Editing Metal (section)

===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}}</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}}</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}}</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]].<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>

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>