Editing Metalloid (section)

===Boron===
{{Main|Boron}}
[[File:Boron R105.jpg|thumb|right|Boron, shown here in the form of its β-[[rhombohedral]] phase (its most thermodynamically stable [[allotrope]])<ref>[[#VanSetten2007|Van Setten et al. 2007, pp.&nbsp;2460–61]]; [[#Oganov2009|Oganov et al. 2009, pp.&nbsp;863–64]]</ref>|alt=Several dozen small angular stone like shapes, grey with scattered silver flecks and highlights.]] Pure boron is a shiny, silver-grey crystalline solid.<ref>[[#Housecroft2008|Housecroft & Sharpe 2008, p.&nbsp;331]]; [[#Oganov2010|Oganov 2010, p.&nbsp;212]]</ref> It is less dense than aluminium (2.34 vs. 2.70 g/cm<sup>3</sup>), and is hard and brittle. It is barely reactive under normal conditions, except for attack by [[fluorine]],<ref>[[#Housecroft2008|Housecroft & Sharpe 2008, p.&nbsp;333]]</ref> and has a melting point of 2076&nbsp;°C (cf. steel ~1370&nbsp;°C).<ref>[[#Kross|Kross 2011]]</ref> Boron is a semiconductor;<ref>[[#Berger1997|Berger 1997, p.&nbsp;37]]</ref> its room temperature electrical conductivity is 1.5 × 10<sup>−6</sup> [[Siemens (unit)|S]]•cm<sup>−1</sup><ref>[[#Greenwood2002|Greenwood & Earnshaw 2002, p.&nbsp;144]]</ref> (about 200 times less than that of tap water)<ref>[[#Kopp|Kopp, Lipták & Eren 2003, p.&nbsp;221]]</ref> and it has a band gap of about 1.56&nbsp;eV.<ref>[[#Prudenziati1977|Prudenziati 1977, p.&nbsp;242]]</ref>{{refn|1=Boron, at 1.56 eV, has the largest band gap amongst the commonly recognised (semiconducting) metalloids. Of nearby elements in periodic table terms, selenium has the next highest band gap (close to 1.8 eV) followed by white phosphorus (around 2.1 eV).<ref>[[#Berger1997|Berger 1997, pp.&nbsp;84, 87]]</ref>|group=n}} Mendeleev commented that, "Boron appears in a free state in several forms which are intermediate between the metals and the nonmmetals."<ref>[[#Mendeléeff1897a|Mendeléeff 1897, p.&nbsp;57]]</ref>
The structural chemistry of boron is dominated by its small atomic size, and relatively high ionization energy. With only three valence electrons per boron atom, simple covalent bonding cannot fulfil the octet rule.<ref name="Rayner-Canham 2006, p. 291">[[#Rayner2006|Rayner-Canham & Overton 2006, p.&nbsp;291]]</ref> Metallic bonding is the usual result among the heavier congenors of boron but this generally requires low ionization energies.<ref>[[#Siekierski2002|Siekierski & Burgess 2002, p.&nbsp;63]]</ref> Instead, because of its small size and high ionization energies, the basic structural unit of boron (and nearly all of its allotropes){{refn|1=The synthesis of B<sub>40</sub> [[borospherene]], a "distorted fullerene with a hexagonal hole on the top and bottom and four heptagonal holes around the waist" was announced in 2014.<ref>[[#Wogan|Wogan 2014]]</ref>|group=n}} is the icosahedral B<sub>12</sub> cluster. Of the 36 electrons associated with 12 boron atoms, 26 reside in 13 delocalized molecular orbitals; the other 10 electrons are used to form two- and three-centre covalent bonds between icosahedra.<ref>[[#Siekierski2002|Siekierski & Burgess 2002, p.&nbsp;86]]</ref> The same motif can be seen, as are [[deltahedron|deltahedral]] variants or fragments, in metal borides and hydride derivatives, and in some halides.<ref>[[#Greenwood2002|Greenwood & Earnshaw 2002, p.&nbsp;141]]; [[#Henderson2000|Henderson 2000, p.&nbsp;58]]; [[#Housecroft2008|Housecroft & Sharpe 2008, pp.&nbsp;360–72]]</ref>

The bonding in boron has been described as being characteristic of behaviour intermediate between metals and nonmetallic [[covalent network]] solids (such as [[diamond]]).<ref>[[#Parry1970|Parry et al. 1970, pp.&nbsp;438, 448–51]]</ref> The energy required to transform B, C, N, Si, and P from nonmetallic to metallic states has been estimated as 30, 100, 240, 33, and 50 kJ/mol, respectively. This indicates the proximity of boron to the metal-nonmetal borderline.<ref name=Fehlner1990>[[#Fehlner1990|Fehlner 1990, p.&nbsp;202]]</ref>

Most of the chemistry of boron is nonmetallic in nature.<ref name=Fehlner1990/> Unlike its heavier congeners, it is not known to form a simple B<sup>3+</sup> or hydrated [B(H<sub>2</sub>O)<sub>4</sub>]<sup>3+</sup> cation.<ref>[[#Owen|Owen & Brooker 1991, p.&nbsp;59]]; [[#Wiberg2001|Wiberg 2001, p.&nbsp;936]]</ref> The small size of the boron atom enables the preparation of many [[interstitial compound|interstitial]] alloy-type borides.<ref name=Greenwood145>[[#Greenwood2002|Greenwood & Earnshaw 2002, p.&nbsp;145]]</ref> Analogies between boron and transition metals have been noted in the formation of [[complex (chemistry)|complexes]],<ref>[[#Houghton1979|Houghton 1979, p.&nbsp;59]]</ref> and [[adduct]]s (for example, BH<sub>3</sub> + [[Carbon monoxide|CO]] →BH<sub>3</sub>CO and, similarly, Fe(CO)<sub>4</sub> + CO →Fe(CO)<sub>5</sub>),{{refn|1=The BH<sub>3</sub> and Fe(CO<sub>4</sub>) species in these reactions are short-lived [[reaction intermediate]]s.<ref>[[#Fehlner1990|Fehlner 1990, p.&nbsp;205]]</ref>|group=n}} as well as in the geometric and electronic structures of [[cluster compound|cluster species]] such as [B<sub>6</sub>H<sub>6</sub>]<sup>2−</sup> and [Ru<sub>6</sub>(CO)<sub>18</sub>]<sup>2−</sup>.<ref>[[#Fehlner1990|Fehlner 1990, pp.&nbsp;204–05, 207]]</ref>{{refn|1=On the analogy between boron and metals, Greenwood<ref>[[#Greenwood2001|Greenwood 2001, p.&nbsp;2057]]</ref> commented that: "The extent to which metallic elements mimic boron (in having fewer electrons than orbitals available for bonding) has been a fruitful cohering concept in the development of metalloborane chemistry&nbsp;... Indeed, metals have been referred to as "honorary boron atoms" or even as "flexiboron atoms". The converse of this relationship is clearly also valid&nbsp;..."|group=n}} The aqueous chemistry of boron is characterised by the formation of many different [[Borate#Polymeric ions|polyborate anions]].<ref>[[#Salentine1987|Salentine 1987, pp.&nbsp;128–32]]; [[#MacKay2002|MacKay, MacKay & Henderson 2002, pp.&nbsp;439–40]]; [[#Kneen1972|Kneen, Rogers & Simpson 1972, p.&nbsp;394]]; [[#Hiller1960|Hiller & Herber 1960, inside front cover; p.&nbsp;225]]</ref> Given its high charge-to-size ratio, boron bonds covalently in nearly all of its compounds;<ref>[[#Sharp1983|Sharp 1983, p.&nbsp;56]]</ref> the exceptions are the [[boride]]s as these include, depending on their composition, covalent, ionic, and metallic bonding components.<ref>[[#Fokwa|Fokwa 2014, p.&nbsp;10]]</ref>{{refn|1=The bonding in [[boron trifluoride]], a gas, has been referred to as predominately ionic<ref name=Gillespie1998>[[#Gillespie1998|Gillespie 1998]]</ref> a description which was subsequently described as misleading.<ref name=Haaland>[[#Haaland|Haaland et al. 2000]]</ref>|group=n}} Simple binary compounds, such as [[boron trichloride]] are [[Lewis acid]]s as the formation of three covalent bonds leaves a hole in the [[octet rule|octet]] which can be filled by an electron-pair donated by a [[Lewis base]].<ref name="Rayner-Canham 2006, p. 291"/> Boron has a strong affinity for [[oxygen]] and a duly extensive [[borate]] chemistry.<ref name=Greenwood145/> The oxide B<sub>2</sub>O<sub>3</sub> is [[polymeric]] in structure,<ref name=Pudd59>[[#Puddephatt1989|Puddephatt & Monaghan 1989, p.&nbsp;59]]</ref> weakly acidic,<ref>[[#Mahan1965|Mahan 1965, p.&nbsp;485]]</ref>{{refn|1=Boron trioxide B<sub>2</sub>O<sub>3</sub> is sometimes described as being (weakly) [[amphoteric]].<ref>[[#Danaith|Danaith 2008, p.&nbsp;81]].</ref> It reacts with [[alkali]]es to give various borates.<ref>[[#Lidin|Lidin 1996, p.&nbsp;28]]</ref> In its [[hydrated]] form (as H<sub>3</sub>BO<sub>3</sub>, [[boric acid]]) it reacts with [[sulfur trioxide]], the [[anhydride]] of [[sulfuric acid]], to form a [[bisulfate]] B(HSO<sub>3</sub>) <sub>4</sub>.<ref>[[#Kondratev|Kondrat'ev & Mel'nikova 1978]]</ref> In its pure (anhydrous) form it reacts with [[phosphoric acid]] to form a "[[phosphate]]" BPO<sub>4</sub>.<ref>[[#Holderness|Holderness & Berry 1979, p.&nbsp;111]]; [[#Wiberg2001|Wiberg 2001, p.&nbsp;980]]</ref> The latter compound may be regarded as a [[mixed oxide]] of B<sub>2</sub>O<sub>3</sub> and [[P2O5|P<sub>2</sub>O<sub>5</sub>]].<ref>[[#Toy|Toy 1975, p.&nbsp;506]]</ref>|group=n}} and a glass former.<ref name=Rao22>[[#Rao2002|Rao 2002, p.&nbsp;22]]</ref> [[Organometallic chemistry|Organometallic compounds]] of boron{{refn|1=Organic derivatives of metalloids are traditionally counted as organometallic compounds.<ref>[[#Fehlner|Fehlner 1992, p.&nbsp;1]]</ref>|group=n}} have been known since the 19th century (see [[organoboron chemistry]]).<ref>[[#Haiduc1985|Haiduc & Zuckerman 1985, p.&nbsp;82]]</ref>