The word metalloid comes from the Latin metallum ("metal") and the Greek oeidḗs ("resembling in form or appearance").Oxford English Dictionary 1989, 'metalloid'; Gordh, Gordh & Headrick 2003, p. 753 However, there is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature.
The six commonly recognised metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. Five elements are less frequently so classified: carbon, aluminium, selenium, polonium and astatine. On a standard periodic table, all eleven elements are in a diagonal region of the p-block extending from boron at the upper left to astatine at lower right. Some periodic tables include a dividing line between metals and nonmetals, and the metalloids may be found close to this line.
Typical metalloids have a metallic appearance, may be brittle and are only fair conductors of electricity. They can form alloys with metals, and many of their other physical properties and chemical properties are intermediate between those of metallic and nonmetallic elements. They and their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors, and electronics.
The term metalloid originally referred to nonmetals. Its more recent meaning, as a category of elements with intermediate or hybrid properties, became widespread in 1940–1960. Metalloids are sometimes called semimetals, a practice that has been discouraged, as the term semimetal has a more common usage as a specific kind of electronic band structure of a substance. In this context, only arsenic and antimony are semimetals, and commonly recognised as metalloids.
Definitions
Judgment-based A metalloid is an element that possesses a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals, 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. Difficulty of categorisation is a key attribute. Most elements have a mixture of metallic and nonmetallic properties,Hopkins & Bailar 1956, p. 458 and can be classified according to which set of properties is more pronounced.Glinka 1965, p. 77 Only the elements at or near the margins, lacking a sufficiently clear preponderance of either metallic or nonmetallic properties, are classified as metalloids.Tyler Miller 1987, p. 59
Boron, silicon, germanium, arsenic, antimony, and tellurium are commonly recognised as metalloids.Goldsmith 1982, p. 526; Kotz, Treichel & Weaver 2009, p. 62; Bettelheim et al. 2010, p. 46 Depending on the author, one or more from selenium, polonium, or astatine are sometimes added to the list.Hawkes 2001, p. 1686; Segal 1989, p. 965; McMurray & Fay 2009, p. 767 Boron sometimes is excluded, by itself, or with silicon.Bucat 1983, p. 26; Brown c. 2007 Sometimes tellurium is not regarded as a metalloid.Swift & Schaefer 1962, p. 100 The inclusion of antimony, polonium, and astatine as metalloids has been questioned.Hawkes 2001, p. 1686; Hawkes 2010; Holt, Rinehart & Wilson c. 2007
Several other elements are occasionally classified as metalloids. These elements includeDunstan 1968, pp. 310, 409. Dunstan lists Be, Al, Ge (maybe), As, Se (maybe), Sn, Sb, Te, Pb, Bi, and Po as metalloids (pp. 310, 323, 409, 419). hydrogen,Tilden 1876, pp. 172, 198–201; Smith 1994, p. 252; Bodner & Pardue 1993, p. 354 beryllium,Bassett et al. 1966, p. 127 nitrogen,Rausch 1960 phosphorus,Thayer 1977, p. 604; Warren & Geballe 1981; Masters & Ela 2008, p. 190 sulfur,Warren & Geballe 1981; Chalmers 1959, p. 72; US Bureau of Naval Personnel 1965, p. 26 zinc,Siebring 1967, p. 513 gallium,Wiberg 2001, p. 282 tin, iodine,Rausch 1960; Friend 1953, p. 68 lead,Murray 1928, p. 1295 bismuth, radon,Hampel & Hawley 1966, p. 950; Stein 1985; Stein 1987, pp. 240, 247–48 flerovium, moscovium, livermorium, tennessine, and oganesson. However, due to relativistic effects and short half-lives, Fl to Og are hard to place into metalloids. The term metalloid has also been used for elements that exhibit metallic lustre and electrical conductivity, and that are amphoteric, such as arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead, and aluminium.Hatcher 1949, p. 223; Secrist & Powers 1966, p. 459 The p-block metals,Taylor 1960, p. 614 and nonmetals (such as carbon or nitrogen) that can form alloys with metalsConsidine & Considine 1984, p. 568; Cegielski 1998, p. 147; The American heritage science dictionary 2005, p. 397 or modify their propertiesWoodward 1948, p. 1 have also occasionally been considered as metalloids.
Criteria-based Element IE(kcal/mol) IE(kJ/mol) ENBand structure Boron 191 801 2.04 semiconductor Silicon 188 787 1.90 semiconductor Germanium 182 762 2.01 semiconductor Arsenic 226 944 2.18 semimetal Antimony 199 831 2.05 semimetal Tellurium 208 869 2.10 semiconductor average 199 832 2.05The elements commonly recognised as metalloids, and their ionization energies (IE);NIST 2010. Values shown in the above table have been converted from the NIST values, which are given in eV. electronegativities (EN, revised Pauling scale); and electronic band structuresBerger 1997; Lovett 1977, p. 3 (most thermodynamically stable forms under ambient conditions). No widely accepted definition of a metalloid exists, nor any division of the periodic table into metals, metalloids, and nonmetals;Goldsmith 1982, p. 526; Hawkes 2001, p. 1686 HawkesHawkes 2001, p. 1687 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 SharpSharp 1981, p. 299 as "arbitrary".
The number and identities of metalloids depend on what classification criteria are used. EmsleyEmsley 1971, p. 1 recognised four metalloids (germanium, arsenic, antimony, and tellurium); James et al.James et al. 2000, p. 480 listed twelve (Emsley's plus boron, carbon, silicon, selenium, bismuth, polonium, moscovium, and livermorium). On average, seven elements are included in such lists; individual classification arrangements tend to share common ground and vary in the ill-definedChatt 1951, p. 417 "The boundary between metals and metalloids is indefinite ..."; Burrows et al. 2009, p. 1192: "Although the elements are conveniently described as metals, metalloids, and nonmetals, the transitions are not exact ..." margins.
A single quantitative criterion such as electronegativity is commonly used,Rochow 1966, pp. 1, 4–7 metalloids having electronegativity values from 1.8 or 1.9 to 2.2.Rochow 1977, p. 76; Mann et al. 2000, p. 2783 Further examples include packing efficiency (the fraction of volume in a crystal structure occupied by atoms) and the Goldhammer–Herzfeld criterion ratio.Askeland, Phulé & Wright 2011, p. 69 The commonly recognised metalloids have packing efficiencies of between 34% and 41%. The Goldhammer–Herzfeld ratio, roughly equal to the cube of the atomic radius divided by the molar volume,Edwards & Sienko 1983, p. 693 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.Edwards & Sienko 1983, p. 695; Edwards et al. 2010 Other authors have relied on, for example, atomic conductanceHill & Holman 2000, p. 160. They characterise metalloids (in part) on the basis that they are "poor conductors of electricity with atomic conductance usually less than 10−3 but greater than 10−5 ohm−1 cm−4". or bulk coordination number.Bond 2005, p. 3: "One criterion for distinguishing semi-metals from true metals under normal conditions is that the 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)."
Masterton and SlowinskiMasterton & Slowinski 1977, p. 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. used three criteria to describe the six elements commonly recognised as metalloids: metalloids have 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.
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).Vernon 2013, p. 1703
Periodic table territory Distribution and recognition statusof elements classified as metalloids 1 2 12 13 14 15 16 17 18 H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Zn Ga Ge As Se Br Kr Rb Sr Cd In Sn Sb Te I Xe Cs Ba Hg Tl Pb Bi Po At Rn Fr Ra Cn Nh Fl Mc Lv Ts Og Metal–nonmetal dividing line: between , , , , , , and Periodic table extract showing groups 1–2 and 12–18, and a dividing line between metals and nonmetals. Percentages are median appearance frequencies in the list of metalloid lists. Sporadically recognised elements show that the metalloid net is sometimes cast very widely; although they do not appear in the list of metalloid lists, isolated references to their designation as metalloids can be found in the literature (as cited in this article).
Location Metalloids lie on either side of the dividing line between metals and nonmetals. This can be found, in varying configurations, on some periodic tables. Elements to the lower left of the line generally display increasing metallic behaviour; elements to the upper right display increasing nonmetallic behaviour. When presented as a regular stairstep, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.Horvath 1973, p. 336
Diagonal relationship
The metalloids illustrate an organizing principle of the periodic table called a diagonal relationship.Rodgers 2011, pp. 232–33; 240–41 The diagonal positioning of the metalloids represents an exception to the observation that elements with similar properties tend to occur in vertical groups.Gray 2009, p. 9 A related effect can be seen in other diagonal similarities between some elements and their lower right neighbours, specifically lithium-magnesium, beryllium-aluminium, and boron-silicon. Rayner-CanhamRayner-Canham 2011 has argued that these similarities extend to carbon-phosphorus, nitrogen-sulfur, and into three d-block series.