Editing Aluminium (section)

=== 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}}