Editing Metalloid (section)

===Allotropes===
[[File:Sn-Alpha-Beta.jpg|thumb|left|[[White tin]] (left) and [[grey tin]] (right). Both forms have a metallic appearance.|alt=Many small, shiny, silver-coloured spheres on the left; many of the same sized spheres on the right are duller and darker than the ones of the left and have a subdued metallic shininess.]]
Different crystalline forms of an element are called [[allotropy|allotropes]]. Some allotropes, particularly those of elements located (in periodic table terms) alongside or near the notional dividing line between metals and nonmetals, exhibit more pronounced metallic, metalloidal or nonmetallic behaviour than others.<ref>[[#Brescia1980|Brescia et al. 1980, pp.&nbsp;166–71]]</ref> The existence of such allotropes can complicate the classification of the elements involved.<ref>[[#Fine|Fine & Beall 1990, p.&nbsp;578]]</ref>

Tin, for example, has two allotropes: [[tetragonal crystal system|tetragonal]] "white" β-tin and cubic "grey" α-tin. White tin is a very shiny, ductile and malleable metal. It is the stable form at or above room temperature and has an electrical conductivity of 9.17 × 10<sup>4</sup>&nbsp;S·cm<sup>−1</sup> (~1/6th that of copper).<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;901]]</ref> Grey tin usually has the appearance of a grey micro-crystalline powder, and can also be prepared in brittle semi-lustrous crystalline or [[polycrystalline]] forms. It is the stable form below 13.2&nbsp;°C and has an electrical conductivity of between (2–5) × 10<sup>2</sup>&nbsp;S·cm<sup>−1</sup> (~1/250th that of white tin).<ref>[[#Berger1997|Berger 1997, p.&nbsp;80]]</ref> Grey tin has the same crystalline structure as that of diamond. It behaves as a semiconductor (as if it had a band gap of 0.08&nbsp;eV), but has the electronic band structure of a semimetal.<ref>[[#Lovett1977|Lovett 1977, p.&nbsp;101]]</ref> It has been referred to as either a very poor metal,<ref>[[#Cohen|Cohen & Chelikowsky 1988, p.&nbsp;99]]</ref> a metalloid,<ref>[[#Taguena|Taguena-Martinez, Barrio & Chambouleyron 1991, p.&nbsp;141]]</ref> a nonmetal<ref>[[#Ebbing|Ebbing & Gammon 2010, p.&nbsp;891]]</ref> or a near metalloid.<ref name=Vernon>[[#Vernon|Vernon 2013, p.&nbsp;1705]]</ref>

The diamond allotrope of carbon is clearly nonmetallic, being translucent and having a low electrical conductivity of 10<sup>−14</sup> to 10<sup>−16</sup>&nbsp;S·cm<sup>−1</sup>.<ref>[[#Asmussen|Asmussen & Reinhard 2002, p.&nbsp;7]]</ref> Graphite has an electrical conductivity of 3 × 10<sup>4</sup>&nbsp;S·cm<sup>−1</sup>,<ref>[[#Deprez1988|Deprez & McLachan 1988]]</ref> a figure more characteristic of a metal. Phosphorus, sulfur, arsenic, selenium, antimony, and bismuth also have less stable allotropes that display different behaviours.<ref>[[#Addison1964|Addison 1964 (P, Se, Sn)]]; [[#Marko1998|Marković, Christiansen & Goldman 1998 (Bi)]]; [[#Nagao2004|Nagao et al. 2004]]</ref>