Editing Metalloid (section)

===Astatine===
{{Main|Astatine}}
As a [[halogen]], astatine tends to be classified as a nonmetal.<ref>[[#Hawkes2010|Hawkes 2010]]; [[#Holt2007|Holt, Rinehart & Wilson c. 2007]]; [[#Hawkes1999|Hawkes 1999, p.&nbsp;14]]; [[#Roza2009|Roza 2009, p.&nbsp;12]]</ref> It has some marked metallic properties<ref>[[#Keller1985|Keller 1985]]</ref> and is sometimes instead classified as either a metalloid<ref>[[#Harding2002|Harding, Johnson & Janes 2002, p.&nbsp;61]]</ref> or (less often) as a metal.{{refn|1=A further option is to include astatine both as a nonmetal and as a metalloid.<ref>[[#Long1986|Long & Hentz 1986, p.&nbsp;58]]</ref>|group=n}} Immediately following its production in 1940, early investigators considered it a metal.<ref>[[#Vasáros1985|Vasáros & Berei 1985, p.&nbsp;109]]</ref> In 1949 it was called the most noble (difficult to [[redox|reduce]]) nonmetal as well as being a relatively noble (difficult to oxidize) metal.<ref>[[#Haissinsky1949|Haissinsky & Coche 1949, p.&nbsp;400]]</ref> In 1950 astatine was described as a halogen and (therefore) a [[reactivity (chemistry)|reactive]] nonmetal.<ref>[[#Brownlee1950|Brownlee et al. 1950, p.&nbsp;173]]</ref> In 2013, on the basis of [[relativistic quantum chemistry|relativistic]] modelling, astatine was predicted to be a monatomic metal, with a [[Face-centred cubic|face-centred cubic crystalline structure]].<ref>[[#Hermann|Hermann, Hoffmann & Ashcroft 2013]]</ref>

Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be a metal if it could form a condensed phase.<ref>[[#Siekierski2002|Siekierski & Burgess 2002, pp.&nbsp;65, 122]]</ref>{{refn|1=A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.<ref>[[#Emsley2001|Emsley 2001, p.&nbsp;48]]</ref>|group=n}} Astatine may be metallic in the liquid state on the basis that elements with an [[enthalpy of vaporization]] (∆H<sub>vap</sub>) greater than ~42 kJ/mol are metallic when liquid.<ref name="Rao & Ganguly 1986">[[#Rao1986|Rao & Ganguly 1986]]</ref> Such elements include boron,{{refn|1=The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.<ref>[[#Krishnan1998|Krishnan et al. 1998]]</ref> found that liquid boron behaved like a metal. Glorieux et al.<ref>[[#Glorieux2001|Glorieux, Saboungi & Enderby 2001]]</ref> characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity. Millot et al.<ref>[[#Millot2002|Millot et al. 2002]]</ref> reported that the emissivity of liquid boron was not consistent with that of a liquid metal.|group=n}} silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆H<sub>vap</sub> of [[Diatomic molecule|diatomic]] astatine are 50 kJ/mol or higher;<ref>[[#Vasáros1985|Vasáros & Berei 1985, p.&nbsp;117]]</ref> diatomic iodine, with a ∆H<sub>vap</sub> of 41.71,<ref>[[#Kaye1973|Kaye & Laby 1973, p.&nbsp;228]]</ref> falls just short of the threshold figure.

"Like typical metals, it [astatine] is precipitated by [[hydrogen sulfide]] even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the [[cathode]] on [[electrolysis]]."<ref>[[#Samsonov1968|Samsonov 1968, p.&nbsp;590]]</ref>{{refn|1=Korenman<ref>[[#Korenman1959|Korenman 1959, p.&nbsp;1368]]</ref> similarly noted that "the ability to precipitate with hydrogen sulfide distinguishes astatine from other halogens and brings it closer to bismuth and other [[heavy metal (chemistry)|heavy metals]]".|group=n}} Further indications of a tendency for astatine to behave like a [[heavy metal (chemistry)|(heavy) metal]] are: "...&nbsp;the formation of [[pseudohalide]] compounds&nbsp;... complexes of astatine cations&nbsp;... complex anions of trivalent astatine&nbsp;... as well as complexes with a variety of organic solvents".<ref>[[#Rossler1985|Rossler 1985, pp.&nbsp;143–44]]</ref> It has also been argued that astatine demonstrates cationic behaviour, by way of stable At<sup>+</sup> and AtO<sup>+</sup> forms, in strongly acidic aqueous solutions.<ref>[[#Champion2010|Champion et al. 2010]]</ref>

Some of astatine's reported properties are nonmetallic. It has been extrapolated to have the narrow liquid range ordinarily associated with nonmetals (mp 302&nbsp;°C; bp 337&nbsp;°C),<ref>[[#Borst1982|Borst 1982, pp.&nbsp;465, 473]]</ref> although experimental indications suggest a lower boiling point of about 230±3&nbsp;°C. Batsanov gives a calculated band gap energy for astatine of 0.7&nbsp;eV;<ref>[[#Batsanov1971|Batsanov 1971, p.&nbsp;811]]</ref> this is consistent with nonmetals (in physics) having separated [[valence band|valence]] and [[conduction band]]s and thereby being either semiconductors or insulators.<ref>[[#Swalin1962|Swalin 1962, p.&nbsp;216]]; [[#Feng2005|Feng & Lin 2005, p.&nbsp;157]]</ref> The chemistry of astatine in aqueous solution is mainly characterised by the formation of various anionic species.<ref>[[#Schwietzer2010|Schwietzer & Pesterfield 2010, pp.&nbsp;258–60]]</ref> Most of its known compounds resemble those of iodine,<ref>[[#Hawkes1999|Hawkes 1999, p.&nbsp;14]]</ref> which is a halogen and a nonmetal.<ref>[[#Olmsted1997|Olmsted & Williams 1997, p.&nbsp;328]]; [[#Daintith2004|Daintith 2004, p.&nbsp;277]]</ref> Such compounds include astatides (XAt), astatates (XAtO<sub>3</sub>), and [[monovalent ion|monovalent]] [[interhalogen compound]]s.<ref>[[#Eberle1985|Eberle1985, pp.&nbsp;213–16, 222–27]]</ref>

Restrepo et al.<ref>[[#Restrepo2004|Restrepo et al. 2004, p.&nbsp;69]]; [[#Restrepo2006|Restrepo et al. 2006, p.&nbsp;411]]</ref> reported that astatine appeared to be more polonium-like than halogen-like. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.
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