Editing Metalloid (section)

==Definitions==
{{See also|Lists of metalloids}}

===Judgment-based===
A metalloid is an element that possesses a preponderance of properties in between, or that are a mixture of, those of [[metal]]s and [[nonmetal]]s, and which is therefore hard to classify as either a [[metal]] or a [[nonmetal]]. This is a generic definition that draws on metalloid attributes consistently cited in the literature.{{refn|1=Definitions and extracts by different authors, illustrating aspects of the generic definition, follow:
*"In chemistry a metalloid is an element with properties intermediate between those of [[metal]]s and [[nonmetal]]s."<ref>[[#Cusack1987|Cusack 1987, p.&nbsp;360]]</ref>
*"Between the [[metal]]s and [[nonmetal]]s in the periodic table we find elements&nbsp;... [that] share some of the characteristic properties of both the [[metal]]s and [[Nonmetal|nonmetals]], making it difficult to place them in either of these two main categories"<ref>[[#Kelter2009|Kelter, Mosher & Scott 2009, p.&nbsp;268]]</ref>
*"Chemists sometimes use the name metalloid&nbsp;... for these elements which are difficult to classify one way or the other."<ref name="Hill 2000, p. 41">[[#Hill2000|Hill & Holman 2000, p.&nbsp;41]]</ref>
*"Because the traits distinguishing [[metal]]s and [[nonmetal]]s are qualitative in nature, some elements do not fall unambiguously in either category. These elements&nbsp;... are called metalloids&nbsp;..."<ref>[[#King1979|King 1979, p.&nbsp;13]]</ref>

More broadly, metalloids have been referred to as:
*"elements that&nbsp;... are somewhat of a cross between [[metal]]s and [[nonmetal]]s";<ref>[[#Moore2011|Moore 2011, p.&nbsp;81]]</ref> or
*"weird in-between elements".<ref>[[#Gray2010|Gray 2010]]</ref>|group=n}} Difficulty of categorisation is a key attribute. Most elements have a mixture of metallic and nonmetallic properties,<ref name=Hopkins>[[#Hopkins1956|Hopkins & Bailar 1956, p.&nbsp;458]]</ref> and can be classified according to which set of properties is more pronounced.<ref>[[#Glinka1965|Glinka 1965, p.&nbsp;77]]</ref>{{refn|1=[[Gold]], for example, has mixed properties but is still recognised as "king of [[metal]]s". Besides metallic behaviour (such as high electrical conductivity, and [[cation]] formation), gold shows nonmetallic behaviour:
*It has the [[table of standard electrode potentials|highest]] [http://www.chemguide.co.uk/physical/redoxeqia/introduction.html electrode potential]
*It has the third-highest [[ionization energy]] among the [[metal]]s (after [[zinc]] and [[mercury (element)|mercury]])
*It has the highest [[electron affinity]]
*Its [[electronegativity]] of 2.54 is highest among the [[metal]]s and exceeds that of some [[nonmetal]]s ([[hydrogen]] 2.2; [[phosphorus]] 2.19; and [[radon]] 2.2)
*It forms the Au<sup>−</sup> auride [[anion]], acting in this way like a [[halogen]]
*It sometimes has a tendency, known as "[[aurophilicity]]", to bond to itself.<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;1279]]</ref>
On halogen character, see also Belpassi et al.,<ref>[[#Belpassi2006|Belpassi et al. 2006, pp.&nbsp;4543–44]]</ref> who conclude that in the aurides MAu (M = [[alkali metal|Li–Cs]]) gold "behaves as a halogen, intermediate between [[bromine|Br]] and [[iodine|I]]"; on aurophilicity, see also Schmidbaur and Schier.<ref>[[#Schmidbaur2008|Schmidbaur & Schier 2008, pp.&nbsp;1931–51]]</ref>|group=n}} Only the elements at or near the margins, lacking a sufficiently clear preponderance of either metallic or nonmetallic properties, are classified as metalloids.<ref>[[#TM1987|Tyler Miller 1987, p.&nbsp;59]]</ref>

[[Boron]], [[silicon]], [[germanium]], [[arsenic]], [[antimony]], and [[tellurium]] are commonly recognised as metalloids.<ref>[[#Goldsmith1982|Goldsmith 1982, p.&nbsp;526]]; [[#Kotz2009|Kotz, Treichel & Weaver 2009, p.&nbsp;62]]; [[#Bettelheim|Bettelheim et al. 2010, p.&nbsp;46]]</ref>{{refn|1=Mann et al.<ref name=Mann/> refer to these elements as "the recognized metalloids".|group=n}} Depending on the author, one or more from [[selenium]], [[polonium]], or [[astatine]] are sometimes added to the list.<ref>[[#Hawkes2001|Hawkes 2001, p.&nbsp;1686]]; [[#Segal1989|Segal 1989, p.&nbsp;965]]; [[#McMurray2009|McMurray & Fay 2009, p.&nbsp;767]]</ref> [[Boron]] sometimes is excluded, by itself, or with [[silicon]].<ref>[[#Bucat1983|Bucat 1983, p.&nbsp;26]]; [[#Brown2007|Brown c. 2007]]</ref> Sometimes [[tellurium]] is not regarded as a metalloid.<ref name="Swift1962,100">[[#Swift1962|Swift & Schaefer 1962, p.&nbsp;100]]</ref> The inclusion of [[antimony]], [[polonium]], and [[astatine]] as metalloids has been questioned.<ref>[[#Hawkes2001|Hawkes 2001, p.&nbsp;1686]]; [[#Hawkes2010|Hawkes 2010]]; [[#Holt2007|Holt, Rinehart & Wilson c. 2007]]</ref>

Other elements are occasionally classified as metalloids. These elements include<ref>[[#Dunstan1968|Dunstan 1968, pp.&nbsp;310, 409]]. Dunstan lists Be, Al, Ge (maybe), As, Se (maybe), Sn, Sb, Te, Pb, Bi, and Po as metalloids (pp.&nbsp;310, 323, 409, 419).</ref> [[hydrogen]],<ref>[[#Tilden1876|Tilden 1876, pp.&nbsp;172, 198–201]]; [[#Smith1994|Smith 1994, p.&nbsp;252]]; [[#Bodner1993|Bodner & Pardue 1993, p.&nbsp;354]]</ref> [[beryllium]],<ref>[[#Bassett1966|Bassett et al. 1966, p.&nbsp;127]]</ref> [[nitrogen]],<ref name=rausch>[[#Rausch1960|Rausch 1960]]</ref> [[phosphorus]],<ref>[[#Thayer1977|Thayer 1977, p.&nbsp;604]]; [[#Warren1981|Warren & Geballe 1981]]; [[#M&E|Masters & Ela 2008, p.&nbsp;190]]</ref> [[sulfur]],<ref>[[#Warren1981|Warren & Geballe 1981]]; [[#Chalmers1959|Chalmers 1959, p.&nbsp;72]]; [[#United1965|US Bureau of Naval Personnel 1965, p.&nbsp;26]]</ref> [[zinc]],<ref>[[#Siebring1967|Siebring 1967, p.&nbsp;513]]</ref> [[gallium]],<ref>[[#Wiberg2001|Wiberg 2001, p.&nbsp;282]]</ref> [[tin]], [[iodine]],<ref>[[#Rausch1960|Rausch 1960]]; [[#Friend1953|Friend 1953, p.&nbsp;68]]</ref> [[lead]],<ref>[[#Murray1928|Murray 1928, p.&nbsp;1295]]</ref> [[bismuth]],<ref name="Swift1962,100"/> and [[radon]].<ref>[[#Hampel&H1966|Hampel & Hawley 1966, p.&nbsp;950]]; [[#Stein1985|Stein 1985]]; [[#Stein1987|Stein 1987, pp.&nbsp;240, 247–48]]</ref> The term metalloid has also been used for elements that exhibit metallic [[Lustre (mineralogy)|lustre]] and [[Electrical resistivity and conductivity|electrical conductivity]], and that are [[amphoterism|amphoteric]], such as [[arsenic]], [[antimony]], [[vanadium]], [[chromium]], [[molybdenum]], [[tungsten]], [[tin]], [[lead]], and [[aluminium]].<ref>[[#Hatcher1949|Hatcher 1949, p.&nbsp;223]]; [[#Secrist|Secrist & Powers 1966, p.&nbsp;459]]</ref> The [[Post-transition metal#p-block metals|p-block metals]],<ref>[[#Taylor1960|Taylor 1960, p.&nbsp;614]]</ref> and nonmetals (such as carbon or nitrogen) that can form [[alloy]]s with metals<ref>[[#Considine1984|Considine & Considine 1984, p.&nbsp;568]]; [[#Cegielski1998|Cegielski 1998, p.&nbsp;147]]; [[#TheAmerican2005|''The American heritage science dictionary 2005'', p.&nbsp;397]]</ref> or modify their properties<ref>[[#Woodward1948|Woodward 1948, p.&nbsp;1]]</ref> have also occasionally been considered as metalloids.

===Criteria-based===
{| class="wikitable floatright" style="width: 75px;"
|-
! Element
! IE<br/>(kcal/mol)
! IE<br/>(kJ/mol)
! EN
! nowrap|[[electronic band structure|Band structure]]
|-
| Boron
| style="text-align:center;"| 191
| style="text-align:center;"| 801
|style="padding-left:1em; padding-right:1em;"| 2.04
|style="padding-left:1em; padding-right:1em;"| [[semiconductor]]
|-
| Silicon
| style="text-align:center;"| 188
| style="text-align:center;"| 787
|style="padding-left:1em; padding-right:1em;"| 1.90
|style="padding-left:1em; padding-right:1em;"| semiconductor
|-
| Germanium
| style="text-align:center;"| 182
| style="text-align:center;"| 762
|style="padding-left:1em; padding-right:1em;"| 2.01
|style="padding-left:1em; padding-right:1em;"| semiconductor
|-
| Arsenic
| style="text-align:center;"| 226
| style="text-align:center;"| 944
|style="padding-left:1em; padding-right:1em;"| 2.18
|style="padding-left:1em; padding-right:1em;"| [[semimetal]]
|-
| Antimony
| style="text-align:center;"| 199
| style="text-align:center;"| 831
|style="padding-left:1em; padding-right:1em;"| 2.05
|style="padding-left:1em; padding-right:1em;"| semimetal
|-
| Tellurium
| style="text-align:center;"| 208
| style="text-align:center;"| 869
|style="padding-left:1em; padding-right:1em;"| 2.10
|style="padding-left:1em; padding-right:1em;"| semiconductor
|-
| style="text-align: right"| ''average''
| style="text-align:center;"| 199
| style="text-align:center;"| 832
|style="padding-left:1em; padding-right:1em;"| 2.05
|
|-
| colspan="5" style="text-align: left; font-size: 90%" |The elements commonly recognised as metalloids, and their [[ionization energy|ionization energies]] (IE);<ref>[[#NIST2010|NIST 2010]]. Values shown in the above table have been converted from the NIST values, which are given in&nbsp;eV.</ref> electronegativities (EN, revised Pauling scale); and electronic band structures<ref>[[#Berger1997|Berger 1997]]; [[#Lovett1977|Lovett 1977, p.&nbsp;3]]</ref> (most thermodynamically stable forms under ambient conditions).
|}
No widely accepted definition of a metalloid exists, nor any division of the periodic table into [[Metal|metals]], metalloids, and [[Nonmetal|nonmetals]];<ref>[[#Goldsmith1982|Goldsmith 1982, p.&nbsp;526]]; [[#Hawkes2001|Hawkes 2001, p.&nbsp;1686]]</ref> Hawkes<ref name=H1687>[[#Hawkes2001|Hawkes 2001, p.&nbsp;1687]]</ref> questioned the feasibility of establishing a specific definition, noting that anomalies can be found in several attempted constructs. Classifying an element as a metalloid has been described by Sharp<ref name="Sharp1981">[[#Sharp1981|Sharp 1981, p.&nbsp;299]]</ref> as "arbitrary".

The number and identities of metalloids depend on what classification criteria are used. Emsley<ref>[[#Emsley1971|Emsley 1971, p.&nbsp;1]]</ref> recognised four metalloids (germanium, arsenic, antimony, and tellurium); James et al.<ref>[[#James2000|James et al. 2000, p.&nbsp;480]]</ref> listed twelve (Emsley's plus boron, carbon, silicon, selenium, bismuth, polonium, [[moscovium]], and [[livermorium]]). On average, seven elements are included in [[lists of metalloids|such lists]]; individual classification arrangements tend to share common ground and vary in the ill-defined<ref>[[#Chatt1951|Chatt 1951, p.&nbsp;417]] "The boundary between metals and metalloids is indefinite&nbsp;..."; [[#Burrows2009|Burrows et al. 2009, p.&nbsp;1192]]: "Although the elements are conveniently described as metals, metalloids, and nonmetals, the transitions are not exact&nbsp;..."</ref> margins.{{refn|1=Jones<ref>[[#Jones2010|Jones 2010, p.&nbsp;170]]</ref> writes: "Though classification is an essential feature in all branches of science, there are always hard cases at the boundaries. Indeed, the boundary of a class is rarely sharp."|group=n}}{{refn|1=The lack of a standard division of the elements into metals, metalloids, and nonmetals is not necessarily an issue. There is more or less, a continuous progression from the metallic to the nonmetallic. A specified subset of this continuum could serve its particular purpose as well as any other.<ref>[[#Kneen1972|Kneen, Rogers & Simpson 1972, pp.&nbsp;218–20]]</ref>|group=n}}

A single quantitative criterion such as [[electronegativity]] is commonly used,<ref>[[#Rochow1966|Rochow 1966, pp.&nbsp;1, 4–7]]</ref> metalloids having electronegativity values from 1.8 or 1.9 to 2.2.<ref>[[#Rochow1977|Rochow 1977, p.&nbsp;76]]; [[#Mann2000|Mann et al. 2000, p.&nbsp;2783]]</ref> Further examples include [[atomic packing factor|packing efficiency]] (the fraction of volume in a [[crystal structure]] occupied by atoms) and the Goldhammer–Herzfeld criterion ratio.<ref>[[#Askeland|Askeland, Phulé & Wright 2011, p.&nbsp;69]]</ref> The commonly recognised metalloids have packing efficiencies of between 34% and 41%.{{refn|1=The packing efficiency of boron is 38%; silicon and germanium 34; arsenic 38.5; antimony 41; and tellurium 36.4.<ref>[[#VanSetten2007|Van Setten et al. 2007, pp.&nbsp;2460–61]]; [[#Russell2005|Russell & Lee 2005, p.&nbsp;7]] (Si, Ge); [[#Pearson1972|Pearson 1972, p.&nbsp;264]] (As, Sb, Te; also black P)</ref> These values are lower than in most metals (80% of which have a packing efficiency of at least 68%),<ref>[[#Russell2005|Russell & Lee 2005, p.&nbsp;1]]</ref> but higher than those of elements usually classified as nonmetals. (Gallium is unusual, for a metal, in having a packing efficiency of just 39%.)<ref>[[#Russell2005|Russell & Lee 2005, pp.&nbsp;6–7, 387]]</ref> Other notable values for metals are 42.9 for bismuth<ref name="ReferenceB">[[#Pearson1972|Pearson 1972, p.&nbsp;264]]</ref> and 58.5 for liquid mercury.<ref>[[#Okajima1972|Okajima & Shomoji 1972, p.&nbsp;258]]</ref>) Packing efficiencies for nonmetals are: graphite 17%,<ref>[[#Kitaĭgorodskiĭ1961|Kitaĭgorodskiĭ 1961, p.&nbsp;108]]</ref> sulfur 19.2,<ref name="Neuburger">[[#Neuburger1936|Neuburger 1936]]</ref> iodine 23.9,<ref name="Neuburger"/> selenium 24.2,<ref name="Neuburger"/> and black phosphorus 28.5.<ref name="ReferenceB"/>|group=n}} The Goldhammer–Herzfeld ratio, roughly equal to the cube of the atomic radius divided by the [[molar volume]],<ref>[[#Edwards1983|Edwards & Sienko 1983, p.&nbsp;693]]</ref>{{refn|1=More specifically, the <span id="Gold"></span>''Goldhammer–[[Karl Herzfeld|Herzfeld]] criterion'' is the ratio of the force holding an individual atom's [[valence electron]]s in place with the forces on the same electrons from interactions ''between'' the atoms in the solid or liquid element. When the interatomic forces are greater than, or equal to, the atomic force, valence electron itinerancy is indicated and metallic behaviour is predicted.<ref>[[#Herzfeld|Herzfeld 1927]]; [[#Edwards2000|Edwards 2000, pp.&nbsp;100–03]]</ref> Otherwise nonmetallic behaviour is anticipated.|group=n}} is a simple measure of how metallic an element is, the recognised metalloids having ratios from around 0.85 to 1.1 and averaging 1.0.<ref>[[#Edwards1983|Edwards & Sienko 1983, p.&nbsp;695]]; [[#Edwards2010|Edwards et al. 2010]]</ref>{{refn|1=As the ratio is based on classical arguments<ref>[[#Edwards1999|Edwards 1999, p.&nbsp;416]]</ref> it does not accommodate the finding that polonium, which has a value of ~0.95, adopts a metallic (rather than [[covalent]]) [[crystalline structure]], on [[relativistic quantum chemistry|relativistic]] grounds.<ref>[[#Steurer2007|Steurer 2007, p.&nbsp;142]]; [[#Pyykkö|Pyykkö 2012, p.&nbsp;56]]</ref> Even so it offers a [[wikt:first-order|first order]] rationalization for the occurrence of metallic character amongst the elements.<ref name=edwards695>[[#Edwards1983|Edwards & Sienko 1983, p.&nbsp;695]]</ref>|group=n}}
Other authors have relied on, for example, atomic conductance{{refn|1=Atomic conductance is the electrical conductivity of one mole of a substance. It is equal to electrical conductivity divided by molar volume.<ref name="Hill 2000, p. 41"/>|group=n}}<ref>[[#Hill2000|Hill & Holman 2000, p.&nbsp;160]]. They characterise metalloids (in part) on the basis that they are "poor conductors of electricity with atomic conductance usually less than 10<sup>−3</sup> but greater than 10<sup>−5</sup>&nbsp;ohm<sup>−1</sup> cm<sup>−4</sup>".</ref> or [[coordination number#Usage in quasicrystal, liquid and other disordered systems|bulk coordination number]].<ref>[[#Bond2005|Bond 2005, p.&nbsp;3]]: "One criterion for distinguishing semi-metals from true metals under normal conditions is that the [[coordination number#Usage in quasicrystal, liquid and other disordered systems|bulk coordination number]] of the former is never greater than eight, while for metals it is usually twelve (or more, if for the body-centred cubic structure one counts next-nearest neighbours as well)."</ref>

Jones, writing on the role of classification in science, observed that "[classes] are usually defined by more than two attributes".<ref>[[#Jones2010|Jones 2010, p.&nbsp;169]]</ref> Masterton and Slowinski<ref>[[#Masterton1977|Masterton & Slowinski 1977, p.&nbsp;160]] list B, Si, Ge, As, Sb, and Te as metalloids, and comment that Po and At are ordinarily classified as metalloids but add that this is arbitrary as so little is known about them.</ref> used three criteria to describe the six elements commonly recognised as metalloids: metalloids have [[ionization energy|ionization energies]] around 200 kcal/mol (837 kJ/mol) and electronegativity values close to 2.0. They also said that metalloids are typically semiconductors, though antimony and arsenic (semimetals from a physics perspective) have electrical conductivities approaching those of metals. Selenium and polonium are suspected as not in this scheme, while astatine's status is uncertain.{{refn|1=Selenium has an ionization energy (IE) of 225&nbsp;kcal/mol (941 kJ/mol) and is sometimes described as a semiconductor. It has a relatively high 2.55 electronegativity (EN). Polonium has an IE of 194&nbsp;kcal/mol (812 kJ/mol) and a 2.0 EN, but has a metallic band structure.<ref>[[#Kraig2004|Kraig, Roundy & Cohen 2004, p.&nbsp;412]]; [[#Alloul2010|Alloul 2010, p.&nbsp;83]]</ref> Astatine has an IE of 215&nbsp;kJ/mol (899 kJ/mol) and an EN of 2.2.<ref>[[#Vernon|Vernon 2013, p.&nbsp;1704]]</ref> Its electronic band structure is not known with any certainty.|group=n}}

In this context, Vernon proposed that a metalloid is a chemical element that, in its standard state, has (a) the electronic band structure  of a semiconductor or a semimetal; and (b) an intermediate first ionization potential "(say 750−1,000 kJ/mol)"; and (c) an intermediate electronegativity (1.9–2.2).<ref>[[#Vernon|Vernon 2013, p.&nbsp;1703]]</ref>
{{clear}}