Editing Metalloid (section)

===Selenium===
{{Main|Selenium}}
[[File:Selenium black (cropped).jpg|thumb|right|upright|Grey [[selenium]], being a [[photoconductor]], conducts electricity around 1,000 times better when light falls on it, a property used since the mid-1870s in various light-sensing applications<ref>[[#Emsley2001|Emsley 2001, p.&nbsp;382]]</ref>|alt=A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.]]
Selenium shows borderline metalloid or nonmetal behaviour.<ref>[[#Young2010|Young et al. 2010, p.&nbsp;9]]; [[#Craig2003|Craig & Maher 2003, p.&nbsp;391]]. Selenium is "near metalloidal".</ref>{{refn|1=[[Eugene G. Rochow|Rochow]],<ref>[[#Rochow1957|Rochow 1957]]</ref> who later wrote his 1966 monograph ''The metalloids'',<ref>[[#Rochow1966|Rochow 1966, p.&nbsp;224]]</ref> commented that, "In some respects selenium acts like a metalloid and tellurium certainly does".|group=n}}

Its most stable form, the grey [[trigonal crystal system|trigonal]] allotrope, is sometimes called "metallic" selenium because its electrical conductivity is several orders of magnitude greater than that of the red [[monoclinic crystal system|monoclinic]] form.<ref>[[#Moss1952|Moss 1952, p.&nbsp;192]]</ref> The metallic character of selenium is further shown by its lustre,<ref name="Glinka 1965, p.356">[[#Glinka1965|Glinka 1965, p.&nbsp;356]]</ref> and its crystalline structure, which is thought to include weakly "metallic" interchain bonding.<ref>[[#Evans1966|Evans 1966, pp.&nbsp;124–25]]</ref> Selenium can be drawn into thin threads when molten and viscous.<ref>[[#Regnault1853|Regnault 1853, p.&nbsp;208]]</ref> It shows reluctance to acquire "the high positive oxidation numbers characteristic of nonmetals".<ref>[[#Scott1962|Scott & Kanda 1962, p.&nbsp;311]]</ref> It can form cyclic polycations (such as Se{{su|b=8|p=2+}}) when dissolved in [[oleum]]s<ref>[[#Cotton1999|Cotton et al. 1999, pp.&nbsp;496, 503–04]]</ref> (an attribute it shares with sulfur and tellurium), and a hydrolysed cationic salt in the form of trihydroxoselenium(IV) perchlorate <span style="white-space: nowrap">[Se(OH)<sub>3</sub>]<sup>+</sup>·ClO{{su|b=4|p=–}}.<ref>[[#Arlman1939|Arlman 1939]]; [[#Bagnall1966|Bagnall 1966, pp.&nbsp;135, 142–43]]</ref></span>

The nonmetallic character of selenium is shown by its brittleness<ref name="Glinka 1965, p.356"/> and the low electrical conductivity (~10<sup>−9</sup> to 10<sup>−12</sup>&nbsp;S•cm<sup>−1</sup>) of its highly purified form.<ref name="Kozyrev">[[#Kozyrev1959|Kozyrev 1959, p.&nbsp;104]]; [[#Chizhikov1968|Chizhikov & Shchastlivyi 1968, p.&nbsp;25]]; [[#Glazov1969|Glazov, Chizhevskaya & Glagoleva 1969, p.&nbsp;86]]</ref> This is comparable to or less than that of [[bromine]] (7.95{{e|–12}}&nbsp;S•cm<sup>−1</sup>),<ref>[[#Chao1964|Chao & Stenger 1964]]</ref> a nonmetal. Selenium has the electronic band structure of a [[semiconductor]]<ref name="Berger 1997, pp.86–7">[[#Berger1997|Berger 1997, pp.&nbsp;86–87]]</ref> and retains its semiconducting properties in liquid form.<ref name="Berger 1997, pp.86–7"/> It has a relatively high<ref>[[#Snyder1966|Snyder 1966, p.&nbsp;242]]</ref> electronegativity (2.55 revised Pauling scale). Its reaction chemistry is mainly that of its nonmetallic anionic forms Se<sup>2−</sup>, SeO{{su|b=3|p=2−}} and SeO{{su|b=4|p=2−}}.<ref>[[#Fritz2008|Fritz & Gjerde 2008, p.&nbsp;235]]</ref>

Selenium is commonly described as a metalloid in the [[environmental chemistry]] literature.<ref>[[#Meyer2005|Meyer et al. 2005, p.&nbsp;284]]; [[#Manahan|Manahan 2001, p.&nbsp;911]]; [[#Szpunar|Szpunar et al. 2004, p.&nbsp;17]]</ref> It moves through the aquatic environment similarly to arsenic and antimony;<ref>[[#USEPA1988|US Environmental Protection Agency 1988, p.&nbsp;1]]; [[#Uden|Uden 2005, pp.&nbsp;347‒48]]</ref> its water-soluble salts, in higher concentrations, have a similar [[toxicology|toxicological profile]] to that of arsenic.<ref>[[#DeZuane|De Zuane 1997, p.&nbsp;93]]; [[#Dev|Dev 2008, pp.&nbsp;2‒3]]</ref>