that are heavier than [[helium]]. In this sense the first four "metals" collecting in stellar cores through nucleosynthesis are [[carbon]], [[nitrogen]], [[oxygen]], and [[neon]]. A star [[nuclear fusion|fuses]] lighter atoms, mostly hydrogen and helium, into heavier atoms over its lifetime. The [[metallicity]] of an astronomical object is the proportion of its matter made up of the heavier chemical elements.<ref name="Martin">{{cite web |author=Martin |first=John C. |title=What we learn from a star's metal content |url=https://sites.google.com/a/uis.edu/john-c-martin-s-homepage/research/rr-lyrae-kinematics/what-we-learn-from-a-star-s-metal-content |access-date=March 25, 2021 |work=John C. Martin's Homepage}}</ref><ref>{{cite journal |last1=Martin |first1=John C. |last2=Morrison |first2=Heather L. |date=May 18, 1998 |orig-year=1998 |title=A New Analysis of RR Lyrae Kinematics in the Solar Neighborhood |url=https://iopscience.iop.org/article/10.1086/300568/fulltext/tal-content |journal=[[The Astronomical Journal]] |language=en |publication-date=October 1, 1998 |volume=116 |issue=4 |pages=1724–1735 |doi=10.1086/300568 |arxiv=astro-ph/9806258 |bibcode=1998AJ....116.1724M |s2cid=18530430 |via=IOPscience}}</ref>

The strength and resilience of some metals has led to their frequent use in, for example, high-rise building and bridge [[construction]], as well as most vehicles, many [[home appliance]]s, tools, pipes, and railroad tracks. [[Precious metal]]s were historically used as [[coin]]age, but in the modern era, [[coinage metals]] have extended to at least 23 of the chemical elements.<ref>{{cite journal |last1=Roe |first1=J. |last2=Roe |first2=M. |date=1992 |title=World's coinage uses 24 chemical elements |journal=World Coinage News |volume=19 |issue=4, 5 |pages=24–25, 18–19}}</ref> There is also extensive use of multi-element metals such as [[titanium nitride]]<ref name="Stampfl-2001">{{Cite journal |last1=Stampfl |first1=C. |last2=Mannstadt |first2=W. |last3=Asahi |first3=R. |last4=Freeman |first4=A. J. |date=2001 |title=Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations |url=https://link.aps.org/doi/10.1103/PhysRevB.63.155106 |journal=Physical Review B |volume=63 |issue=15 |pages=155106 |doi=10.1103/PhysRevB.63.155106|bibcode=2001PhRvB..63o5106S |url-access=subscription }}</ref> or [[degenerate semiconductor]]s in the semiconductor industry.

The history of refined metals is thought to begin with the use of copper about 11,000 years ago. Gold, silver, iron (as meteoric iron), lead, and brass were likewise in use before the first known appearance of bronze in the fifth millennium BCE. Subsequent developments include the production of early forms of steel; the discovery of [[sodium]]—the first [[light metal]]—in 1809; the rise of modern [[alloy steel]]s; and, since the end of World War II, the development of more sophisticated alloys.

==Properties==

===Form and structure===
[[File:Gallium crystals.jpg|thumb|[[Gallium]] crystals|alt=Gallium crystals on a table]]
Most metals are shiny and [[lustrous]], at least when polished, or fractured. Sheets of metal thicker than a few [[micrometre]]s appear opaque, but [[gold leaf]] transmits green light. This is due to the freely moving electrons which reflect light.<ref name="Kittel-2018" /><ref name="Ashcroft-1976" />

Although most elemental metals have higher [[density|densities]] than [[nonmetal]]s,<ref name="morty">{{cite book |author=Mortimer, Charles E. |title=Chemistry: A Conceptual Approach |location=New York |publisher=D. Van Nostrad Company |edition=3rd |year=1975}}</ref> there is a wide variation in their densities, [[lithium]] being the least dense (0.534&nbsp;g/cm<sup>3</sup>) and [[osmium]] (22.59&nbsp;g/cm<sup>3</sup>) the most dense. Some of the [[Superheavy element|6d transition metals]] are expected to be denser than osmium, but their known isotopes are too unstable for bulk production to be possible<ref>{{Cite conference|last1=Moller|first1=P.|last2=Nix|first2=J. R.|date=1994|title=Fission properties of the heaviest elements|url=https://digital.library.unt.edu/ark:/67531/metadc674703/m2/1/high_res_d/32502.pdf|conference=Dai 2 Kai Hadoron Tataikei no Simulation Symposium, Tokai-mura, Ibaraki, Japan|publisher=[[University of North Texas]]|access-date=2020-02-16}}</ref> Magnesium, aluminium and titanium are [[light metal]]s of significant commercial importance. Their respective densities of 1.7, 2.7, and 4.5&nbsp;g/cm<sup>3</sup> can be compared to those of the older structural metals, like iron at 7.9 and copper at 8.9&nbsp;g/cm<sup>3</sup>. The most common lightweight metals are [[Aluminium alloy|aluminium]]<ref>{{Citation |last=Benedyk |first=J. C. |title=3 - Aluminum alloys for lightweight automotive structures |date=2010-01-01 |work=Materials, Design and Manufacturing for Lightweight Vehicles |pages=79–113 |editor-last=Mallick |editor-first=P. K. |url=https://www.sciencedirect.com/science/article/pii/B9781845694630500031 |access-date=2024-07-23 |series=Woodhead Publishing Series in Composites Science and Engineering |publisher=Woodhead Publishing |doi=10.1533/9781845697822.1.79 |isbn=978-1-84569-463-0|url-access=subscription }}</ref><ref>{{Cite journal |last1=Li |first1=Shuang–Shuang |last2=Yue |first2=Xin |last3=Li |first3=Qing–Yuan |last4=Peng |first4=He–Li |last5=Dong |first5=Bai–Xin |last6=Liu |first6=Tian–Shu |last7=Yang |first7=Hong–Yu |last8=Fan |first8=Jun |last9=Shu |first9=Shi–Li |last10=Qiu |first10=Feng |last11=Jiang |first11=Qi–Chuan |date=2023-11-01 |title=Development and applications of aluminum alloys for aerospace industry |journal=Journal of Materials Research and Technology |volume=27 |pages=944–983 |doi=10.1016/j.jmrt.2023.09.274 |issn=2238-7854|doi-access=free }}</ref> and [[Magnesium alloy|magnesium]]<ref>{{Cite journal |last1=Gupta |first1=M. |last2=Wong |first2=W. L. E. |date=2015-07-01 |title=Magnesium-based nanocomposites: Lightweight materials of the future |url=https://www.sciencedirect.com/science/article/pii/S104458031500131X |journal=Materials Characterization |volume=105 |pages=30–46 |doi=10.1016/j.matchar.2015.04.015 |issn=1044-5803|url-access=subscription }}</ref><ref>{{Cite journal |last1=Ogawa |first1=Yukiko |last2=Ando |first2=Daisuke |last3=Sutou |first3=Yuji |last4=Koike |first4=Junichi |date=2016-07-22 |title=A lightweight shape-memory magnesium alloy |url=https://www.science.org/doi/10.1126/science.aaf6524 |journal=Science |language=en |volume=353 |issue=6297 |pages=368–370 |doi=10.1126/science.aaf6524 |pmid=27463668 |bibcode=2016Sci...353..368O |issn=0036-8075|url-access=subscription }}</ref> alloys.

[[File:Ductility.svg|thumb|right|Schematic appearance of round metal bars after tensile testing.<br />
(a) Brittle fracture<br />
(b) Ductile fracture<br />
(c) Completely ductile fracture]]

Metals are typically malleable and ductile, deforming under stress without [[cleavage (crystal)|cleaving]].<ref name="morty"/> The nondirectional nature of metallic bonding contributes to the ductility of most metallic solids, where the [[Peierls stress]] is relatively low allowing for [[dislocation]] motion, and there are also many combinations of planes and directions for [[Plastic Deformation|plastic deformation]].<ref name="Weertman-1992">{{Cite book |last1=Weertman |first1=Johannes |title=Elementary dislocation theory |last2=Weertman |first2=Julia R. |date=1992 |publisher=Oxford University Press |isbn=978-0-19-506900-6 |location=New York}}</ref> Due to their having close packed arrangements of atoms the [[Burgers vector]] of the dislocations are fairly small, which also means that the energy needed to produce one is small.<ref name="Callister-1997" /><ref name="Weertman-1992" /> In contrast, in an ionic compound like table salt, the Burgers vectors are much larger and the energy to move a dislocation is far higher.<ref name="Callister-1997" /> Reversible [[deformation (engineering)|elastic deformation]] in metals can be described well by [[Hooke's law]] for the restoring forces, where the [[stress (mechanics)|stress]] is linearly proportional to the [[deformation (mechanics)|strain]], up to the [[proportional limit]] of the material.<ref>{{Cite book |last=Timoshenko |first=Stephen |url=https://books.google.com/books?id=tkScQmyhsb8C&dq=introduction+to+elasticity+timoshenko&pg=PA7 |title=History of Strength of Materials: With a Brief Account of the History of Theory of Elasticity and Theory of Structures |date=1983-01-01 |publisher=Courier Corporation |isbn=978-0-486-61187-7 |language=en}}</ref><ref>{{Cite web |last=Roylance |first=David |date=2001-08-23 |title=Stress-strain curves |url=https://resources.saylor.org/wwwresources/archived/site/wp-content/uploads/2012/09/ME1022.2.4.pdf |page=2}}</ref>

A temperature change may lead to the movement of [[crystallographic defect|structural defects]] in the metal such as [[grain boundaries]], [[vacancy defect|point vacancies]], [[dislocations|line and screw dislocations]], [[stacking fault]]s and [[crystal twinning|twins]] in both [[crystalline]] and [[amorphous solid|non-crystalline]] metals. Internal [[slip (materials science)|slip]], [[creep (deformation)|creep]], and [[fatigue (material)|metal fatigue]] may also ensue.<ref name="Callister-1997" /><ref name="Weertman-1992" />

The atoms of simple metallic substances are often in one of three common [[crystal structure]]s, namely [[body-centered cubic]] (bcc), [[face-centered cubic]] (fcc), and [[hexagonal close-packed]] (hcp). In bcc, each atom is positioned at the center of a cube of eight others. In fcc and hcp, each atom is surrounded by twelve others, but the stacking of the layers differs. Some metals adopt different structures depending on the temperature.<ref>{{cite book |last1=Holleman |first1=A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego |year=2001 |isbn=0-12-352651-5}}</ref>
<gallery widths="135" heights="135">
File:Cubic-body-centered.svg|Body-centered cubic crystal structure, with a 2-atom unit cell, as found in e.g. chromium, iron, and tungsten
File:Cubic-face-centered.svg|Face-centered cubic crystal structure, with a 4-atom unit cell, as found in e.g. aluminium, copper, and gold
File:Hexagonal close packed.svg|Hexagonal close-packed crystal structure, with a 6-atom unit cell, as found in e.g. titanium, cobalt, and zinc
File:PSM V87 D113 Arrangement of atoms in a rock salt crystal.png|Arrangement of atoms in a rock salt crystal such as TiN
</gallery>

Many other metals with different elements have more complicated structures, such as [[rock-salt structure]] in [[titanium nitride]] or [[perovskite (structure)]] in some nickelates.<ref>{{Cite book |last=Koster |first=G. |title=Epitaxial growth of complex metal oxides |date=2015 |publisher=Elsevier |isbn=978-1-78242-245-7 |location=Boston, MA}}</ref>

===Electrical and thermal===
[[File:Band filling diagram.svg|thumb|{{{1|right}}}|300px|

The energy states available to electrons in different kinds of solids at [[thermodynamic equilibrium]].
<div style="height:5px;font-size:1px;">&nbsp;</div>
Here, height is energy while width is the [[density of states|density of available states]] for a certain energy in the material listed. The shading follows the [[Fermi–Dirac statistics|Fermi–Dirac distribution]] ('''black'''=all states filled, '''white'''=no state filled).
<div style="height:5px;font-size:1px;">&nbsp;</div>
The [[Fermi level]] ''E''<sub>F</sub> is the energy level at which the electrons are in a position to interact with energy levels above them. In metals and [[semimetal]]s the [[Fermi level]] ''E''<sub>F</sub> lies inside at least one band of energy states.
<div style="height:6px;font-size:1px;">&nbsp;</div>
In [[insulator (electricity)|insulators]] and [[semiconductor]]s the Fermi level is inside a [[band gap]]; however, in semiconductors the bands are near enough to the Fermi level to be [[Fermi–Dirac statistics|thermally populated]] with electrons or [[electron hole|holes]].
]]

The electronic structure of metals makes them good [[electrical conductivity|conductors of electricity]]. In general, electrons in a material all have different [[Momentum|momenta]], which average to zero when there is no external [[voltage]]. In metals, when a voltage is applied, some electrons shift to states with slightly higher momentum in the direction of the electric field, while others slow down slightly. This creates a net [[drift velocity]] that leads to an electric current.<ref name="Kittel-2018" /><ref name="Ashcroft-1976" /><ref name="h887">{{cite book |last=Simon |first=Steven H. |title=The Oxford Solid State Basics |date=2013-06-21 |publisher=OUP Oxford |isbn=978-0-19-150210-1 |publication-place=Oxford |pages=173-174}}</ref> This involves small changes in which [[Quantum state|wavefunctions]] the electrons are in, changing to those with the higher momenta. According to the [[Pauli exclusion principle]], no two electrons can occupy the same quantum state.<ref>{{Cite book |last=Schiff |first=Leonard |author-link=Leonard I. Schiff |url=https://ia601609.us.archive.org/11/items/ost-physics-schiff-quantummechanics/Schiff-QuantumMechanics.pdf |title=Quantum Mechanics |publisher=[[McGraw-Hill]] |year=1959}}</ref> Therefore, for the electrons to shift to higher-momentum states, such states must be unoccupied. In metals, these empty [[Delocalized electron|delocalized electron states]] are available at energies near the highest occupied levels, as shown in the Figure.

By contrast, semiconductors like silicon and nonmetals like [[strontium titanate]] have an [[Band gap|energy gap]] between the highest filled electron states (the valence band) and the lowest empty states (the conduction band). A small electric field is insufficient to excite electrons across this gap, making these materials poor electrical conductors.<ref name="h887" /> However, semiconductors can carry some current when [[Doping (semiconductor)|doped]] with elements that introduce additional partially occupied energy states, or when thermal excitation enables electrons to cross the energy gap.<ref name="Solymar-2004">{{Cite book |last1=Solymar |first1=L. |title=Electrical properties of materials |last2=Walsh |first2=D. |date=2004 |publisher=Oxford University Press |isbn=978-0-19-926793-4 |edition=7th |location=Oxford; New York}}</ref>

The elemental metals have electrical conductivity values of from 6.9 × 10<sup>3</sup> [[Siemens (unit)|S]]/cm for [[manganese]] to 6.3 × 10<sup>5</sup> S/cm for [[silver]]. In contrast, a [[semiconductor|semiconducting]] metalloid such as [[boron]] has an electrical conductivity 1.5 × 10<sup>−6</sup> S/cm. Typically, the electrical conductivity of metals decreases with heating because the increased thermal motion of the atoms makes it harder for electrons to flow.<ref>{{Cite book |title=Springer Handbook of Electronic and Photonic Materials |date=2017 |publisher=Springer International Publishing : Imprint: Springer |isbn=978-3-319-48933-9 |editor-last=Capper |editor-first=Peter |edition=2nd ed. 2017 |series=Springer Handbooks |location=Cham |pages=23 |editor-last2=Kasap |editor-first2=Safa}}</ref> Exceptionally, [[plutonium]]'s electrical conductivity increases when heated in the temperature range of around −175 to +125&nbsp;°C, with anomalously large thermal expansion coefficient and a phase change from monoclinic to face-centered cubic near 100&nbsp;°C.<ref name="HeckerPlutonium">{{Cite journal |last=Hecker |first=Siegfried S. |date=2000 |title=Plutonium and its alloys: from atoms to microstructure |url=https://fas.org/sgp/othergov/doe/lanl/pubs/00818035.pdf |url-status=live |journal=Los Alamos Science |volume=26 |pages=290–335 |archive-url=https://web.archive.org/web/20090224204042/http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818035.pdf |archive-date=February 24, 2009 |access-date=February 15, 2009}}</ref> This behavior, along with similar phenomena observed in other transuranic elements, is attributed to more complex relativistic and spin interactions which are not captured in simple models.<ref>{{Cite journal |last1=Tsiovkin |first1=Yu. Yu. |last2=Lukoyanov |first2=A. V. |last3=Shorikov |first3=A. O. |last4=Tsiovkina |first4=L. Yu. |last5=Dyachenko |first5=A. A. |last6=Bystrushkin |first6=V. B. |last7=Korotin |first7=M. A. |last8=Anisimov |first8=V. I. |last9=Dremov |first9=V. V. |date=2011 |title=Electrical resistivity of pure transuranium metals under pressure |url=https://linkinghub.elsevier.com/retrieve/pii/S0022311511003369 |journal=Journal of Nuclear Materials |volume=413 |issue=1 |pages=41–46 |doi=10.1016/j.jnucmat.2011.03.053 |bibcode=2011JNuM..413...41T |issn=0022-3115|url-access=subscription }}</ref>
[[File:TiN_DOS.tif|thumb|Density of states of TiN, with the occupied states shaded in blue and the Fermi level at the x origin. All the states, as well as those associated with the Ti and N atoms are shown.|left|250x250px]]
All of the metallic alloys as well as conducting ceramics and polymers are metals by the same definition; for instance [[titanium nitride]] has delocalized states at the Fermi level. They have electrical conductivities similar to those of elemental metals. Liquid forms are also metallic conductors or electricity, for instance [[Mercury (element)|mercury]]. In normal conditions no gases are metallic conductors. However, a [[plasma (physics)|plasma]] is a metallic conductor and the charged particles in a plasma have many properties in common with those of electrons in elemental metals, particularly for white dwarf stars.<ref>{{Cite journal |last1=Koester |first1=D |last2=Chanmugam |first2=G |date=1990 |title=Physics of white dwarf stars |url=https://iopscience.iop.org/article/10.1088/0034-4885/53/7/001 |journal=Reports on Progress in Physics |volume=53 |issue=7 |pages=837–915 |doi=10.1088/0034-4885/53/7/001 |issn=0034-4885|url-access=subscription }}</ref>

Metals are relatively good [[thermal conductivity|conductors of heat]], which in metals is transported mainly by the conduction electrons.<ref name=bassani>{{cite book | last=Skośkiewicz | first=T. | title=Encyclopedia of Condensed Matter Physics | chapter=Thermal Conductivity at Low Temperatures | publisher=Elsevier | date=2005 | pages=159–164 | isbn=978-0-12-369401-0 | doi=10.1016/b0-12-369401-9/01168-2}}</ref> At higher temperatures the electrons can occupy slightly higher energy levels given by [[Fermi–Dirac statistics]].<ref name="Ashcroft-1976" /><ref name="Solymar-2004" /> These have slightly higher momenta ([[kinetic energy]]) and can pass on thermal energy. The empirical [[Wiedemann–Franz law]] states that in many metals the ratio between thermal and electrical conductivities is proportional to temperature, with a proportionality constant that is roughly the same for all metals.<ref name="Ashcroft-1976" />
[[File:Battery Demonstration Unit - DPLA (cropped).jpg|alt=Battery demonstration unit for conducting polymers built by nobel laureate Alan MacDiarmid|thumb|Battery demonstration unit for conducting polymers built by nobel laureate [[Alan MacDiarmid]]<ref>{{Cite web |title=The Nobel Prize in Chemistry 2000 |url=https://www.nobelprize.org/prizes/chemistry/2000/summary/ |access-date=2024-07-23 |website=NobelPrize.org |language=en-US}}</ref>]]
The contribution of a metal's electrons to its heat capacity and thermal conductivity, and the electrical conductivity of the metal itself can be approximately calculated from the [[free electron model]].<ref name="Ashcroft-1976" /> However, this does not take into account the detailed structure of the metal's ion lattice. Taking into account the positive potential caused by the arrangement of the ion cores enables consideration of the [[electronic band structure]] and [[binding energy]] of a metal. Various models are applicable, the simplest being the [[nearly free electron model]].<ref name="Ashcroft-1976" /> Modern methods such as [[density functional theory]] are typically used.<ref>{{Cite web |last=Burke |first=Kieron |date=2007 |title=The ABC of DFT |url=https://dft.uci.edu/doc/g1.pdf}}</ref><ref>{{Cite book |last1=Gross |first1=Eberhard K. U. |url=https://books.google.com/books?id=aG4ECAAAQBAJ&q=density+functional+theory |title=Density Functional Theory |last2=Dreizler |first2=Reiner M. |date=2013 |publisher=Springer Science & Business Media |isbn=978-1-4757-9975-0 |language=en}}</ref>

===Chemical===
The elements which form metals usually form [[cations]] through electron loss.<ref name="morty"/> Most will react with oxygen in the air to form [[oxide]]s over various timescales ([[potassium]] burns in seconds while iron [[rust]]s over years) which depend upon whether the native oxide forms a [[Passivation (chemistry)|passivation layer]] that acts as a [[diffusion barrier]].<ref>{{Cite book |last1=Bockris |first1=J. O'M |title=Modern electrochemistry. 2 |last2=Reddy |first2=Amulya K. N. |date=1977 |publisher=Plenum Pr |isbn=978-0-306-25002-6 |edition=3. print |location=New York}}</ref><ref>{{Cite book |last1=Kelly |first1=Robert G. |url=https://www.taylorfrancis.com/books/9780203909133 |title=Electrochemical Techniques in Corrosion Science and Engineering |last2=Scully |first2=John R. |last3=Shoesmith |first3=David |last4=Buchheit |first4=Rudolph G. |date=2002-09-13 |publisher=CRC Press |isbn=978-0-203-90913-3 |edition=0 |language=en |doi=10.1201/9780203909133}}</ref> Some others, like [[palladium]], [[platinum]], and [[gold]], do not react with the atmosphere at all; gold can form compounds where it gains an electron (aurides, e.g. [[caesium auride]]). The [[oxide]]s of elemental metals are often [[base (chemistry)|basic]]. However, oxides with very high [[oxidation state]]s such as CrO<sub>3</sub>, Mn<sub>2</sub>O<sub>7</sub>, and OsO<sub>4</sub> often have strictly acidic reactions; and oxides of the less electropositive metals such as BeO, Al<sub>2</sub>O<sub>3</sub>, and PbO, can display both basic and acidic properties. The latter are termed [[amphoteric]] oxides.

==Periodic table distribution of elemental metals==
{{See also|Periodic table}}
The elements that form exclusively metallic structures under ordinary conditions are shown in yellow on the periodic table below. The remaining elements either form [[network covalent bonding|covalent network]] structures (light blue), molecular covalent structures (dark blue), or remain as single atoms (violet).<ref>{{Greenwood&Earnshaw2nd}}</ref> Astatine (At), francium (Fr), and the elements from fermium (Fm) onwards are shown in gray because they are extremely radioactive and have never been produced in bulk. Theoretical and experimental evidence suggests that these uninvestigated elements should be metals,<ref name=Siekierski>{{cite book|title=Concise Chemistry of the Elements|year=2002|publisher=Horwood|isbn=978-1-898563-71-6|last1=Siekierski|first1=S.|last2=Burgess|first2 =J. |pages=60–66}}</ref> except for oganesson (Og) which DFT calculations indicate would be a semiconductor.<ref name="semiconductor">{{cite journal |last1=Mewes |first1=Jan-Michael |last2=Smits |first2=Odile Rosette |first3=Paul |last3=Jerabek |first4=Peter |last4=Schwerdtfeger |date=25 July 2019 |title=Oganesson is a Semiconductor: On the Relativistic Band-Gap Narrowing in the Heaviest Noble-Gas Solids |journal=Angewandte Chemie |volume=58 |issue=40 |pages=14260–14264|doi=10.1002/anie.201908327|pmid=31343819|pmc=6790653}}</ref>
{{Periodic table (simple substance bonding)}}
The situation changes with pressure: at extremely high pressures, all elements (and indeed all substances) are expected to metallize.<ref name=Siekierski/> Arsenic (As) has both a stable metallic allotrope and a metastable semiconducting allotrope at standard conditions. A similar situation affects carbon (C): [[graphite]] is metallic, but [[diamond]] is not.

==Alloys==
{{Main|Alloy}}
[[File:Pigs and Bars of Babbitt.jpg|thumb|left|Samples of [[babbitt (alloy)|babbitt metal]], an alloy of [[tin]], [[antimony]], and [[copper]], used in bearings to reduce friction|alt=Three bars of babbitt metal]]

In the context of metals, an alloy is a substance having metallic properties which is composed of two or more [[chemical element|elements]]. Often at least one of these is a metallic element; the term "alloy" is sometimes used more generally as in [[silicon–germanium]] alloys. An alloy may have a variable or fixed composition. For example, gold and silver form an alloy in which the proportions of gold or silver can be varied; titanium and silicon form an alloy [[Titanium disilicide|TiSi<sub>2</sub>]] in which the ratio of the two components is fixed (also known as an [[intermetallic compound]]<ref>{{Cite journal |last1=Liu |first1=C. T. |last2=Stiegler |first2=J. O. |date=1984-11-09 |title=Ductile Ordered Intermetallic Alloys |url=https://www.science.org/doi/10.1126/science.226.4675.636 |journal=Science |language=en |volume=226 |issue=4675 |pages=636–642 |doi=10.1126/science.226.4675.636 |pmid=17774926 |bibcode=1984Sci...226..636L |issn=0036-8075|url-access=subscription }}</ref><ref>{{Cite journal |last1=Cinca |first1=Nuria |last2=Lima |first2=Carlos Roberto Camello |last3=Guilemany |first3=Jose Maria |date=2013 |title=An overview of intermetallics research and application: Status of thermal spray coatings |journal=Journal of Materials Research and Technology |volume=2 |issue=1 |pages=75–86 |doi=10.1016/j.jmrt.2013.03.013 |issn=2238-7854|doi-access=free }}</ref>).

[[File:Willem Lenssinck - Formula I Racing Horse.jpg|thumb|150px|A sculpture cast in [[nickel silver]]—an alloy of copper, nickel, and zinc that looks like silver|alt=A metal sculpture]]
Most pure metals are either too soft, brittle, or chemically reactive for practical use. Combining different ratios of metals and other elements in alloys modifies the properties to produce desirable characteristics, for instance more ductile, harder, resistant to corrosion, or have a more desirable color and luster. Of all the metallic alloys in use today, the alloys of [[iron]] ([[steel]], [[stainless steel]], [[cast iron]], [[tool steel]], [[alloy steel]]) make up the largest proportion both by quantity and commercial value.<ref>{{Cite web |title=Steel use by sector |url=https://worldsteel.org/data/steel-use-by-sector/ |access-date=2024-09-18 |website=worldsteel.org |language=en-US}}</ref> Iron alloyed with various proportions of carbon gives low-, mid-, and high-carbon steels, with increasing carbon levels reducing ductility and toughness. The addition of [[silicon]] will produce cast irons, while the addition of [[chromium]], [[nickel]], and [[molybdenum]] to carbon steels (more than 10%) results in stainless steels with enhanced corrosion resistance.

Other significant metallic alloys are those of [[aluminium]], [[titanium]], [[copper]], and [[magnesium]]. Copper alloys have been known since prehistory—[[bronze]] gave the [[Bronze Age]] its name—and have many applications today, most importantly in electrical wiring. The alloys of the other three metals have been developed relatively recently; due to their chemical reactivity they need [[electrolysis|electrolytic]] extraction processes. The alloys of aluminium, titanium, and magnesium are valued for their high strength-to-weight ratios; magnesium can also provide [[electromagnetic shielding]].<ref>{{Cite journal |date=December 17, 2020 |title=Electromagnetic Shielding Performance of Different Metallic Coatings Deposited by Arc Thermal Spray Process |pmc=7767199 |last1=Jang |first1=J. M. |last2=Lee |first2=H. S. |last3=Singh |first3=J. K. |journal=Materials |volume=13 |issue=24 |page=5776 |doi=10.3390/ma13245776 |doi-access=free |pmid=33348891 |bibcode=2020Mate...13.5776J }}</ref><ref>{{Cite web |title=Metals Program Overview |url=https://arpa-e.energy.gov/sites/default/files/documents/files/METALS_ProgramSummary.pdf |access-date=June 4, 2024 |website=arpa-e.energy.gov}}</ref> These materials are ideal for situations where high strength-to-weight ratio is more important than material cost, such as in aerospace and some automotive applications.<ref>{{Cite book |last1=Gialanella |first1=Stefano |url=http://link.springer.com/10.1007/978-3-030-24440-8 |title=Aerospace Alloys |last2=Malandruccolo |first2=Alessio |date=2020 |publisher=Springer International Publishing |isbn=978-3-030-24439-2 |series=Topics in Mining, Metallurgy and Materials Engineering |location=Cham |language=en |doi=10.1007/978-3-030-24440-8}}</ref>

Alloys specially designed for highly demanding applications, such as [[jet engine]]s, may contain more than ten elements.

==Categories==
Metals can be categorised by their composition, physical or chemical properties. Categories described in the subsections below include [[ferrous metallurgy|ferrous]] and [[non-ferrous metals|non-ferrous]] metals; brittle metals and [[refractory metal]]s; white metals; [[heavy metals|heavy]] and [[light metal|light]] metals; [[base metal|base]], [[noble metal|noble]], and [[precious metal|precious]] metals as well as both metallic [[#Ceramic metals|ceramics]] and [[#Polymer metals|polymers]].

===Ferrous and non-ferrous metals===
{{Main|Ferrous metallurgy|Non-ferrous metal}}
The term "ferrous" is derived from the [[Latin language|Latin]] word meaning "containing iron". This can include pure iron, such as [[wrought iron]], or an alloy such as [[steel]]. Ferrous metals are often [[magnetism|magnetic]], but not exclusively. Non-ferrous metals and alloys lack appreciable amounts of iron.

===Brittle elemental metal===
While nearly all elemental metals are malleable or ductile, a few—beryllium, chromium, manganese, gallium, and bismuth—are brittle.<ref>{{cite book |title=Structure–Property Relations in Nonferrous Metals |last1=Russell |first1=A. M. |last2=Lee |first2=K. L. |year=2005 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=978-0-471-64952-6 |pages=passim |bibcode=2005srnm.book.....R}}</ref> Arsenic and antimony, if admitted as metals, are brittle. Low values of the ratio of bulk [[elastic modulus]] to [[shear modulus]] ([[Pugh's criterion]]) are indicative of intrinsic brittleness.<ref>{{Cite journal |last1=Senkov |first1=O. N. |last2=Miracle |first2=D. B. |date=2021-02-25 |title=Generalization of intrinsic ductile-to-brittle criteria by Pugh and Pettifor for materials with a cubic crystal structure |journal=Scientific Reports |language=en |volume=11 |issue=1 |page=4531 |doi=10.1038/s41598-021-83953-z |issn=2045-2322 |pmc=7907099 |pmid=33633140|bibcode=2021NatSR..11.4531S }}</ref> A material is brittle if it is hard for dislocations to move, which is often associated with large [[Burgers vector]]s and only a limited number of slip planes.<ref>{{Cite book |url=https://linkinghub.elsevier.com/retrieve/pii/B9780750646819X50007 |title=Introduction to Dislocations |date=2001 |publisher=Elsevier |isbn=978-0-7506-4681-9 |language=en |doi=10.1016/b978-0-7506-4681-9.x5000-7}}</ref>

===Refractory metal===
{{Main|Refractory metal}}
A refractory metal is a metal that is very resistant to heat and wear. Which metals belong to this category varies; the most common definition includes niobium, molybdenum, tantalum, tungsten, and rhenium as well as their alloys. They all have melting points above 2000&nbsp;°C, and a high [[hardness]] at room temperature. Several compounds such as titanium nitride are also described as refractory metals.
<gallery widths="165" heights="165">
File:Niobium crystals and 1cm3 cube.jpg|Niobium crystals and a 1&nbsp;cm<sup>3</sup> [[anodizing|anodized]] niobium cube for comparison
File:Macro Photographs of 3D Print of NASA Meatball - Made out of GRX-810, an Oxide Dispersion Strengthened (ODS) High Temperature Alloy (GRC-2023-C-02309).jpg|3D print of NASA meatball, made out of GRX-810, an oxide dispersion strengthened high temperature alloy
File:Rhenium single crystal bar and 1cm3 cube.jpg|Rhenium single crystal, a remelted bar, and a 1&nbsp;cm<sup>3</sup> rhenium cube for comparison
File:Titanium nitride TiN.jpg|Titanium nitride powder
</gallery>

===White metal===
{{Main|White metal}}
A [[white metal]] is any of a range of white-colored alloys with relatively low melting points used mainly for decorative purposes.<ref>{{Cite web |date=2019-04-17 |title=Belmont Metals - White Metals |url=https://www.belmontmetals.com/product-category/white-metals/ |access-date=2024-07-08 |website=Belmont Metals |language=en-US}}</ref><ref>{{Cite web |last=Roden |first=Arabella |date=2019-11-04 |title=A closer look at the world of white metals |url=https://www.jewellermagazine.com/Article2/8576/A-closer-look-at-the-world-of-white-metals |access-date=2024-07-08 |website=jewellermagazine.com}}</ref> In Britain, the fine art trade uses the term "white metal" in auction catalogues to describe foreign silver items which do not carry British Assay Office marks,<ref>{{Cite book |url=https://www.assayofficelondon.co.uk/media/1818/2128-hallmarking-guide-pressready_oct2016.pdf |title=Practical guidance in relation to the hallmarking act 1973 |publisher=Assay offices of Great Britain}}</ref> but which are nonetheless understood to be silver and are priced accordingly.

===Heavy and light metals===
{{Main|Heavy metals|Light metals}}
A heavy metal is any relatively dense metal, either single element or multielement.<ref>{{cite book |url=https://books.google.com/books?id=GXYT3JKBm9EC&q=%C2%A0%C2%A0A+heavy+metal+is+any+relatively+dense+metal+or+metalloid&pg=PA3 |title=Metal contamination |date=2006 |publisher=Editions Quae |isbn=978-2-7592-0011-5 |language=en}}</ref> [[Magnesium]], [[aluminium]] and [[titanium]] alloys are light metals of significant commercial importance.<ref>Brandes EA & Brook GB (eds) 1998, ''Light Metals Handbook,'' Butterworth Heinemann, Oxford, {{ISBN|0-7506-3625-4}}, p.&nbsp;viii</ref> Their densities of 1.7, 2.7 and 4.5 g/cm<sup>3</sup> range from 19 to 56% of the densities of other structural metals,<ref>Polmear I 2006, ''Light Alloys: From Traditional Alloys to Nanocrystals,'' 4th ed., Butterworth Heinemann, Oxford, {{ISBN|0-7506-6371-5}}, p.&nbsp;1</ref> such as [[iron]] (7.9) and [[copper]] (8.9) and their alloys.

===Base, noble, and precious metals===
{{main|Base metal|noble metal|precious metal}}
The term ''base metal'' refers to a metal that is easily [[oxidation|oxidized]] or [[corrosion|corroded]], such as reacting easily with dilute [[hydrochloric acid]] (HCl) to form a metal chloride and [[hydrogen]]. The term is normally used for the elements, and examples include iron, [[nickel]], [[lead]], and zinc. Copper is considered a base metal as it is oxidized relatively easily, although it does not react with HCl.

[[File:Rhodium powder pressed melted.jpg|thumb|[[Rhodium]], a [[noble metal]], shown here as 1&nbsp;g of powder, a 1&nbsp;g pressed cylinder, and a 1&nbsp;g pellet|alt=Rhodium powder, a rhodium cylinder, and a rhodium pellet in a row]]
The term [[noble metal]] (also for elements) is commonly used in opposition to ''base metal''. Noble metals are less reactive, resistant to [[corrosion]] or [[oxidation]],<ref>{{cite book |url=https://books.google.com/books?id=Tiv9Ba4dDI4C&q=Noble+metals+are+metals+that+are+resistant+to+corrosion+or+oxidation&pg=PA267 |title=Chemical Oxidation Applications for Industrial Wastewaters |last1=Tunay |first1=Olcay |last2=Kabdasli |first2=Isik |last3=Arslan-Alaton |first3=Idil |last4=Olmez-Hanci |first4=Tugba |date=2010 |publisher=IWA Publishing |isbn=978-1-84339-307-8 |language=en}}</ref> unlike most [[base metal]]s. They tend to be precious metals, often due to perceived rarity. Examples include gold, platinum, silver, [[rhodium]], iridium, and palladium.

In [[alchemy]] and [[numismatics]], the term base metal is contrasted with [[precious metal]], that is, those of high economic value.<ref>{{cite book |url=https://books.google.com/books?id=dWMqaOTqSqcC&q=A+precious+metal+rare+metal+of+high+economic+value&pg=PA203 |title=Earth's Natural Resources |last=Walther |first=John V. |date=2013 |publisher=Jones & Bartlett Publishers |isbn=978-1-4496-3234-2 |language=en}}</ref> Most coins today are made of base metals with [[fiat currency|low intrinsic value]]; in the past, coins frequently derived their value primarily from their [[precious metal]] content; [[gold]], [[silver]], [[platinum]], and [[palladium]] each have an [[ISO 4217]] currency code. Currently they have industrial uses such as platinum and palladium in [[catalytic converters]], are used in [[jewellery]] and also a role as investments and a [[store of value]].<ref>{{cite book |url=https://books.google.com/books?id=qaBmBgAAQBAJ&q=The+demand+for+precious+metals+is+driven+not+only+by+their+practical+use%2C+but+also+by+their+role+as+investments+and+a+store+of+value&pg=PA105 |title=The Art of RF (Riba-Free) Islamic Banking and Finance: Tools and Techniques for Community-Based Banking |last=Abdul-Rahman |first=Yahia |date=2014 |publisher=John Wiley & Sons |isbn=978-1-118-77096-2 |language=en}}</ref> Palladium and platinum, as of summer 2024, were valued at slightly less than half the price of gold, while silver is substantially less expensive.

===Valve metals===
{{Main|Valve metals}}
In electrochemistry, a valve metal is a metal which passes current in only one direction due to the formation of any insulating oxide later.<ref>{{Cite journal |last1=Yasuda |first1=Kouji |last2=Macak |first2=Jan M. |last3=Berger |first3=Steffen |last4=Ghicov |first4=Andrei |last5=Schmuki |first5=Patrik |date=2007 |title=Mechanistic Aspects of the Self-Organization Process for Oxide Nanotube Formation on Valve Metals |url=https://iopscience.iop.org/article/10.1149/1.2749091 |journal=Journal of the Electrochemical Society |language=en |volume=154 |issue=9 |pages=C472 |doi=10.1149/1.2749091|bibcode=2007JElS..154C.472Y |url-access=subscription }}</ref>

=== Metallic ceramics ===
[[File:Titanium nitride coating 90.jpg|thumb|TiN coated drill bit]]
There are many ceramic compounds which have metallic electrical conduction, but are not simple combinations of metallic elements. (They are not the same as [[cermet]]s which are composites of a non-conducting ceramic and a conducting metal.) One set, the transition metal nitrides has significant ionic character to the bonding, so can be classified as both ceramics and metals.<ref name="Stampfl-2001" /> They have partially filled states at the Fermi level<ref name="Stampfl-2001" /> so are good thermal and electrical conductors, and there is often significant charge transfer from the transition metal atoms to the nitrogen.<ref name="Stampfl-2001" /> However, unlike most elemental metals, ceramic metals are often not particularly ductile. Their uses are widespread, for instance [[titanium nitride]] finds use in orthopedic devices<ref>{{Cite journal |last1=van Hove |first1=Ruud P. |last2=Sierevelt |first2=Inger N. |last3=van Royen |first3=Barend J. |last4=Nolte |first4=Peter A. |date=2015 |title=Titanium-Nitride Coating of Orthopaedic Implants: A Review of the Literature |journal=BioMed Research International |language=en |volume=2015 |pages=1–9 |doi=10.1155/2015/485975 |doi-access=free |issn=2314-6133 |pmc=4637053 |pmid=26583113}}</ref> and as a wear resistant coating.<ref>{{Cite journal |last1=Santecchia |first1=Eleonora |last2=Hamouda |first2=A. M. S. |last3=Musharavati |first3=Farayi |last4=Zalnezhad |first4=Erfan |last5=Cabibbo |first5=Marcello |last6=Spigarelli |first6=Stefano |date=2015 |title=Wear resistance investigation of titanium nitride-based coatings |url=https://www.sciencedirect.com/science/article/pii/S0272884215009116 |journal=Ceramics International |volume=41 |issue=9, Part A |pages=10349–10379 |doi=10.1016/j.ceramint.2015.04.152 |issn=0272-8842|url-access=subscription }}</ref> In many cases their utility depends upon there being effective deposition methods so they can be used as thin film coatings.<ref>{{Cite journal |last=Matthews |first=A. |date=1985 |title=Titanium Nitride PVD Coating Technology |url=http://journals.sagepub.com/doi/10.1179/sur.1985.1.2.93 |journal=Surface Engineering |language=en |volume=1 |issue=2 |pages=93–104 |doi=10.1179/sur.1985.1.2.93 |issn=0267-0844|url-access=subscription }}</ref>

=== Metallic polymers ===
[[File:Conducting polymers.svg|thumb|Several of the conducting polymers<ref>{{Cite journal |last1=K |first1=Namsheer |last2=Rout |first2=Chandra Sekkha |date=2021 |title=Conducting polymers: a comprehensive review on recent advances in synthesis, properties and applications |journal=RSC Advances |language=en |volume=11 |issue=10 |pages=5659–5697 |doi=10.1039/D0RA07800J|pmid=35686160 |pmc=9133880 |bibcode=2021RSCAd..11.5659K }}</ref>]]
{{Main|Conductive polymer}}{{See also|Conductive metal−organic frameworks}}
There are many polymers which have metallic electrical conduction,<ref>{{Cite journal |last1=Das |first1=Tapan K. |last2=Prusty |first2=Smita |date=2012 |title=Review on Conducting Polymers and Their Applications |url=http://www.tandfonline.com/doi/abs/10.1080/03602559.2012.710697 |journal=Polymer-Plastics Technology and Engineering |language=en |volume=51 |issue=14 |pages=1487–1500 |doi=10.1080/03602559.2012.710697 |issn=0360-2559|url-access=subscription }}</ref><ref>{{Cite journal |last=Swager |first=Timothy M. |date=2017 |title=50th Anniversary Perspective : Conducting/Semiconducting Conjugated Polymers. A Personal Perspective on the Past and the Future |url=https://pubs.acs.org/doi/10.1021/acs.macromol.7b00582 |journal=Macromolecules |language=en |volume=50 |issue=13 |pages=4867–4886 |doi=10.1021/acs.macromol.7b00582 |bibcode=2017MaMol..50.4867S |issn=0024-9297|hdl=1721.1/116306 |hdl-access=free }}</ref> typically associated with extended aromatic components such as in the polymers indicated in the Figure. The conduction of the aromatic regions is similar to that of graphite, so is highly directional.<ref>{{Cite journal |last1=Beygisangchin |first1=Mahnoush |last2=Abdul Rashid |first2=Suraya |last3=Shafie |first3=Suhaidi |last4=Sadrolhosseini |first4=Amir Reza |last5=Lim |first5=Hong Ngee |date=2021-06-18 |title=Preparations, Properties, and Applications of Polyaniline and Polyaniline Thin Films—A Review |journal=Polymers |volume=13 |issue=12 |pages=2003 |doi=10.3390/polym13122003 |doi-access=free |issn=2073-4360 |pmc=8234317 |pmid=34207392}}</ref>

=== Half metal ===
{{Main|Half-metal}}
A '''half-metal''' is any substance that acts as a [[Electrical conductor|conductor]] to [[electron]]s of one [[Spin (physics)|spin]] orientation, but as an [[Electrical insulation|insulator]] or [[semiconductor]] to those of the opposite spin. They were first described in 1983, as an explanation for the electrical properties of [[manganese]]-based [[Heusler alloy]]s.<ref>{{Cite journal |last1=de Groot |first1=R. A. |last2=Mueller |first2=F. M. |last3=Engen |first3=P. G. van |last4=Buschow |first4=K. H. J. |date=1983-06-20 |title=New Class of Materials: Half-Metallic Ferromagnets |url=https://link.aps.org/doi/10.1103/PhysRevLett.50.2024 |journal=Physical Review Letters |language=en |volume=50 |issue=25 |pages=2024–2027 |doi=10.1103/PhysRevLett.50.2024 |bibcode=1983PhRvL..50.2024D |issn=0031-9007}}</ref> Although all half-metals are [[Ferromagnetism|ferromagnetic]] (or [[Ferrimagnetism|ferrimagnetic]]), most ferromagnets are not half-metals. Many of the known examples of half-metals are [[oxide]]s, [[sulfide]]s, or [[Heusler alloy]]s.<ref>{{Cite journal |last1=Coey |first1=J. M. D. |last2=Venkatesan |first2=M. |date=2002-05-15 |title=Half-metallic ferromagnetism: Example of CrO2 (invited) |url=https://pubs.aip.org/jap/article/91/10/8345/485889/Half-metallic-ferromagnetism-Example-of-CrO2 |journal=Journal of Applied Physics |language=en |volume=91 |issue=10 |pages=8345–8350 |doi=10.1063/1.1447879 |issn=0021-8979|url-access=subscription }}</ref>

=== Semimetal ===
{{Main|Semimetal}}
A '''semimetal''' is a material with a small energy overlap between the bottom of the [[Electrical conduction|conduction]] [[Electronic band structure|band]] and the top of the [[valence band]], but they do not overlap in [[momentum space]].<ref>{{Cite journal |last1=Zhai |first1=Enzi |last2=Liang |first2=Tianyu |last3=Liu |first3=Ruizi |last4=Cai |first4=Mingyang |last5=Li |first5=Ran |last6=Shao |first6=Qiming |last7=Su |first7=Cong |last8=Lin |first8=Yuxuan Cosmi |date=2024-08-01 |title=The rise of semi-metal electronics |url=https://www.nature.com/articles/s44287-024-00068-z |journal=Nature Reviews Electrical Engineering |language=en |volume=1 |issue=8 |pages=497–515 |doi=10.1038/s44287-024-00068-z |issn=2948-1201|url-access=subscription }}</ref> Unlike a regular metal, semimetals have charge carriers of both types (holes and electrons), although the charge carriers typically occur in much smaller numbers than in a real metal. In this respect they resemble [[degenerate semiconductor]]s. This explains why the electrical properties of semimetals are partway between those of metals and [[semiconductors]]. There are additional types, in particular [[Weyl semimetal|Weyl]] and [[Dirac semi-metal|Dirac semimetals]].<ref>{{Cite journal |last1=Armitage |first1=N. P. |last2=Mele |first2=E. J. |last3=Vishwanath |first3=Ashvin |date=2018-01-22 |title=Weyl and Dirac semimetals in three-dimensional solids |url=https://link.aps.org/doi/10.1103/RevModPhys.90.015001 |journal=Reviews of Modern Physics |language=en |volume=90 |issue=1 |page=015001 |doi=10.1103/RevModPhys.90.015001 |issn=0034-6861|arxiv=1705.01111 |bibcode=2018RvMP...90a5001A }}</ref>

The classic elemental semimetallic elements are [[arsenic]], [[antimony]], [[bismuth]], α-[[tin]] (gray tin) and [[graphite]]. There are also [[chemical compound]]s, such as [[mercury telluride]] (HgTe),<ref>{{cite journal |last=Wang |first=Yang |author2=N. Mansour |author3=A. Salem |author4=K.F. Brennan |author5=P.P. Ruden |name-list-style=amp |date=1992 |title=Theoretical study of a potential low-noise semimetal-based avalanche photodetector |journal=IEEE Journal of Quantum Electronics |volume=28 |issue=2 |pages=507–513 |bibcode=1992IJQE...28..507W |doi=10.1109/3.123280}}</ref> and some [[conductive polymers]].<ref>{{cite journal |last1=Bubnova |first1=Olga |last2=Zia |first2=Ullah Khan |last3=Wang |first3=Hui |date=2014 |title=Semi-Metallic Polymers |url=https://liu.diva-portal.org/smash/get/diva2:698183/FULLTEXT01 |journal=Nature Materials |volume=13 |issue=2 |pages=190–4 |bibcode=2014NatMa..13..190B |doi=10.1038/nmat3824 |pmid=24317188 |s2cid=205409397}}</ref>

==Lifecycle==
===Formation===
{{See also|Nucleosynthesis}}
{{Periodic table (metal abundance in Earth crust)}}

Metallic elements up to the [[iron peak|vicinity of iron]] (in the periodic table) are largely made via [[stellar nucleosynthesis]]. In this process, lighter elements from hydrogen to [[silicon]] undergo successive [[nuclear fusion|fusion]] reactions inside stars, releasing light and heat and forming heavier elements with higher atomic numbers.<ref name="Cox">{{harvnb|Cox|1997|pp=73–89}}</ref>

Heavier elements are not usually formed this way since fusion reactions involving such nuclei would consume rather than release energy.<ref>{{harvnb|Cox|1997|pp=32, 63, 85}}</ref> Rather, they are largely synthesised (from elements with a lower atomic number) by [[neutron capture]], with the two main modes of this repetitive capture being the [[s-process]] and the [[r-process]]. In the s-process ("s" stands for "slow"), singular captures are separated by years or decades, allowing the less stable nuclei to [[beta decay]],<ref>{{harvnb|Podosek|2011|p=482}}</ref> while in the r-process ("rapid"), captures happen faster than nuclei can decay. Therefore the s-process takes a more-or-less clear path: for example, stable cadmium-110 nuclei are successively bombarded by free neutrons inside a star until they form cadmium-115 nuclei which are unstable and decay to form indium-115 (which is nearly stable, with a half-life {{val|30000}} times the age of the universe). These nuclei capture neutrons and form indium-116, which is unstable, and decays to form tin-116, and so on.<ref name="Cox" /><ref>{{harvnb|Padmanabhan|2001|p=234}}</ref>{{#tag:ref|In some cases, for example in the presence of [[photodisintegration|high energy gamma rays]] or in a [[rp-process|very high temperature hydrogen rich environment]], the subject nuclei may experience neutron loss or proton gain resulting in the production of (comparatively rare) [[p-nuclei|neutron deficient isotopes]].<ref>{{harvnb|Rehder|2010|pp=32, 33}}</ref>|group=n}} In contrast, there is no such path in the r-process. The s-process stops at bismuth due to the short half-lives of the next two elements, polonium and astatine, which decay to bismuth or lead. The r-process is so fast it can skip this zone of instability and go on to create heavier elements such as [[thorium]] and uranium.<ref>{{harvnb|Hofmann|2002|pp=23–24}}</ref>

Metals condense in planets as a result of stellar evolution and destruction processes. Stars lose much of their mass when it is [[stellar mass loss|ejected]] late in their lifetimes, and sometimes thereafter as a result of a [[neutron star]] merger,<ref>{{harvnb|Hadhazy|2016}}</ref>{{#tag:ref|The ejection of matter when two neutron stars collide is attributed to the interaction of their [[tidal force]]s, possible crustal disruption, and shock heating (which is what happens if you floor the accelerator in car when the engine is cold).<ref>{{harvnb|Choptuik|Lehner|Pretorias|2015|p=383}}</ref>|group=n}} thereby increasing the abundance of elements heavier than helium in the [[interstellar medium]]. When gravitational attraction causes this matter to coalesce and collapse [[nebular hypothesis|new stars and planets are formed]].<ref>{{harvnb|Cox|1997|pp=83, 91, 102–103}}</ref>

===Abundance and occurrence===
{{See also|Abundance of the chemical elements}}
[[File:Diaspore-Margarite-rare-09-05b.jpg|thumb|left|A sample of [[diaspore]], an aluminium oxide hydroxide mineral, α-AlO(OH)|alt=A sample of diaspore]]
The [[Earth's crust]] is made of approximately 25% of metallic elements by weight, of which 80% are light metals such as sodium, magnesium, and aluminium. Despite the overall scarcity of some heavier metals such as copper, they can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes.

Metallic elements are primarily found as lithophiles (rock-loving) or chalcophiles (ore-loving). Lithophile elements are mainly the s-block elements, the more reactive of the d-block elements, and the f-block elements. They have a strong affinity for oxygen and mostly exist as relatively low-density silicate minerals. Chalcophile elements are mainly the less reactive d-block elements, and the period 4–6 p-block metals. They are usually found in (insoluble) sulfide minerals. Being denser than the lithophiles, hence sinking lower into the crust at the time of its solidification, the chalcophiles tend to be less abundant than the lithophiles.

On the other hand, gold is a siderophile, or iron-loving element. It does not readily form compounds with either oxygen or sulfur. At the time of the Earth's formation, and as the most noble (inert) of metallic elements, gold sank into the core due to its tendency to form high-density metallic alloys. Consequently, it is relatively rare. Some other (less) noble ones—molybdenum, rhenium, the platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum), germanium, and tin—can be counted as siderophiles but only in terms of their primary occurrence in the Earth (core, mantle, and crust), rather the crust. These otherwise occur in the crust, in small quantities, chiefly as chalcophiles (less so in their native form).{{#tag:ref|Iron, cobalt, nickel, and tin are also siderophiles from a whole of Earth perspective.|group=n}}

The rotating fluid outer core of the Earth's interior, which is composed mostly of iron, is thought to be the source of Earth's protective magnetic field.{{#tag:ref|Another life-enabling role for iron is as a key constituent of [[hemoglobin]], which enables the transportation of oxygen from the lungs to the rest of the body.|group=n}} The core lies above Earth's solid inner core and below its mantle. If it could be rearranged into a column having a {{cvt|5|m2}} footprint it would have a height of nearly 700 light years. The magnetic field shields the Earth from the charged particles of the solar wind, and cosmic rays that would otherwise strip away the upper atmosphere (including the ozone layer that limits the transmission of ultraviolet radiation).

===Extraction===
{{main|Ore|Mining|Extractive metallurgy}}
Metallic elements are often extracted from the Earth by mining ores that are rich sources of the requisite elements, such as [[bauxite]]. Ores are located by [[prospecting]] techniques, followed by the exploration and examination of deposits. Mineral sources are generally divided into [[surface mining|surface mines]], which are mined by excavation using heavy equipment, and [[underground mining (hard rock)|subsurface mines]]. In some cases, the sale price of the metal(s) involved make it economically feasible to mine lower concentration sources.

Once the ore is mined, the elements must be [[extractive metallurgy|extracted]], usually by chemical or electrolytic reduction. [[Pyrometallurgy]] uses high temperatures to convert ore into raw metals, while [[hydrometallurgy]] employs [[aqueous]] chemistry for the same purpose.

When a metallic ore is an ionic compound, the ore must usually be [[smelting|smelted]]—heated with a reducing agent—to extract the pure metal. Many common metals, such as iron, are smelted using [[carbon]] as a reducing agent. Some metals, such as aluminium and [[sodium]], have no commercially practical reducing agent, and are extracted using [[electrolysis]] instead.<ref name="losal">{{cite web |url=https://periodic.lanl.gov/11.shtml |title=Los Alamos National Laboratory – Sodium |access-date=2007-06-08}}</ref><ref>{{cite web |url=https://periodic.lanl.gov/13.shtml |title=Los Alamos National Laboratory – Aluminum |access-date=2007-06-08}}</ref>

[[Sulfide]] ores are not reduced directly to the metal but are roasted in air to convert them to oxides.

===Recycling===
[[File:CompactedSteelScraps.jpg|thumb|left|A pile of compacted steel scraps, ready for recycling|alt=A pile of compacted steel scraps]]
Demand for metals is closely linked to economic growth given their use in infrastructure, construction, manufacturing, and consumer goods. During the 20th century, the variety of metals used in society grew rapidly. Today, the development of major nations, such as China and India, and technological advances, are fueling ever more demand. The result is that mining activities are expanding, and more and more of the world's metal stocks are above ground in use, rather than below ground as unused reserves. An example is the in-use stock of [[copper]]. Between 1932 and 1999, copper in use in the U.S. rose from 73 g to 238 g per person.<ref name="unep.org">[http://www.unep.org/resourcepanel/Publications/tabid/54044/Default.aspx ''The Recycling Rates of Metals: A Status Report''] {{webarchive|url=https://web.archive.org/web/20160101194304/http://www.unep.org/resourcepanel/Publications/tabid/54044/Default.aspx |date=2016-01-01}} 2010, [[International Resource Panel]], [[United Nations Environment Programme]]</ref>

Metals are inherently recyclable, so in principle, can be used over and over again, minimizing these negative environmental impacts and saving energy. For example, 95% of the energy used to make aluminium from bauxite ore is saved by using recycled material.<ref>[https://www.theguardian.com/environment/2008/feb/22/pledges.waste ''Tread lightly: Aluminium attack''] Carolyn Fry, Guardian.co.uk, 22 February 2008.</ref>

Globally, metal recycling is generally low. In 2010, the [[International Resource Panel]], hosted by the [[United Nations Environment Programme]] published reports on metal stocks that exist within society<ref>[http://www.unep.org/resourcepanel/Publications/tabid/54044/Default.aspx ''Metal Stocks in Society: Scientific Synthesis''] {{webarchive|url=https://web.archive.org/web/20160101194304/http://www.unep.org/resourcepanel/Publications/tabid/54044/Default.aspx |date=2016-01-01}} 2010, [[International Resource Panel]], [[United Nations Environment Programme]]</ref> and their recycling rates.<ref name="unep.org"/> The authors of the report observed that the metal stocks in society can serve as huge mines above ground. They warned that the recycling rates of some rare metals used in applications such as mobile phones, battery packs for hybrid cars and fuel cells are so low that unless future end-of-life recycling rates are dramatically stepped up these critical metals will become unavailable for use in modern technology.

==History==
===Prehistory===
Copper, which occurs in native form, may have been the first metal discovered given its distinctive appearance, heaviness, and malleability. Gold, silver, iron (as meteoric iron), and lead were likewise discovered in prehistory. Forms of [[brass]], an alloy of copper and zinc made by concurrently smelting the ores of these metals, originate from this period (although pure zinc was not isolated until the 13th century). The malleability of the solid metals led to the first attempts to craft metal ornaments, tools, and weapons. Meteoric iron containing nickel was discovered from time to time and, in some respects this was superior to any industrial steel manufactured up to the 1880s when alloy steels become prominent.<ref>{{Cite book |last=Reardon |first=Arthur C. |url=https://www.asminternational.org/documents/10192/3212401/05306G_Sample_BuyNow.pdf/ab60c086-2c71-4de0-91f6-aad1112cf4dc |title=Metallurgy for the non-metallurgist |date=2011 |publisher=ASM International |isbn=978-1-61503-845-9 |location=Materials Park, Ohio |pages=73–84 |oclc=780082219}}</ref>

<gallery widths="165px" heights="165px">
File:NatCopper.jpg|[[Native metal#Copper|Native copper]]
File:Gold-crystals.jpg|Gold crystals
File:Silver crystal.jpg|Crystalline silver
File:Widmanstatten hand.jpg|A slice of meteoric iron
File:Lead electrolytic and 1cm3 cube.jpg|alt=Three, dark broccoli shaped clumps of oxidised lead with grossly distended buds, and a cube of lead which has a dull silvery appearance.|<div align="center">[[redox|oxidised]] [[lead]]<br />nodules and 1&nbsp;cm<sup>3</sup> cube</div>
File:Akan MHNT.AC.AF.29.jpg|A brass weight (35 g)
</gallery>

===Antiquity===
[[File:Netuno19b.jpg|thumb|The [[Artemision Bronze]]{{#tag:ref|[[Bronze]] is an alloy consisting primarily of copper, commonly with about 12% tin and often with the addition of other metals (such as aluminium, manganese, nickel, or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus, or silicon.|group=n}} showing either [[Poseidon]] or [[Zeus]], c.&nbsp;460&nbsp;BCE, [[National Archaeological Museum, Athens|National Archaeological Museum]], [[Athens]]. The figure is more than 2&nbsp;m in height.|alt=Refer to caption]]

The discovery of [[bronze]] (an alloy of copper with arsenic or tin) enabled people to create metal objects which were harder and more durable than previously possible. Bronze tools, weapons, armor, and [[building material]]s such as decorative tiles were harder and more durable than their stone and copper ("[[Chalcolithic]]") predecessors. Initially, bronze was made of copper and [[arsenic]] (forming [[arsenic bronze]]) by smelting naturally or artificially mixed ores of copper and arsenic.<ref>
{{cite book |title=A History of Metallurgy, Second Edition |last=Tylecote |first=R. F. |year=1992 |publisher=Maney Publishing, for the Institute of Materials |location=London |isbn=978-1-902653-79-2 |url=http://m.friendfeed-media.com/450c6f3cdc92be9e19ecd285bd7f809a9ae1d4d5 |url-status=dead |archive-url=https://web.archive.org/web/20150402174311/http://m.friendfeed-media.com/450c6f3cdc92be9e19ecd285bd7f809a9ae1d4d5 |archive-date=2015-04-02}}</ref> The earliest [[Artifact (archaeology)|artifacts]] so far known come from the [[Iranian plateau]] in the fifth millennium BCE.<ref name="Thornton, 2002" >{{cite journal |last1=Thornton |first1=C. |last2=Lamberg-Karlovsky |first2=C. C. |last3=Liezers |first3=M. |last4=Young |first4=S. M. M. |year=2002 |title=On pins and needles: tracing the evolution of copper-based alloying at Tepe Yahya, Iran, via ICP-MS analysis of Common-place items. |journal=Journal of Archaeological Science |volume=29 |issue=12 |pages=1451–1460 |ref=Thornton, 2002 |doi=10.1006/jasc.2002.0809|bibcode=2002JArSc..29.1451T }}</ref> It was only later that [[tin]] was used, becoming the major non-copper ingredient of bronze in the late third millennium BCE.<ref>{{cite journal |last1=Kaufman |first1=Brett |title=Metallurgy and Archaeological Change in the Ancient Near East |journal=Backdirt: Annual Review |volume=2011 |page=86}}</ref> Pure tin itself was first isolated in 1800 BCE by Chinese and Japanese metalworkers.

Mercury was known to ancient Chinese and Indians before 2000 BCE, and found in Egyptian tombs dating from 1500 BCE.

The earliest known production of steel, an iron-carbon alloy, is seen in pieces of ironware excavated from an [[archaeological site]] in [[Anatolia]] ([[Kaman-Kalehöyük]]) which are nearly 4,000 years old, dating from 1800 BCE.<ref>{{cite journal |last=Akanuma |first=H. |title=The significance of the composition of excavated iron fragments taken from Stratum III at the site of Kaman-Kalehöyük, Turkey |journal=Anatolian Archaeological Studies |volume=14 |pages=147–158 |year=2005 |publisher=Japanese Institute of Anatolian Archaeology |place=Tokyo}}</ref><ref>{{cite news |title=Ironware piece unearthed from Turkey found to be oldest steel |url=http://www.hindu.com/thehindu/holnus/001200903261611.htm |access-date=2009-03-27 |location=Chennai, India |work=The Hindu |date=2009-03-26 |url-status=dead |archive-url=https://web.archive.org/web/20090329111924/http://www.hindu.com/thehindu/holnus/001200903261611.htm |archive-date=2009-03-29}}</ref>

From about 500 BCE sword-makers of [[Toledo, Spain]], were making early forms of [[alloy steel]] by adding a mineral called [[wolframite]], which contained tungsten and manganese, to iron ore (and carbon). The resulting [[Toledo steel]] came to the attention of Rome when used by Hannibal in the [[Punic Wars]]. It soon became the basis for the weaponry of Roman legions; such swords were, "stronger in composition than any existing sword and, because… [they] would not break, provided a psychological advantage to the Roman soldier."<ref>{{cite book |last1=Gabriel |first1=RA | date= 1990|title= The Culture of War: Invention and Early Development|location=Westport CT |publisher= Greenwood Publishing Group|page=108 |isbn=978-0-313-26664-5}}</ref>

[[Metallurgy in pre-Columbian America|In pre-Columbian America]], objects made of [[tumbaga]], an alloy of copper and gold, started being produced in Panama and Costa Rica between 300 and 500 CE. Small metal sculptures were common and an extensive range of tumbaga (and gold) ornaments comprised the usual regalia of persons of high status.

At around the same time indigenous Ecuadorians were combining gold with a naturally-occurring platinum alloy containing small amounts of palladium, rhodium, and iridium, to produce miniatures and masks of a white gold-platinum alloy. The metal workers involved heated gold with [[grain (metal)|grains]] of the platinum alloy until the gold melted. After cooling, the resulting conglomeration was hammered and reheated repeatedly until it became homogenous, equivalent to melting all the metals (attaining the melting points of the platinum group metals concerned was beyond the technology of the day).<ref>{{cite book |title=The Metalsmiths, revised edition |last=Knauth |first=P. |year=1976 |publisher=Time-Life International |location=London |pages=133, 137}}</ref>{{#tag:ref|In Damascus, Syria, blade-smiths forged knives and swords with a distinctive surface pattern composed of swirling patterns of light-etched regions on a nearly black background. These blades had legendary cutting abilities. [[Wootz|The iron the smiths were using]] was sourced from India, and contained one or more carbide-forming elements, such as V, Mo, Cr, Mn, and Nb. Modern analysis of these weapons has shown that these elements supported the catalytic formation of carbon nanotubes, which in turn promoted the formation of [[cementite]] (Fe<small>3</small>C) nanowires. The malleability of the carbon nanotubes offset the brittle nature of the cementite, and endowed the resulting steel with a unique combination of strength and flexibility. Knowledge of how to make what came to called [[Damascus steel]] died out in the eighteenth century possibly due to exhausting ore sources with the right combination of impurities. The techniques involved were not rediscovered until 2009.|group=n}}

<gallery widths="165px" heights="165px">
File:Tin-2.jpg|A droplet of solidified molten tin
File:Pouring liquid mercury bionerd.jpg|alt=A silvery molasses-like liquid being poured into a circular container with a height equivalent to a smaller coin on its edge|<div align="center">[[Mercury (element)|Mercury]] being<br />poured into a [[petri dish]]</div>
File:25 litrai en électrum représentant un trépied delphien.jpg|Electrum, a natural alloy of silver and gold, was often used for making coins. Shown is the Greek god [[Apollo]], and on the obverse, a [[Sacrificial tripod#Ancient Greece|Delphi tripod]] ({{circa|310}}–305 BCE).
File:Passover Plate (4047010755).jpg|A plate made of [[pewter]], an alloy of 85–99% tin and (usually) copper. Pewter was first used around the beginning of the Bronze Age in the Near East.
File:Museo del Oro - Tolima pectoral.jpg|A pectoral (ornamental breastplate) made of [[tumbaga]], an alloy of gold and copper
</gallery>

== Ancient Greece ==
[[File:Aristoteles Louvre.jpg|right|thumb|upright=1.0|alt=A stone sculpture of the head of a bearded man|Greek philosopher [[Aristotle]] (384–322 BCE) categorized substances found in the earth as either metals or minerals.]]
Around 340 BCE, in Book III of his treatise ''[[Meteorology (Aristotle)|Meteorology]]'', the ancient Greek philosopher [[Aristotle]] categorized substances found within the Earth into metals and minerals. The latter category included various minerals such as [[realgar]], [[ochre]], [[red ochre|ruddle]], sulfur, [[cinnabar]], and other substances that he referred to as "stones which cannot be melted".<ref>{{Cite journal |last=Jordan |first=J. M. |date=April 2016 |title='Ancient episteme' and the nature of fossils: a correction of a modern scholarly error |url=http://link.springer.com/10.1007/s40656-015-0094-6 |journal=History and Philosophy of the Life Sciences |language=en |volume=38 |issue=1 |pages=90–116 |doi=10.1007/s40656-015-0094-6 |issn=0391-9714|url-access=subscription }}</ref>

===Middle Ages===
{{Quote box
| quote=Gold is for the mistress—silver for the maid—<br />Copper for the craftsman cunning at his trade.<br />"Good!" said the Baron, sitting in his hall,<br />"But Iron—Cold Iron—is master of them all."
| salign=right| source=from [[Cold Iron (poem)|Cold Iron]] by [[Rudyard Kipling]]<ref>Published in ''[[The Delineator]]'', Sept. 1909. Reprinted as the introduction to [[Rewards and Fairies]] in 1910.</ref>| bgcolor=Cornsilk
| quoted=1
| width=25em
| align=right
}}
Arabic and medieval [[alchemy|alchemists]] believed that all metals and matter were composed of the principle of sulfur, the father of all metals and carrying the combustible property, and the principle of mercury, the mother of all metals{{#tag:ref|In ancient times, lead was regarded as the father of all metals.|group=n}} and carrier of the liquidity, fusibility, and volatility properties. These principles were not necessarily the common substances [[sulfur]] and [[mercury (element)|mercury]] found in most laboratories. This theory reinforced the belief that all metals were destined to become gold in the bowels of the earth through the proper combinations of heat, digestion, time, and elimination of contaminants, all of which could be developed and hastened through the knowledge and methods of alchemy.{{#tag:ref|[[Paracelsus]], a later [[German Renaissance]] writer, added the third principle of salt, carrying the nonvolatile and incombustible properties, in his [[Paracelsus#Philosophy|''tria prima'' doctrine]]. These theories retained the four classical elements as underlying the composition of sulfur, mercury, and salt.|group=n}}

Arsenic, zinc, antimony, and bismuth became known, although these were at first called semimetals or bastard metals on account of their immalleability. [[Albertus Magnus]] is believed to have been the first to isolate arsenic from a compound in 1250, by heating soap together with [[arsenic trisulfide]]. Metallic zinc, which is brittle if impure, was isolated in India by 1300 AD. The first description of a procedure for isolating antimony is in the 1540 book ''[[De la pirotechnia]]'' by [[Vannoccio Biringuccio]]. Bismuth was described by Agricola in ''[[De Natura Fossilium]]'' (c. 1546); it had been confused in early times with tin and lead because of its resemblance to those elements.

<gallery widths="165px" heights="165px">
File:Arsen 1a.jpg|Arsenic, sealed in a container to prevent tarnishing
File:Zinc fragment sublimed and 1cm3 cube.jpg|Zinc fragments and a 1&nbsp;cm<sup>3</sup> cube
File:Antimony-4.jpg|Antimony, showing its brilliant lustre
File:Wismut Kristall und 1cm3 Wuerfel.jpg|Bismuth in crystalline form, with a very thin oxidation layer, and a 1&nbsp;cm<sup>3</sup> bismuth cube
</gallery>

===The Renaissance===
[[File:Georg Agricola-Titelblatt.jpg|thumb|left|200px|''[[De re metallica]]'', 1555|alt=The title page of De re metallica, which is written in Latin]]
[[File:Platinum crystals.jpg|thumb|left|200px|Platinum crystals|alt=Refer to caption]]
[[File:HEUraniumC.jpg|thumb|left|200px|A disc of highly enriched uranium that was recovered from scrap processed at the [[Y-12 National Security Complex]], in [[Oak Ridge, Tennessee]]|alt=A disc of uranium being held by gloved hands]]
[[File:Cerium2.jpg|thumb|left|200px|Ultrapure cerium under argon, 1.5 gm|alt=Ultrapure cerium under argon]]

The first systematic text on the arts of mining and metallurgy was [[De la pirotechnia|''De la Pirotechnia'']] (1540) by [[Vannoccio Biringuccio]], which treats the examination, fusion, and working of metals.

Sixteen years later, [[Georgius Agricola]] published ''[[De Re Metallica]]'' in 1556, an account of the profession of mining, metallurgy, and the accessory arts and sciences, an extensive treatise on the chemical industry through the sixteenth century.

He gave the following description of a metal in his ''[[De Natura Fossilium]]'' (1546):
<blockquote><div style="font-size:90%">
Metal is a mineral body, by nature either liquid or somewhat hard. The latter may be melted by the heat of the fire, but when it has cooled down again and lost all heat, it becomes hard again and resumes its proper form. In this respect it differs from the stone which melts in the fire, for although the latter regain its hardness, yet it loses its pristine form and properties.

Traditionally there are six different kinds of metals, namely gold, silver, copper, iron, tin, and lead. There are really others, for [[mercury (element)|quicksilver]] is a metal, although the Alchemists disagree with us on this subject, and [[bismuth]] is also. The ancient Greek writers seem to have been ignorant of bismuth, wherefore Ammonius rightly states that there are many species of metals, animals, and plants which are unknown to us. [[Stibium]] when smelted in the crucible and refined has as much right to be regarded as a proper metal as is accorded to lead by writers. If when smelted, a certain portion be added to tin, a bookseller's alloy is produced from which the type is made that is used by those who print books on paper.

Each metal has its own form which it preserves when separated from those metals which were mixed with it. Therefore neither [[electrum]] nor Stannum [not meaning our tin] is of itself a real metal, but rather an alloy of two metals. Electrum is an alloy of gold and silver, Stannum of lead and silver. And yet if silver be parted from the electrum, then gold remains and not electrum; if silver be taken away from Stannum, then lead remains and not Stannum.

Whether brass, however, is found as a native metal or not, cannot be ascertained with any surety. We only know of the artificial brass, which consists of copper tinted with the colour of the mineral [[calamine]]. And yet if any should be dug up, it would be a proper metal. Black and white copper seem to be different from the red kind.

Metal, therefore, is by nature either solid, as I have stated, or fluid, as in the unique case of quicksilver.

But enough now concerning the simple kinds.<ref>Georgius Agricola, [https://www.gutenberg.org/files/38015/38015-h/38015-h.htm ''De Re Metallica''] (1556) Tr. Herbert Clark Hoover & Lou Henry Hoover (1912); Footnote quoting ''De Natura Fossilium'' (1546), p. 180</ref>
</div>
</blockquote>

Platinum, the third precious metal after gold and silver, was discovered in Ecuador during the period 1736 to 1744 by the Spanish astronomer Antonio de Ulloa and his colleague the mathematician Jorge Juan y Santacilia. Ulloa was the first person to write a scientific description of the metal, in 1748.

In 1789, the German chemist Martin Heinrich Klaproth isolated an oxide of uranium, which he thought was the metal itself. Klaproth was subsequently credited as the discoverer of uranium. It was not until 1841, that the French chemist Eugène-Melchior Péligot, prepared the first sample of uranium metal. Henri Becquerel subsequently discovered radioactivity in 1896 using uranium.

In the 1790s, Joseph Priestley and the Dutch chemist Martinus van Marum observed the effect of metal surfaces on the dehydrogenation of alcohol, a development which subsequently led, in 1831, to the industrial scale synthesis of sulphuric acid using a platinum catalyst.

In 1803, cerium was the first of the [[Lanthanide|lanthanide metals]] to be discovered, in Bastnäs, Sweden by Jöns Jakob Berzelius and Wilhelm Hisinger, and independently by Martin Heinrich Klaproth in Germany. The lanthanide metals were regarded as oddities until the 1960s when methods were developed to more efficiently separate them from one another. They have subsequently found uses in cell phones, magnets, lasers, lighting, batteries, catalytic converters, and in other applications enabling modern technologies.

Other metals discovered and prepared during this time were cobalt, nickel, manganese, molybdenum, tungsten, and chromium; and some of the [[platinum group]] metals, palladium, osmium, iridium, and rhodium.

===Light metallic elements===
All elemental metals discovered before 1809 had relatively high densities; their heaviness was regarded as a distinguishing criterion. From 1809 onward, light metals such as sodium, potassium, and strontium were isolated. Their low densities challenged conventional wisdom as to the nature of metals. They behaved chemically as metals however, and were subsequently recognized as such.

Aluminium was discovered in 1824 but it was not until 1886 that an industrial large-scale production method was developed. Prices of aluminium dropped and aluminium became widely used in jewelry, everyday items, eyeglass frames, optical instruments, tableware, and foil in the 1890s and early 20th century. Aluminium's ability to form hard yet light alloys with other metals provided the metal many uses at the time. During World War I, major governments demanded large shipments of aluminium for light and strong airframes.

While pure metallic titanium (99.9%) was first prepared in 1910 it was not used outside the laboratory until 1932. In the 1950s and 1960s, the Soviet Union pioneered the use of titanium in military and submarine applications as part of programs related to the Cold War. Starting in the early 1950s, titanium came into use in military aviation, particularly in high-performance jets, starting with aircraft such as the [[F-100 Super Sabre]] and [[Lockheed A-12]] and [[SR-71]].

Metallic scandium was produced for the first time in 1937. The first pound of 99% pure scandium metal was produced in 1960. Production of aluminium-scandium alloys began in 1971 following a U.S. patent. Aluminium-scandium alloys were also developed in the USSR.
<gallery widths="165" heights="165">
File:Na (Sodium).jpg|Chunks of sodium
File:Potassium-2.jpg|Potassium pearls under paraffin oil. Size of the largest pearl is 0.5&nbsp;cm.
File:Strontium destilled crystals.jpg|Strontium crystals
File:Aluminium-4.jpg|Aluminium chunk,<br />2.6&nbsp;grams, {{nowrap|1=1 x 2 cm}}
File:Titan-crystal bar.JPG|A bar of titanium crystals
File:Scandium sublimed dendritic and 1cm3 cube.jpg|Scandium, including a 1&nbsp;cm<sup>3</sup> cube
</gallery>

===The age of steel===
[[File:Allegheny Ludlum steel furnace.jpg|thumb|right|White-hot steel pours like water from a 35-ton electric furnace, at the Allegheny Ludlum Steel Corporation, in [[Brackenridge, Pennsylvania|Brackenridge]], [[Pennsylvania]].]]

The modern era in [[steelmaking]] began with the introduction of [[Henry Bessemer]]'s [[Bessemer process]] in 1855, the raw material for which was pig iron. His method let him produce steel in large quantities cheaply, thus [[mild steel]] came to be used for most purposes for which wrought iron was formerly used. The [[Gilchrist-Thomas process]] (or ''basic Bessemer process'') was an improvement to the Bessemer process, made by lining the converter with a [[basic (chemistry)|basic]] material to remove phosphorus.

Due to its high [[tensile strength]] and low cost, steel came to be a major component used in [[building]]s, [[infrastructure]], [[tool]]s, [[ship]]s, [[automobile]]s, [[machine]]s, appliances, and [[weapon]]s.

In 1872, the Englishmen Clark and Woods patented an alloy that would today be considered a [[stainless steel]]. The corrosion resistance of iron-chromium alloys had been recognized in 1821 by French metallurgist [[Pierre Berthier]]. He noted their resistance against attack by some acids and suggested their use in cutlery. Metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical. It was not until 1912 that the industrialization of stainless steel alloys occurred in England, Germany, and the United States.

===The last stable metallic elements===
By 1900 three metals with [[atomic number]]s less than lead (#82), the heaviest stable metal, remained to be discovered: elements 71, 72, 75.

[[Carl Auer von Welsbach|Von Welsbach]], in 1906, proved that the old ytterbium also contained a new element (#71), which he named ''cassiopeium''. [[Georges Urbain|Urbain]] proved this simultaneously, but his samples were very impure and only contained trace quantities of the new element. Despite this, his chosen name ''[[lutetium]]'' was adopted.

In 1908, Ogawa found element 75 in thorianite but assigned it as element 43 instead of 75 and named it ''nipponium''. In 1925 Walter Noddack, Ida Eva Tacke, and Otto Berg announced its separation from gadolinite and gave it the present name, ''[[rhenium]]''.

Georges Urbain claimed to have found element 72 in rare-earth residues, while Vladimir Vernadsky independently found it in orthite. Neither claim was confirmed due to World War I, and neither could be confirmed later, as the chemistry they reported does not match that now known for ''hafnium''. After the war, in 1922, Coster and Hevesy found it by X-ray spectroscopic analysis in Norwegian zircon. [[Hafnium]] was thus the last stable element to be discovered, though rhenium was the last to be correctly recognized.

<gallery widths="165px" heights="165px">
File:Lutetium sublimed dendritic and 1cm3 cube.jpg|Lutetium, including a 1&nbsp;cm<sup>3</sup> cube
File:Rhenium single crystal bar and 1cm3 cube.jpg|Rhenium, including a 1&nbsp;cm<sup>3</sup> cube
File:Hf-crystal bar.jpg|Hafnium, in the form of a 1.7&nbsp;kg bar
</gallery>

By the end of World War II scientists had synthesized four post-uranium elements, all of which are radioactive (unstable) metals: neptunium (in 1940), plutonium (1940–41), and curium and americium (1944), representing elements 93 to 96. The first two of these were eventually found in nature as well. Curium and americium were by-products of the Manhattan project, which produced the world's first atomic bomb in 1945. The bomb was based on the nuclear fission of uranium, a metal first thought to have been discovered nearly 150 years earlier.

===Post-World War II developments===

====Superalloys====
{{Main|Superalloy}}

[[File:ریخته گري دقیق سوپرآلیاژهای نیکل.jpg|thumb|Heat treating superalloy turbine blades]]
Superalloys composed of combinations of Fe, Ni, Co, and Cr, and lesser amounts of W, Mo, Ta, Nb, Ti, and Al were developed shortly after World War II for use in high performance engines, operating at elevated temperatures (above 650&nbsp;°C (1,200&nbsp;°F)). They retain most of their strength under these conditions, for prolonged periods, and combine good low-temperature ductility with resistance to corrosion or oxidation. Superalloys can now be found in a wide range of applications including land, maritime, and aerospace turbines, and chemical and petroleum plants.

====Transcurium metals====
The successful development of the atomic bomb at the end of World War II sparked further efforts to synthesize new elements, nearly all of which are, or are expected to be, metals, and all of which are radioactive. It was not until 1949 that element 97 ([[Berkelium]]), next after element 96 ([[Curium]]), was synthesized by firing alpha particles at an americium target. In 1952, element 100 ([[Fermium]]) was found in the debris of the first hydrogen bomb explosion; hydrogen, a nonmetal, had been identified as an element nearly 200 years earlier. Since 1952, elements 101 ([[Mendelevium]]) to 118 ([[Oganesson]]) have been synthesized.

====Bulk metallic glasses====
{{Main|Metallic Glass}}
[[File:Metalic Glas Vitreloy4.jpg|thumb|Metallic Glass Vitreloy4]]

A metallic glass (also known as an amorphous or glassy metal) is a solid metallic material, usually an alloy, with a disordered atomic-scale structure. Most pure and alloyed metals, in their solid state, have atoms arranged in a highly ordered crystalline structure. In contrast these have a non-crystalline glass-like structure. But unlike common glasses, such as window glass, which are typically electrical insulators, amorphous metals have good electrical conductivity. Amorphous metals are produced in several ways, including extremely rapid cooling, physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying. The first reported metallic glass was an alloy (Au<sub>75</sub>Si<sub>25</sub>) produced at [[Caltech]] in 1960. More recently, batches of amorphous steel with three times the strength of conventional steel alloys have been produced. Currently, the most important applications rely on the special magnetic properties of some ferromagnetic metallic glasses. The low magnetization loss is used in high-efficiency transformers. Theft control ID tags and other article surveillance schemes often use metallic glasses because of these magnetic properties.

====Shape-memory alloys====
{{Main|Shape-memory alloy}}
A shape-memory alloy (SMA) is an alloy that "remembers" its original shape and when deformed returns to its pre-deformed shape when heated. While the shape memory effect had been first observed in 1932, in an Au-Cd alloy, it was not until 1962, with the accidental discovery of the effect in a Ni-Ti alloy that research began in earnest, and another ten years before commercial applications emerged. SMA's have applications in robotics and automotive, aerospace, and biomedical industries. There is another type of SMA, called a ferromagnetic shape-memory alloy (FSMA), that changes shape under strong magnetic fields. These materials are of interest as the magnetic response tends to be faster and more efficient than temperature-induced responses.

====Quasicrystalline alloys====
{{Main|Quasicrystal}}
[[File:Ho-Mg-ZnQuasicrystal.jpg|thumb|A Ho-Mg-Zn icosahedral quasicrystal formed as a pentagonal [[dodecahedron]], the [[dual polyhedron|dual]] of the [[icosahedron]]|alt=A metallic regular dodecahedron]]
In 1984, Israeli metallurgist [[Dan Shechtman]] found an aluminium-manganese alloy having five-fold symmetry, in breach of crystallographic convention at the time which said that crystalline structures could only have two-, three-, four-, or six-fold symmetry.<ref name="s">{{cite journal |last1=Shechtman |first1=D. |last2=Blech |first2=I. |last3=Gratias |first3=D. |last4=Cahn |first4=J. |year=1984 |title=Metallic Phase with Long-Range Orientational Order and No Translational Symmetry |journal=Physical Review Letters |volume=53 |issue=20 |pages=1951–1953 |bibcode=1984PhRvL..53.1951S |doi=10.1103/PhysRevLett.53.1951 |doi-access=free}}</ref> Due to reservation about the scientific community's reaction, it took Shechtman two years to publish the results&nbsp;for which he was awarded the Nobel Prize in Chemistry in 2011.<ref name="bloomberg">{{cite news |last=Gerlin |first=Andrea |date=October 5, 2011 |title=Tecnion's Shechtman Wins Nobel in Chemistry for Quasicrystals Discovery |url=https://www.bloomberg.com/news/2011-10-05/technion-s-shechtman-wins-chemistry-nobel-for-discovery-of-quasicrystals.html |archive-url=https://web.archive.org/web/20141205014945/http://www.bloomberg.com/news/2011-10-05/technion-s-shechtman-wins-chemistry-nobel-for-discovery-of-quasicrystals.html |archive-date=December 5, 2014 |access-date=January 4, 2019 |website=[[Bloomberg News|Bloomberg]]}}</ref> Since this time, hundreds of quasicrystals have been reported and confirmed. They exist in many metallic alloys (and some polymers). Quasicrystals are found most often in aluminium alloys (Al-Li-Cu, Al-Mn-Si, Al-Ni-Co, Al-Pd-Mn, Al-Cu-Fe, Al-Cu-V, etc.), but numerous other compositions are also known (Cd-Yb, Ti-Zr-Ni, Zn-Mg-Ho, Zn-Mg-Sc, In-Ag-Yb, Pd-U-Si, etc.).<ref name=":0">{{Cite journal |last=Dubois |first=Jean-Marie |date=2012-09-24 |title=Properties- and applications of quasicrystals and complex metallic alloys |url=https://pubs.rsc.org/en/content/articlelanding/2012/cs/c2cs35110b |journal=Chemical Society Reviews |language=en |volume=41 |issue=20 |pages=6760–6777 |doi=10.1039/C2CS35110B |issn=1460-4744}}</ref> [[Quasicrystal]]s effectively have infinitely large unit cells. [[Icosahedrite]] Al<sub>63</sub>Cu<sub>24</sub>Fe<sub>13</sub>, the first quasicrystal found in nature, was discovered in 2009. Most quasicrystals have ceramic-like properties including low electrical conductivity (approaching values seen in insulators) and low thermal conductivity, high hardness, brittleness, and resistance to corrosion, and non-stick properties. Quasicrystals have been used to develop heat insulation,&nbsp;LEDs, diesel engines, and new materials that convert heat to electricity. New applications may take advantage of the low coefficient of friction and the hardness of some quasicrystalline materials, for example embedding particles in plastic to make strong, hard-wearing, low-friction plastic gears. Other potential applications include selective solar absorbers for power conversion, broad-wavelength reflectors, and bone repair and prostheses applications where biocompatibility, low friction, and corrosion resistance are required.<ref name=":0" />

====Complex metallic alloys====
{{Main|Complex metallic alloy}}
Complex metallic alloys (CMAs) are intermetallic compounds characterized by large unit cells comprising some tens up to thousands of atoms; the presence of well-defined clusters of atoms (frequently with icosahedral symmetry); and partial disorder within their crystalline lattices. They are composed of two or more metallic elements, sometimes with metalloids or [[chalcogenides]] added. They include, for example, NaCd2, with 348 sodium atoms and 768 cadmium atoms in the unit cell. [[Linus Pauling]] attempted to describe the structure of NaCd<sub>2</sub> in 1923,<ref>{{cite journal |last=Pauling |first=Linus |author-link=Linus Pauling |year=1923 |title=The Crystal Structure of Magnesium Stannide |journal=Journal of the American Chemical Society |publisher=American Chemical Society (ACS) |volume=45 |issue=12 |pages=2777–2780 |doi=10.1021/ja01665a001 |issn=0002-7863}}</ref> but did not succeed until 1955.<ref>{{cite journal |last=Pauling |first=Linus |author-link=Linus Pauling |year=1955 |title=The Stochastic Method and the Structure of Proteins |journal=American Scientist |volume=43 |issue=2 |pages=285–297 |jstor=27826614}}</ref> Potential applications of CMAs include as heat insulation; solar heating; magnetic refrigerators; using waste heat to generate electricity; and coatings for turbine blades in military engines.<ref name=":0" />

====High-entropy alloys====
{{Main|High-entropy alloy}}
High entropy alloys (HEAs) such as AlLiMgScTi are composed of equal or nearly equal quantities of five or more metals. The term "high-entropy alloys" was coined by [[Taiwan|Taiwanese]] scientist [[Jein-Wei Yeh|Jien-Wei Yeh]]<ref>{{Cite journal |last1=Yeh |first1=J.-W. |last2=Chen |first2=S.-K. |last3=Lin |first3=S.-J. |last4=Gan |first4=J.-Y. |last5=Chin |first5=T.-S. |last6=Shun |first6=T.-T. |last7=Tsau |first7=C.-H. |last8=Chang |first8=S.-Y. |date=May 2004 |title=Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes |url=http://dx.doi.org/10.1002/adem.200300567 |journal=Advanced Engineering Materials |volume=6 |issue=5 |pages=299–303 |doi=10.1002/adem.200300567 |issn=1438-1656 |s2cid=137380231}}</ref> because the [[Entropy of mixing|entropy increase of mixing]] is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal.<ref>{{cite journal |last1=Ye |first1=Y.F. |last2=Wang |first2=Q. |last3=Lu |first3=J. |last4=Liu |first4=C.T. |last5=Yang |first5=Y. |date=July 2016 |title=High-entropy alloy: challenges and prospects |journal=Materials Today |volume=19 |issue=6 |pages=349–362 |doi=10.1016/j.mattod.2015.11.026 |doi-access=free}}</ref> Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also used in the literature.<ref>{{Cite journal |last1=Miracle |first1=D.B. |last2=Senkov |first2=O.N. |date=January 2017 |title=A critical review of high entropy alloys and related concepts |journal=Acta Materialia |volume=122 |pages=448–511 |bibcode=2017AcMat.122..448M |doi=10.1016/j.actamat.2016.08.081 |issn=1359-6454 |doi-access=free}}</ref><ref name=":9">{{Cite journal |last1=George |first1=Easo P. |last2=Raabe |first2=Dierk |last3=Ritchie |first3=Robert O. |date=2019-06-18 |title=High-entropy alloys |url=http://dx.doi.org/10.1038/s41578-019-0121-4 |journal=Nature Reviews Materials |volume=4 |issue=8 |pages=515–534 |bibcode=2019NatRM...4..515G |doi=10.1038/s41578-019-0121-4 |issn=2058-8437 |osti=1550755 |s2cid=196206754}}</ref> These alloys are currently the focus of significant attention in [[materials science]] and engineering because they have potentially desirable properties.<ref name="tsai">{{cite journal |last1=Tsai |first1=Ming-Hung |last2=Yeh |first2=Jien-Wei |date=30 April 2014 |title=High-Entropy Alloys: A Critical Review |journal=Materials Research Letters |volume=2 |issue=3 |pages=107–123 |doi=10.1080/21663831.2014.912690 |doi-access=free}}</ref> Furthermore, research indicates that some HEAs have considerably better [[Strength-to-weight ratio|strength-to-weight ratios]], with a higher degree of [[Fracture mechanics|fracture resistance]], [[tensile strength]], and [[corrosion]] and [[oxidation]] resistance than conventional alloys.<ref>{{Cite journal |last1=Raabe |first1=Dierk |last2=Tasan |first2=Cemal Cem |last3=Springer |first3=Hauke |last4=Bausch |first4=Michael |date=2015-07-21 |title=From High-Entropy Alloys to High-Entropy Steels |url=http://dx.doi.org/10.1002/srin.201500133 |journal=Steel Research International |volume=86 |issue=10 |pages=1127–1138 |doi=10.1002/srin.201500133 |issn=1611-3683 |s2cid=53702488}}</ref><ref>{{Cite journal |last1=Gludovatz |first1=Bernd |last2=Hohenwarter |first2=Anton |last3=Catoor |first3=Dhiraj |last4=Chang |first4=Edwin H. |last5=George |first5=Easo P. |last6=Ritchie |first6=Robert O. |date=2014-09-05 |title=A fracture-resistant high-entropy alloy for cryogenic applications |url=https://www.science.org/doi/10.1126/science.1254581 |journal=Science |language=en |volume=345 |issue=6201 |pages=1153–1158 |bibcode=2014Sci...345.1153G |doi=10.1126/science.1254581 |issn=0036-8075 |pmid=25190791 |s2cid=1851195}}</ref><ref>{{Cite journal |last1=Li |first1=Zezhou |last2=Zhao |first2=Shiteng |last3=Ritchie |first3=Robert O. |last4=Meyers |first4=Marc A. |date=2019-05-01 |title=Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys |url=https://www.sciencedirect.com/science/article/pii/S0079642518301178 |journal=Progress in Materials Science |language=en |volume=102 |pages=296–345 |doi=10.1016/j.pmatsci.2018.12.003 |issn=0079-6425 |osti=1634203 |s2cid=140083145}}</ref> Although HEAs have been studied since the 1980s, research substantially accelerated starting in the 2010s.<ref name="tsai" /><ref name="lavine">{{cite journal |last1=Lavine |first1=M. S. |date=4 September 2014 |title=A metal alloy that is stronger when cold |journal=Science |volume=345 |issue=6201 |pages=1131 |bibcode=2014Sci...345Q1131L |doi=10.1126/science.345.6201.1131-b}}</ref>
====MAX phase====
{| class="wikitable plainrowheaders" style="float:right; margin-left:2em; margin-top:-2em;"
|+ MAX phase<br />alloy examples
!scope="col"| MAX
!scope="col"| M
!scope="col"| A
!scope="col"| X
|-
!scope="row"| Hf<sub>2</sub>SnC
| Hf || Sn || C
|-
!scope="row"| Ti<sub>4</sub>AlN<sub>3</sub>
| Ti || Al || N
|-
!scope="row"| Ti<sub>3</sub>SiC<sub>2</sub>
| Ti || Si || C
|-
!scope="row"| Ti<sub>2</sub>AlC
| Ti || Al || C
|-
!scope="row"| Cr<sub>2</sub>AlC<sub>2</sub>
| Cr || Al || C
|-
!scope="row"| Ti<sub>3</sub>AlC<sub>2</sub>
| Ti || Al || C
|}
{{Main|MAX phases}}
In a Max phase, '''M''' is an early transition metal, '''A''' is an A group element (mostly group IIIA and IVA, or groups 13 and 14), and '''X''' is either carbon or nitrogen. Examples are Hf<sub>2</sub>SnC and Ti<sub>4</sub>AlN<sub>3</sub>. Such alloys have high electrical and thermal conductivity, thermal shock resistance, damage tolerance, machinability, high elastic stiffness, and low thermal expansion coefficients.<ref>{{Cite journal |last1=Hanaor |first1=D.A.H. |last2=Hu |first2=L. |last3=Kan |first3=W.H. |last4=Proust |first4=G. |last5=Foley |first5=M. |last6=Karaman |first6=I. |last7=Radovic |first7=M. |date=2016 |title=Compressive performance and crack propagation in Al alloy/Ti2AlC composites |url=https://linkinghub.elsevier.com/retrieve/pii/S0921509316307419 |journal=Materials Science and Engineering: A |language=en |volume=672 |pages=247–256 |doi=10.1016/j.msea.2016.06.073|arxiv=1908.08757 }}</ref> They can be polished to a metallic luster because of their excellent electrical conductivities. Some MAX phases are also highly resistant to chemical attack (e.g. Ti<sub>3</sub>SiC<sub>2</sub>) and high-temperature oxidation in air (Ti<sub>2</sub>AlC, Cr<sub>2</sub>AlC<sub>2</sub>, and Ti<sub>3</sub>AlC<sub>2</sub>). Potential applications for MAX phase alloys include: as tough, machinable, thermal shock-resistant refractories; high-temperature heating elements; coatings for electrical contacts; and neutron irradiation resistant parts for nuclear applications.<ref>{{Cite journal |last=Gonzalez-Julian |first=Jesus |date=2021 |title=Processing of MAX phases: From synthesis to applications |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jace.17544 |journal=Journal of the American Ceramic Society |language=en |volume=104 |issue=2 |pages=659–690 |doi=10.1111/jace.17544 |issn=1551-2916}}</ref><ref>{{Cite journal |last=Dahlqvist |first=Martin |last2=Barsoum |first2=Michel W. |last3=Rosen |first3=Johanna |date=2024-01-01 |title=MAX phases – Past, present, and future |url=https://www.sciencedirect.com/science/article/pii/S1369702123003577 |journal=Materials Today |volume=72 |pages=1–24 |doi=10.1016/j.mattod.2023.11.010 |issn=1369-7021}}</ref>

==See also==
{{Div col|colwidth=22em}}
* [[Bimetal]]
* [[Colored gold]]
* [[Ductility]]
* [[Ferrous metallurgy]]
* [[Metal theft]]
* [[Metal toxicity]]
* [[Metallic hydrogen]]
* [[Metallurgy]]
* [[Metals of antiquity]] 
* [[Metalworking]]
* [[Mineral (nutrient)]]
* [[Polymetallic ore]]
* [[Properties of metals, metalloids, and nonmetals]]
* [[Structural steel]]
* [[Transition metal]]
{{Div col end}}

==Note==
{{Reflist|group=n}}

==References==
{{Reflist}}

==Further reading==
* {{anchor|{{harvid|Choptuik|Lehner|Pretorias|2015}}}}[[Matthew Choptuik|Choptuik M. W.]], Lehner L. & Pretorias F. 2015, "Probing strong-field gravity through numerical simulation", in [[Abhay Ashtekar|A. Ashtekar]], [[Beverly Berger|B. K. Berger]], J. Isenberg & M. MacCallum (eds), ''General Relativity and Gravitation: A Centennial Perspective'', Cambridge University Press, Cambridge, {{ISBN|978-1-107-03731-1}}.
* {{cite book |last=Cox |first=P. A. |date=1997 |title=The elements: Their origin, abundance and distribution |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-855298-7}}
* Crow J. M. 2016, "[https://www.newscientist.com/article/mg23230950-500-a-mixedup-recipe-is-redefining-metals-into-impossible-alloys/ Impossible alloys: How to make never-before-seen metals]", ''New Scientist,'' 12 October
* {{anchor|{{harvid|Hadhazy|2016}}}}Hadhazy A. 2016, "[https://kavlifoundation.org/news/cosmic-heavy-metals Galactic 'Gold Mine' Explains the Origin of Nature's Heaviest Elements]", ''Science Spotlights'', 10 May 2016, accessed 11 July 2016.
* {{anchor|{{harvid|Hofmann|2002}}}}Hofmann S. 2002, ''On Beyond Uranium: Journey to the End of the Periodic Table'', [[Taylor & Francis]], London, {{ISBN|978-0-415-28495-0}}.
* {{anchor|{{harvid|Padmanabhan|2001}}}}[[Thanu Padmanabhan|Padmanabhan T.]] 2001, ''Theoretical Astrophysics'', vol. 2, Stars and Stellar Systems, [[Cambridge University Press]], Cambridge, {{ISBN|978-0-521-56241-6}}.
* Parish R. V. 1977, ''The metallic elements,'' Longman, London, {{ISBN|978-0-582-44278-8}}
* {{anchor|{{harvid|Podosek|2011}}}}Podosek F. A. 2011, "Noble gases", in H. D. Holland & [[Karl Turekian|K. K. Turekian]] (eds), ''Isotope Geochemistry: From the Treatise on Geochemistry'', Elsevier, Amsterdam, pp.&nbsp;467–492, {{ISBN|978-0-08-096710-3}}.
* Raymond R. 1984, ''Out of the fiery furnace: The impact of metals on the history of mankind,'' Macmillan Australia, Melbourne, {{ISBN|978-0-333-38024-6}}
* {{anchor|{{harvid|Rehder|2010}}}}Rehder D. 2010, ''Chemistry in Space: From Interstellar Matter to the Origin of Life'', Wiley-VCH, Weinheim, {{ISBN|978-3-527-32689-1}}.
* Russell A. M. & Lee K. L. 2005, ''Structure–property relations in nonferrous metals,'' John Wiley & Sons, Hoboken, New Jersey, {{ISBN|978-0-471-64952-6}}
* Street A. & Alexander W. 1998, ''Metals in the service of man,'' 11<sub>th</sub> ed., Penguin Books, London, {{ISBN|978-0-14-025776-2}}
* Wilson A. J. 1994, ''The living rock: The story of metals since earliest times and their impact on developing civilization'', Woodhead Publishing, Cambridge, {{ISBN|978-1-85573-154-7}}

==External links==
{{Wikiquote}}{{Wiktionary|metal}}{{AmCyc poster|Metal}}
* {{official website|https://www.asminternational.org/}} of ASM International (formerly the American Society for Metals)
* {{official website|https://www.tms.org/}} of The Minerals, Metals & Materials Society

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{{Periodic table (navbox)}}

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