Editing Aluminium (section)

== Physical characteristics ==
=== Isotopes ===
{{Main|Isotopes of aluminium}}
Of aluminium isotopes, only {{SimpleNuclide|Aluminium}} is stable. This situation is common for elements with an odd atomic number.{{efn|No elements with odd atomic numbers have more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having the highest number of stable isotopes of all elements, ten. The single exception is [[beryllium]] which is even-numbered but has only one stable isotope.<ref name="IAEA" /> See [[Even and odd atomic nuclei]] for more details.}} It is the only [[primordial nuclide|primordial]] aluminium isotope, i.e. the only one that has existed on Earth in its current form since the formation of the planet. It is therefore a [[mononuclidic element]] and its [[standard atomic weight]] is virtually the same as that of the isotope. This makes aluminium very useful in [[nuclear magnetic resonance]] (NMR), as its single stable isotope has a high NMR sensitivity.{{sfn|Greenwood|Earnshaw|1997|pp=242–252}} The standard atomic weight of aluminium is low in comparison with many other metals.{{efn|Most other metals have greater standard atomic weights: for instance, that of iron is {{val|55.845}}; copper {{val|63.546}}; lead {{val|207.2}}.{{CIAAW2021}} which has consequences for the element's properties (see [[#Bulk|below]])}}

All other isotopes of aluminium are [[radioactive decay|radioactive]]. The most stable of these is [[Aluminium-26|<sup>26</sup>Al]]: while it was present along with stable <sup>27</sup>Al in the interstellar medium from which the Solar System formed, having been produced by [[stellar nucleosynthesis]] as well, its [[half-life]] is only 717,000&nbsp;years and therefore a detectable amount has not survived since the formation of the planet.<ref name="CIAAWaluminium"/> However, minute traces of <sup>26</sup>Al are produced from [[argon]] in the [[Earth's atmosphere|atmosphere]] by [[spallation]] caused by [[cosmic ray]] protons. The ratio of <sup>26</sup>Al to [[beryllium-10|<sup>10</sup>Be]] has been used for [[Radiometric dating|radiodating]] of geological processes over 10<sup>5</sup> to 10<sup>6</sup>&nbsp;year time scales, in particular transport, deposition, [[sediment]] storage, burial times, and erosion.<ref>{{cite book
|chapter-url=http://www.onafarawayday.com/Radiogenic/Ch14/Ch14-6.htm
|title=Radiogenic Isotope Geology
|last1=Dickin|first1=A.P.|date=2005
|publisher=Cambridge University Press|isbn=978-0-521-53017-0|chapter=''In situ'' Cosmogenic Isotopes
|archive-url=https://web.archive.org/web/20081206010805/http://www.onafarawayday.com/Radiogenic/Ch14/Ch14-6.htm|archive-date=6 December 2008|url-status=dead
|access-date=16 July 2008}}
</ref> Most meteorite scientists believe that the energy released by the decay of <sup>26</sup>Al was responsible for the melting and [[planetary differentiation|differentiation]] of some [[asteroids]] after their formation 4.55&nbsp;billion years ago.<ref>{{cite book
|title=Thunderstones and Shooting Stars
|url=https://archive.org/details/thunderstonessho00dodd_673|url-access=limited
|last1=Dodd|first1=R.T.|date=1986
|publisher=Harvard University Press|isbn=978-0-674-89137-1|pages=[https://archive.org/details/thunderstonessho00dodd_673/page/n99 89]–90}}</ref>

The remaining isotopes of aluminium, with [[mass number]]s ranging from 21 to 43, all have half-lives well under an hour. Three [[metastable]] states are known, all with half-lives under a minute.<ref name="IAEA">{{cite web
|url=https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
|title=Livechart – Table of Nuclides – Nuclear structure and decay data
|author=IAEA – Nuclear Data Section|year=2017|website=www-nds.iaea.org|publisher=[[International Atomic Energy Agency]]|access-date=31 March 2017
|archive-date=23 March 2019|archive-url=https://web.archive.org/web/20190323230752/https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html|url-status=live}}</ref>

=== Electron shell ===

An aluminium atom has 13 electrons, arranged in an [[electron configuration]] of {{nowrap|{{bracket|[[Neon|Ne]]}} 3s<sup>2</sup> 3p<sup>1</sup>}},{{sfn|Dean|1999|p=4.2}} with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three [[ionization energy|ionization energies]] of aluminium are far lower than the fourth ionization energy alone.{{sfn|Dean|1999|p=4.6}} Such an electron configuration is shared with the other well-characterized members of its group, [[boron]], [[gallium]], [[indium]], and [[thallium]]; it is also expected for [[nihonium]]. Aluminium can surrender its three outermost electrons in many chemical reactions (see [[#Chemistry|below]]). The [[electronegativity]] of aluminium is 1.61 (Pauling scale).{{sfn|Dean|1999|p=4.29}}
[[File:Aluminium Atomic lattice.png|alt=M. Tunes & S. Pogatscher, Montanuniversität Leoben 2019 No copyrights =)|left|thumb|upright=1.2|High-resolution [[Scanning transmission electron microscopy|STEM]]-[[Annular dark-field imaging|HAADF]] micrograph of Al atoms viewed along the [001] zone axis.]]
A free aluminium atom has a [[atomic radius|radius]] of 143&nbsp;[[picometer|pm]].{{sfn|Dean|1999|p=4.30}} With the three outermost electrons removed, the [[ionic radius|radius]] shrinks to 39&nbsp;pm for a 4-coordinated atom or 53.5&nbsp;pm for a 6-coordinated atom.{{sfn|Dean|1999|p=4.30}} At [[standard temperature and pressure]], aluminium atoms (when not affected by atoms of other elements) form a [[Cubic crystal system|face-centered cubic crystal system]] bound by [[metallic bonding]] provided by atoms' outermost electrons; hence aluminium (at these conditions) is a metal.<ref name="Enghag2008" /> This crystal system is shared by many other metals, such as [[lead]] and [[copper]]; the size of a unit cell of aluminium is comparable to that of those other metals.<ref name="Enghag2008">{{cite book
|last=Enghag|first=Per|title=Encyclopedia of the Elements: Technical Data – History – Processing – Applications
|url=https://books.google.com/books?id=fUmTX8yKU4gC|date=2008
|publisher=John Wiley & Sons|isbn=978-3-527-61234-5|pages=139, 819, 949|access-date=7 December 2017
|archive-date=25 December 2019|archive-url=https://web.archive.org/web/20191225132056/https://books.google.com/books?id=fUmTX8yKU4gC|url-status=live}}
</ref> The system, however, is not shared by the other members of its group: boron has ionization energies too high to allow metallization, thallium has a [[hexagonal close-packed]] structure, and gallium and indium have unusual structures that are not close-packed like those of aluminium and thallium. The few electrons that are available for [[metallic bonding]] in aluminium are a probable cause for it being soft with a low melting point and low [[electrical resistivity]].{{sfn|Greenwood|Earnshaw|1997|pp= 222–224}}

=== Bulk ===
[[File:Lingot aluminium.jpg|thumb|left|Aluminium ingot from furnace]]

Aluminium metal has an appearance ranging from silvery white to dull gray depending on its [[surface roughness]].{{efn|The two sides of aluminium foil differ in their luster: one is shiny and the other is dull. The difference is due to the small mechanical damage on the surface of dull side arising from the technological process of aluminium foil manufacturing.<ref name="ReynoldsKitchens">{{Cite web
|title=Heavy Duty Foil
|url=https://www.reynoldskitchens.com/products/aluminum-foil/heavy-duty-foil/|website=Reynolds Kitchens|language=en|access-date=20 September 2020
|archive-date=23 September 2020|archive-url=https://web.archive.org/web/20200923185810/https://www.reynoldskitchens.com/products/aluminum-foil/heavy-duty-foil/ |url-status=live}}</ref> Both sides reflect similar amounts of visible light, but the shiny side reflects a far greater share of visible light [[specular reflection|specularly]] whereas the dull side almost exclusively [[Diffuse reflection|diffuses]] light. Both sides of aluminium foil serve as good [[Reflectance|reflectors]] (approximately 86%) of [[visible light]] and an excellent reflector (as much as 97%) of medium and far [[infrared]] radiation.<ref name="Pozzobon">{{Cite journal
|last1=Pozzobon|first1=V.|last2=Levasseur|first2=W.|last3=Do|first3=Kh.-V.|display-authors=3|last4=Palpant|first4=Bruno|last5=Perré|first5=Patrick|date=2020
|title=Household aluminum foil matte and bright side reflectivity measurements: Application to a photobioreactor light concentrator design
|journal=Biotechnology Reports|language=en|volume=25|pages=e00399|doi=10.1016/j.btre.2019.e00399|pmc=6906702|pmid=31867227 | issn=2215-017X }}</ref>}} Aluminium mirrors provides high reflectivity for light in the [[ultraviolet]], visible (on par with silver),<ref>{{Cite journal |last=Hummel |first=R.E. |date=1981 |title=Reflectivity of silver- and aluminium-based alloys for solar reflectors |url=https://linkinghub.elsevier.com/retrieve/pii/0038092X81900402 |journal=Solar Energy |volume=27 |issue=6 |pages=453 |doi=10.1016/0038-092x(81)90040-2 |bibcode=1981SoEn...27..449H |issn=0038-092X}}</ref> and the far [[infrared]] region.<ref>{{Cite book |last1=Hass |first1=G. |last2=Heaney |first2=J. B. |last3=Hunter |first3=W. R. |title=Physics of Thin Films - Advances in Research and Development |date=1982 |editor-last=Hass |editor-first=Georg |editor2-last=Francombe |editor2-first=Maurice H. |editor3-last=Vossen |editor3-first=John L. |chapter=Reflectance and Preparation of Front Surface Mirrors for Use at Various Angles of Incidence from the Ultraviolet to the Far Infrared |chapter-url=https://linkinghub.elsevier.com/retrieve/pii/S0079197013700082 |publisher=Elsevier |volume=12 |pages=8 |doi=10.1016/s0079-1970(13)70008-2|isbn=978-0-12-533012-1 }}</ref> Aluminium is also good at reflecting [[solar radiation]], although prolonged exposure to sunlight in air can deteriorate the reflectivity of the metal;<ref>{{Cite journal |last=Hummel |first=R. E. |date=1981-01-01 |title=Reflectivity of silver- and aluminium-based alloys for solar reflectors |url=https://linkinghub.elsevier.com/retrieve/pii/0038092X81900402 |journal=Solar Energy |volume=27 |issue=6 |pages=449–455 |doi=10.1016/0038-092X(81)90040-2 |bibcode=1981SoEn...27..449H |issn=0038-092X}}</ref> this may be prevented if aluminium is [[anodization|anodized]], which adds a protective layer of oxide on the surface.{{citation needed|date= March 2025|reason =While may be intuitive, this should have a source supporting this claim}}

The density of aluminium is 2.70&nbsp;g/cm<sup>3</sup>, about 1/3 that of steel, much lower than other commonly encountered metals, making aluminium parts easily identifiable through their lightness.{{sfn|Lide|2004|p=4-3}} Aluminium's low density compared to most other metals arises from the fact that its nuclei are much lighter, while difference in the unit cell size does not compensate for this difference. The only lighter metals are the metals of [[alkali metal|groups 1]] and [[alkaline earth metal|2]], which apart from [[beryllium]] and [[magnesium]] are too reactive for structural use (and beryllium is very toxic).<ref>{{cite journal
|title=A brighter beryllium|date=2011|last1=Puchta|first1=Ralph
|journal=Nature Chemistry|volume=3|issue=5|pages=416|pmid=21505503|bibcode=2011NatCh...3..416P|doi=10.1038/nchem.1033|doi-access=free}}
</ref> Aluminium is not as strong or stiff as steel, but the low density makes up for this in the [[aerospace]] industry and for many other applications where light weight and relatively high strength are crucial.{{sfn|Davis|1999|pp=1–3}}

Pure aluminium is quite soft and lacking in strength. In most applications various [[aluminium alloys]] are used instead because of their higher strength and hardness.{{sfn|Davis|1999|p=2}} The [[yield (engineering)|yield strength]] of pure aluminium is 7–11 [[Pascal (unit)|MPa]], while [[aluminium alloy]]s have yield strengths ranging from 200 MPa to 600 MPa.<ref name="Polmear1995">{{cite book
|last1=Polmear|first1=I.J.|date=1995
|title=Light Alloys: Metallurgy of the Light Metals
|edition=3|publisher=[[Butterworth-Heinemann]]|isbn=978-0-340-63207-9}}
</ref> Aluminium is [[ductility|ductile]], with a percent elongation of 50–70%,<ref name="Cardarelli 2008 p158-163">{{Cite book
|last=Cardarelli|first=François|title=Materials handbook : a concise desktop reference|date=2008
|publisher=Springer|isbn=978-1-84628-669-8|edition=2nd|location=London|pages=158–163|oclc=261324602}}
</ref> and [[malleable]] allowing it to be easily [[drawing (metalworking)|drawn]] and [[extrusion|extruded]].{{sfn|Davis|1999|p=4}} It is also easily [[machining|machined]] and [[casting (metalworking)|cast]].{{sfn|Davis|1999|p=4}}

Aluminium is an excellent [[Heat conduction|thermal]] and [[electrical conductor]], having around 60% the conductivity of [[copper]], both thermal and electrical, while having only 30% of copper's density.{{sfn|Davis|1999|pp=2–3}} Aluminium is capable of [[superconductor|superconductivity]], with a superconducting critical temperature of 1.2 [[kelvin]] and a critical magnetic field of about 100 [[gauss (unit)|gauss]] (10 [[millitesla]]s).<ref>
{{cite journal
 |last1=Cochran |first1=J.F.
 |last2=Mapother |first2=D.E.
 |date=1958
 |title=Superconducting Transition in Aluminum
 |journal=[[Physical Review]]
 |volume=111 |issue=1 |pages=132–142
 |bibcode=1958PhRv..111..132C
 |doi=10.1103/PhysRev.111.132
}}</ref> It is [[paramagnetic]] and thus essentially unaffected by static magnetic fields.{{sfn|Schmitz|2006|p=6}} The high electrical conductivity, however, means that it is strongly affected by alternating magnetic fields through the induction of [[eddy currents]].{{sfn|Schmitz|2006|p=161}}