Editing Metalloid (section)

===Aluminium===
{{Main|Aluminium}}
[[File:Aluminium-4.jpg|thumb|left|High purity [[aluminium]] is much softer than its familiar [[aluminium alloys|alloys]]. People who handle it for the first time often ask if it is the real thing.<ref>[[#Russell2005|Russell & Lee 2005, pp.&nbsp;358–59]]</ref>|alt=A silvery white steam-iron shaped lump with semi-circular striations along the width of its top surface and rough furrows in the middle portion of its left edge.]]
Aluminium is ordinarily classified as a metal.<ref>[[#Keevil|Keevil 1989, p.&nbsp;103]]</ref> It is lustrous, malleable and ductile, and has high electrical and thermal conductivity. Like most metals it has a [[close-packed]] crystalline structure,<ref>[[#Russell2005|Russell & Lee 2005, pp.&nbsp;358–60 et seq]]</ref>  and forms a cation in aqueous solution.<ref>[[#Harding|Harding, Janes & Johnson 2002, p.&nbsp;118]]</ref>

It has some properties that are unusual for a metal; taken together,<ref name="Metcalfe et al. 1974, p.539">[[#Metcalfe1974|Metcalfe, Williams & Castka 1974, p.&nbsp;539]]</ref> these are sometimes used as a basis to classify aluminium as a metalloid.<ref>[[#Cobb2005|Cobb & Fetterolf 2005, p.&nbsp;64]]; [[#Metcalfe1974|Metcalfe, Williams & Castka 1974, p.&nbsp;539]]</ref> Its crystalline structure shows some evidence of [[Bonding in solids#Properties|directional bonding]].<ref>[[#Ogata2002|Ogata, Li & Yip 2002]]; [[#Boyer2004|Boyer et al. 2004, p.&nbsp;1023]]; [[#Russell2005|Russell & Lee 2005, p.&nbsp;359]]</ref> Aluminium bonds covalently in most compounds.<ref>[[#Cooper1968|Cooper 1968, p.&nbsp;25]]; [[#Henderson2000|Henderson 2000, p.&nbsp;5]]; [[#Silberberg2006|Silberberg 2006, p.&nbsp;314]]</ref> The oxide [[aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]] is amphoteric<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;1014]]</ref> and a conditional glass-former.<ref name=Rao22/> Aluminium can form anionic [[aluminate]]s,<ref name="Metcalfe et al. 1974, p.539"/> such behaviour being considered nonmetallic in character.<ref name="Hamm 1969, p.653">[[#Hamm1969|Hamm 1969, p.&nbsp;653]]</ref>

Classifying aluminium as a metalloid has been disputed<ref>[[#Daub1996|Daub & Seese 1996, pp.&nbsp;70, 109]]: "Aluminum is not a metalloid but a metal because it has mostly metallic properties."; [[#Denniston2004|Denniston, Topping & Caret 2004, p.&nbsp;57]]: "Note that aluminum (Al) is classified as a metal, not a metalloid."; [[#Hasan2009|Hasan 2009, p.&nbsp;16]]: "Aluminum does not have the characteristics of a metalloid but rather those of a metal."</ref> given its many metallic properties. It is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal–nonmetal dividing line are metalloids.<ref>[[#Holt2007|Holt, Rinehart & Wilson c. 2007]]</ref>{{refn|1=A mnemonic that captures the elements commonly recognised as metalloids goes: ''Up, up-down, up-down, up&nbsp;... are the metalloids!''<ref>[[#Tuthill2011|Tuthill 2011]]</ref>|group=n}}

Stott<ref>[[#Stott1956|Stott 1956, p.&nbsp;100]]</ref> labels aluminium as a weak metal. It has the physical properties of a metal but some of the chemical properties of a nonmetal. Steele<ref>[[#Steele1966|Steele 1966, p.&nbsp;60]]</ref> notes the paradoxical chemical behaviour of aluminium: "It resembles a weak metal in its amphoteric oxide and in the covalent character of many of its compounds&nbsp;... Yet it is a highly [[electronegativity#Electropositivity|electropositive]] metal&nbsp;... [with] a [[table of standard electrode potentials|high negative]] electrode potential". Moody<ref>[[#Moody|Moody 1991, p.&nbsp;303]]</ref> says that, "aluminium is on the 'diagonal borderland' between metals and non-metals in the chemical sense."