Metalloid

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Chemical element with metallic and nonmetallic properties

Elements recognized as metalloids

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• t
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	13	14	15	16	17
2	B Boron	C Carbon	N Nitrogen	O Oxygen	F Fluorine
3	Al Aluminium	Si Silicon	P Phosphorus	S Sulfur	Cl Chlorine
4	Ga Gallium	Ge Germanium	As Arsenic	Se Selenium	Br Bromine
5	In Indium	Sn Tin	Sb Antimony	Te Tellurium	I Iodine
6	Tl Thallium	Pb Lead	Bi Bismuth	Po Polonium	At Astatine
Commonly recognized (86–99%): B, Si, Ge, As, Sb, Te   Irregularly recognized (40–49%): Po, At   Less commonly recognized (24%): Se   Rarely recognized (8–10%): C, Al   (All other elements cited in less than 6% of sources)   Arbitrary metal-nonmetal dividing line: between Be and B, Al and Si, Ge and As, Sb and Te, Po and At
Recognition status, as metalloids, of some elements in the p-block of the periodic table. Percentages are median appearance frequencies in the lists of metalloids.[n 1] The staircase-shaped line is a typical example of the arbitrary metal–nonmetal dividing line found on some periodic tables.

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A metalloid is a type of

nonmetals. 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 of chemistry

.

The six commonly recognised metalloids are

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, but they are brittle and only fair

catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors

, and electronics.

The electrical properties of silicon and germanium enabled the establishment of the semiconductor industry in the 1950s and the development of solid-state electronics from the early 1960s.^[1]

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,^[2] as the term semimetal has a different meaning in physics than in chemistry. In physics, it refers to 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

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 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.^{[n 2]} Difficulty of categorisation is a key attribute. Most elements have a mixture of metallic and nonmetallic properties,^[9] and can be classified according to which set of properties is more pronounced.^{[10][n 3]} Only the elements at or near the margins, lacking a sufficiently clear preponderance of either metallic or nonmetallic properties, are classified as metalloids.^[14]

Boron, silicon, germanium, arsenic, antimony, and tellurium are commonly recognised as metalloids.^{[15][n 4]} Depending on the author, one or more from selenium, polonium, or astatine are sometimes added to the list.^[17] Boron sometimes is excluded, by itself, or with silicon.^[18] Sometimes tellurium is not regarded as a metalloid.^[19] The inclusion of antimony, polonium, and astatine as metalloids has been questioned.^[20]

Other elements are occasionally classified as metalloids. These elements include^[21] hydrogen,^[22] beryllium,^[23] nitrogen,^[24] phosphorus,^[25] sulfur,^[26] zinc,^[27] gallium,^[28] tin, iodine,^[29] lead,^[30] bismuth,^[19] and radon.^[31] 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.^[32] The p-block metals,^[33] and nonmetals (such as carbon or nitrogen) that can form alloys with metals^[34] or modify their properties^[35] have also occasionally been considered as metalloids.

Criteria-based

Element	IE (kcal/mol)	IE (kJ/mol)	EN	Band 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	2.05
The elements commonly recognised as metalloids, and their ionization energies (IE);[36] electronegativities (EN, revised Pauling scale); and electronic band structures[37] (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;^[38] Hawkes^[39] 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^[40] as "arbitrary".

The number and identities of metalloids depend on what classification criteria are used. Emsley^[41] recognised four metalloids (germanium, arsenic, antimony, and tellurium); James et al.^[42] 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-defined^[43] margins.^{[n 5][n 6]}

A single quantitative criterion such as electronegativity is commonly used,^[46] metalloids having electronegativity values from 1.8 or 1.9 to 2.2.^[47] Further examples include packing efficiency (the fraction of volume in a crystal structure occupied by atoms) and the Goldhammer–Herzfeld criterion ratio.^[48] The commonly recognised metalloids have packing efficiencies of between 34% and 41%.^{[n 7]} The Goldhammer–Herzfeld ratio, roughly equal to the cube of the atomic radius divided by the molar volume,^{[56][n 8]} 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.^{[58][n 9]} Other authors have relied on, for example, atomic conductance^{[n 10][62]} or bulk coordination number.^[63]

Jones, writing on the role of classification in science, observed that "[classes] are usually defined by more than two attributes".^[64] Masterton and Slowinski^[65] 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.^{[n 11]}

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).^[68]

Periodic table territory

Distribution and recognition status of 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
	Commonly (93%) to rarely (9%) recognised as a metalloid: B, C, Al, Si, Ge, As, Se, Sb, Te, Po, At   Very rarely (1–5%): H, Be, P, S, Ga, Sn, I, Pb, Bi, Fl, Mc, Lv, Ts   Sporadically: N, Zn, Rn   Metal–nonmetal dividing line: between H and Li, Be and B, Al and Si, Ge and As, Sb and Te, Po and At, and Ts and Og
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.^[69] 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.^[70]

The diagonal positioning of the metalloids represents an exception to the observation that elements with similar properties tend to occur in vertical

d-block

series.

This exception arises due to competing horizontal and vertical trends in the

nuclear charge. Going along a period, the nuclear charge increases with atomic number as do the number of electrons. The additional pull on outer electrons as nuclear charge increases generally outweighs the screening effect of having more electrons. With some irregularities, atoms therefore become smaller, ionization energy increases, and there is a gradual change in character, across a period, from strongly metallic, to weakly metallic, to weakly nonmetallic, to strongly nonmetallic elements.^[73] Going down a main group, the effect of increasing nuclear charge is generally outweighed by the effect of additional electrons being further away from the nucleus. Atoms generally become larger, ionization energy falls, and metallic character increases.^[74] The net effect is that the location of the metal–nonmetal transition zone shifts to the right in going down a group,^[71] and analogous diagonal similarities are seen elsewhere in the periodic table, as noted.^[75]

Alternative treatments

Elements bordering the metal–nonmetal dividing line are not always classified as metalloids, noting a binary classification can facilitate the establishment of rules for determining bond types between metals and nonmetals.[76] In such cases, the authors concerned focus on one or more attributes of interest to make their classification decisions, rather than being concerned about the marginal nature of the elements in question. Their considerations may or not be made explicit and may, at times, seem arbitrary.^{[40][n 12]} Metalloids may be grouped with metals;^[77] or regarded as nonmetals;^[78] or treated as a sub-category of nonmetals.^{[79][n 13]} Other authors have suggested classifying some elements as metalloids "emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table".^[81] Some periodic tables distinguish elements that are metalloids and display no formal dividing line between metals and nonmetals. Metalloids are instead shown as occurring in a diagonal band^[82] or diffuse region.^[83] The key consideration is to explain the context for the taxonomy in use.

Properties

Metalloids usually look like metals but behave largely like nonmetals. Physically, they are shiny, brittle solids with intermediate to relatively good electrical conductivity and the electronic band structure of a semimetal or semiconductor. Chemically, they mostly behave as (weak) nonmetals, have intermediate ionization energies and electronegativity values, and amphoteric or weakly acidic oxides. They can form alloys with metals. Most of their other physical and chemical properties are intermediate in nature.

Compared to metals and nonmetals

Main article: Properties of metals, metalloids and nonmetals

Characteristic properties of metals, metalloids, and nonmetals are summarized in the table.^[84] Physical properties are listed in order of ease of determination; chemical properties run from general to specific, and then to descriptive.

Properties of metals, metalloids and nonmetals

Physical property	Metals	Metalloids	Nonmetals
Form	solid; a few liquid at or near room temperature (Ga, Hg, Rb, Cs, Fr)[85][n 14]	solid[87]	majority gaseous[88]
Appearance	lustrous (at least when freshly fractured)	lustrous[87]	several colourless; others coloured, or metallic grey to black
Elasticity	typically elastic, ductile, malleable (when solid)	brittle[89]	brittle, if solid
Electrical conductivity	good to high[n 15]	intermediate[91] to good[n 16]	poor to good[n 17]
Band structure	metallic (Bi = semimetallic)	are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms[95]	semiconductor or insulator[96]
Chemical property	Metals	Metalloids	Nonmetals
General chemical behaviour	metallic	nonmetallic[97]	nonmetallic
Ionization energy	relatively low	intermediate ionization energies,[98] usually falling between those of metals and nonmetals[99]	relatively high
Electronegativity	usually low	have electronegativity values close to 2[100] (revised Pauling scale) or within the range of 1.9–2.2 (Allen scale)[16][n 18]	high
When mixed with metals	give alloys	can form alloys[103]	ionic or interstitial compounds formed
Oxides	lower oxides basic; higher oxides increasingly acidic	amphoteric or weakly acidic[104]	acidic

The above table reflects the hybrid nature of metalloids. The properties of form, appearance, and behaviour when mixed with metals are more like metals. Elasticity and general chemical behaviour are more like nonmetals. Electrical conductivity, band structure, ionization energy, electronegativity, and oxides are intermediate between the two.

Common applications

The focus of this section is on the recognised metalloids. Elements less often recognised as metalloids are ordinarily classified as either metals or nonmetals; some of these are included here for comparative purposes.

Metalloids are too brittle to have any structural uses in their pure forms.^[105] They and their compounds are used as (or in) alloying components, biological agents (toxicological, nutritional, and medicinal), catalysts, flame retardants, glasses (oxide and metallic), optical storage media and optoelectronics, pyrotechnics, semiconductors, and electronics.^{[n 19]}

Alloys

Writing early in the history of

B metals

, "are probably best classed as alloys".

Among the lighter metalloids, alloys with

case hardening compositions for the engineering industry. Alloys of silicon with iron and with aluminium are widely used by the steel and automotive industries, respectively. Germanium forms many alloys, most importantly with the coinage metals.^[111]

The heavier metalloids continue the theme. Arsenic can form alloys with metals, including

copper tellurium (40–50% tellurium).^[116] Ferrotellurium is used as a stabilizer for carbon in steel casting.^[117] Of the non-metallic elements less often recognised as metalloids, selenium – in the form of ferroselenium (50–58% selenium) – is used to improve the machinability of stainless steels.^[118]

Biological agents

antileukaemic properties of white arsenic were first reported in 1878.^[119]

All six of the elements commonly recognised as metalloids have toxic, dietary or medicinal properties.^[120]

Arsenic and antimony compounds are especially toxic; boron, silicon, and possibly arsenic, are essential trace elements. Boron, silicon, arsenic, and antimony have medical applications, and germanium and tellurium are thought to have potential.

Boron is used in insecticides[121] and herbicides.^[122] It is an essential trace element.^[123] As boric acid, it has antiseptic, antifungal, and antiviral properties.^[124]

Silicon is present in

silatrane, a highly toxic rodenticide.^[125] Long-term inhalation of silica dust causes silicosis, a fatal disease of the lungs. Silicon is an essential trace element.^[123] Silicone gel can be applied to badly burned patients to reduce scarring.^[126]

Salts of germanium are potentially harmful to humans and animals if ingested on a prolonged basis.^[127] There is interest in the pharmacological actions of germanium compounds but no licensed medicine as yet.^[128]

Arsenic is notoriously poisonous and may also be an

acute promyelocytic leukaemia, a cancer of the blood and bone marrow.^[131] Arsenic in drinking water, which causes lung and bladder cancer, has been associated with a reduction in breast cancer mortality rates.^[132]

Metallic antimony is relatively non-toxic, but most antimony compounds are poisonous.[133] Two antimony compounds, sodium stibogluconate and stibophen, are used as antiparasitical drugs.^[134]

Elemental tellurium is not considered particularly toxic; two grams of sodium tellurate, if administered, can be lethal.^[135] People exposed to small amounts of airborne tellurium exude a foul and persistent garlic-like odour.^[136] Tellurium dioxide has been used to treat seborrhoeic dermatitis; other tellurium compounds were used as antimicrobial agents before the development of antibiotics.^[137] In the future, such compounds may need to be substituted for antibiotics that have become ineffective due to bacterial resistance.^[138]

Of the elements less often recognised as metalloids, beryllium and lead are noted for their toxicity;

antibacterials.^[144]

Catalysts

alumina have been used as catalysts for the removal of sulfur contaminants from natural gas.^[155] Titanium doped aluminium has been identified as a substitute for expensive noble metal catalysts used in the production of industrial chemicals.^[156]

Flame retardants

Compounds of boron, silicon, arsenic, and antimony have been used as

silica, and silicates, some of which were developed as alternatives to more toxic halogenated products, can considerably improve the flame retardancy of plastic materials.^[158]

Arsenic compounds such as

halogenated hydrocarbon compounds.^[162]

Glass formation

Optical fibers, usually made of pure silicon dioxide glass, with additives such as boron trioxide or germanium dioxide

for increased sensitivity

The oxides

glass fibre additive,^[166] and is also a component of borosilicate glass, widely used for laboratory glassware and domestic ovenware for its low thermal expansion.^[167] Most ordinary glassware is made from silicon dioxide.^[168] Germanium dioxide is used as a glass fibre additive, as well as in infrared optical systems.^[169] Arsenic trioxide is used in the glass industry as a decolourizing and fining agent (for the removal of bubbles),^[170] as is antimony trioxide.^[171] Tellurium dioxide finds application in laser and nonlinear optics.^[172]

metallic glasses are generally most easily prepared if one of the components is a metalloid or "near metalloid" such as boron, carbon, silicon, phosphorus or germanium.^{[173][n 20]} Aside from thin films deposited at very low temperatures, the first known metallic glass was an alloy of composition Au₇₅Si₂₅ reported in 1960.^[175] A metallic glass having a strength and toughness not previously seen, of composition Pd_82.5P₆Si_9.5Ge₂, was reported in 2011.^[176]

Phosphorus, selenium, and lead, which are less often recognised as metalloids, are also used in glasses.

sodium lamps.^[177] Selenium compounds can be used both as decolourising agents and to add a red colour to glass.^[178] Decorative glassware made of traditional lead glass contains at least 30% lead(II) oxide (PbO); lead glass used for radiation shielding may have up to 65% PbO.^[179] Lead-based glasses have also been extensively used in electronic components, enamelling, sealing and glazing materials, and solar cells. Bismuth based oxide glasses have emerged as a less toxic replacement for lead in many of these applications.^[180]

Optical storage and optoelectronics

Varying compositions of

crystalline states. The change in optical and electrical properties can be used for information storage purposes.^[181] Future applications for GeSbTe may include, "ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses, and artificial retina devices."^[182]

Pyrotechnics

The recognised metalloids have either pyrotechnic applications or associated properties. Boron and silicon are commonly encountered;

blasting cap initiator compositions.^[192]

Carbon, aluminium, phosphorus, and selenium continue the theme. Carbon, in

black powder, is a constituent of fireworks rocket propellants, bursting charges, and effects mixtures, and military delay fuses and igniters.^{[193][n 22]} Aluminium is a common pyrotechnic ingredient,^[184] and is widely employed for its capacity to generate light and heat,^[195] including in thermite mixtures.^[196] Phosphorus can be found in smoke and incendiary munitions, paper caps used in toy guns, and party poppers.^[197] Selenium has been used in the same way as tellurium.^[192]

Semiconductors and electronics

GaAs, for example) or as doping agents (B, Sb, Te

, for example).

All the elements commonly recognised as metalloids (or their compounds) have been used in the semiconductor or solid-state electronic industries.[198]

Some properties of boron have limited its use as a semiconductor. It has a high melting point, single crystals are relatively hard to obtain, and introducing and retaining controlled impurities is difficult.^[199]

Silicon is the leading commercial semiconductor; it forms the basis of modern electronics (including standard solar cells)^[200] and information and communication technologies.^[201] This was despite the study of semiconductors, early in the 20th century, having been regarded as the "physics of dirt" and not deserving of close attention.^[202]

Germanium has largely been replaced by silicon in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process.

silicon-germanium "alloys" and these have been growing in use, particularly for wireless communication devices; such alloys exploit the higher carrier mobility of germanium.^[107] The synthesis of gram-scale quantities of semiconducting germanane was reported in 2013. This consists of one-atom thick sheets of hydrogen-terminated germanium atoms, analogous to graphane. It conducts electrons more than ten times faster than silicon and five times faster than germanium, and is thought to have potential for optoelectronic and sensing applications.^[203] The development of a germanium-wire based anode that more than doubles the capacity of lithium-ion batteries was reported in 2014.^[204] In the same year, Lee et al. reported that defect-free crystals of graphene large enough to have electronic uses could be grown on, and removed from, a germanium substrate.^[205]

Arsenic and antimony are not semiconductors in their

lithium-ion batteries to be replaced by more powerful sodium ion batteries.^[207]

Tellurium, which is a semiconductor in its standard state, is used mainly as a component in type II/VI semiconducting-chalcogenides; these have applications in electro-optics and electronics.^[208] Cadmium telluride (CdTe) is used in solar modules for its high conversion efficiency, low manufacturing costs, and large band gap of 1.44 eV, letting it absorb a wide range of wavelengths.^[200] Bismuth telluride (Bi₂Te₃), alloyed with selenium and antimony, is a component of thermoelectric devices used for refrigeration or portable power generation.^[209]

Five metalloids – boron, silicon, germanium, arsenic, and antimony – can be found in cell phones (along with at least 39 other metals and nonmetals).

n-type semiconductors.^[212] The commercial use of gallium compounds is dominated by semiconductor applications – in integrated circuits, cell phones, laser diodes, light-emitting diodes, photodetectors, and solar cells.^[213] Selenium is used in the production of solar cells^[214] and in high-energy surge protectors.^[215]

Boron, silicon, germanium, antimony, and tellurium,

quantum computing applications.^[221]

Nomenclature and history

Derivation and other names

The word metalloid comes from the

chromium dioxide) or alloy that can act as a conductor and an insulator. "Meta-metal" is sometimes used instead to refer to certain metals (Be, Zn, Cd, Hg, In, Tl, β-Sn, Pb) located just to the left of the metalloids on standard periodic tables.^[225] These metals are mostly diamagnetic^[233] and tend to have distorted crystalline structures, electrical conductivity values at the lower end of those of metals, and amphoteric (weakly basic) oxides.^[234] "Semimetal" sometimes refers, loosely or explicitly, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure. Examples include gallium,^[235] ytterbium,^[236] bismuth^[237] and neptunium.^[238] The names amphoteric element and semiconductor are problematic as some elements referred to as metalloids do not show marked amphoteric behaviour (bismuth, for example)^[239] or semiconductivity (polonium)^[240]

in their most stable forms.

Origin and usage

Main article: Origin and use of the term metalloid

The origin and usage of the term metalloid is convoluted. Its origin lies in attempts, dating from antiquity, to describe metals and to distinguish between typical and less typical forms. It was first applied in the early 19th century to metals that floated on water (sodium and potassium), and then more popularly to nonmetals. Earlier usage in mineralogy, to describe a mineral having a metallic appearance, can be sourced to as early as 1800.^[241] Since the mid-20th century it has been used to refer to intermediate or borderline chemical elements.^{[242][n 23]} The International Union of Pure and Applied Chemistry (IUPAC) previously recommended abandoning the term metalloid, and suggested using the term semimetal instead.^[244] Use of this latter term has more recently been discouraged by Atkins et al.^[2] as it has a different meaning in physics – one that more specifically refers to the electronic band structure of a substance rather than the overall classification of an element. The most recent IUPAC publications on nomenclature and terminology do not include any recommendations on the usage of the terms metalloid or semimetal.^[245]

Elements commonly recognised as metalloids

Properties noted in this section refer to the elements in their most thermodynamically stable forms under ambient conditions.

Boron

Main article: Boron

allotrope)^[246]

Pure boron is a shiny, silver-grey crystalline solid.^[247] It is less dense than aluminium (2.34 vs. 2.70 g/cm³), and is hard and brittle. It is barely reactive under normal conditions, except for attack by fluorine,^[248] and has a melting point of 2076 °C (cf. steel ~1370 °C).^[249] Boron is a semiconductor;^[250] its room temperature electrical conductivity is 1.5 × 10⁻⁶ S•cm^−1[251] (about 200 times less than that of tap water)^[252] and it has a band gap of about 1.56 eV.^{[253][n 24]} Mendeleev commented that, "Boron appears in a free state in several forms which are intermediate between the metals and the nonmmetals."^[255]

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.[256] Metallic bonding is the usual result among the heavier congenors of boron but this generally requires low ionization energies.^[257] Instead, because of its small size and high ionization energies, the basic structural unit of boron (and nearly all of its allotropes)^{[n 25]} is the icosahedral B₁₂ 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.^[259] The same motif can be seen, as are deltahedral variants or fragments, in metal borides and hydride derivatives, and in some halides.^[260]

The bonding in boron has been described as being characteristic of behaviour intermediate between metals and nonmetallic

Silicon is a crystalline solid with a blue-grey metallic lustre.^[284] Like boron, it is less dense (at 2.33 g/cm³) than aluminium, and is hard and brittle.^[285] It is a relatively unreactive element.^[284] According to Rochow,^[286] the massive crystalline form (especially if pure) is "remarkably inert to all acids, including hydrofluoric".^{[n 31]} Less pure silicon, and the powdered form, are variously susceptible to attack by strong or heated acids, as well as by steam and fluorine.^[290] Silicon dissolves in hot aqueous alkalis with the evolution of hydrogen, as do metals^[291] such as beryllium, aluminium, zinc, gallium or indium.^[292] It melts at 1414 °C. Silicon is a semiconductor with an electrical conductivity of 10⁻⁴ S•cm^−1[293] and a band gap of about 1.11 eV.^[287] When it melts, silicon becomes a reasonable metal^[294] with an electrical conductivity of 1.0–1.3 × 10⁴ S•cm⁻¹, similar to that of liquid mercury.^[295]

The chemistry of silicon is generally nonmetallic (covalent) in nature.

Germanium is a shiny grey-white solid.^[309] It has a density of 5.323 g/cm³ and is hard and brittle.^[310] It is mostly unreactive at room temperature^{[n 34]} but is slowly attacked by hot concentrated sulfuric or nitric acid.^[312] Germanium also reacts with molten caustic soda to yield sodium germanate Na₂GeO₃ and hydrogen gas.^[313] It melts at 938 °C. Germanium is a semiconductor with an electrical conductivity of around 2 × 10⁻² S•cm^−1[312] and a band gap of 0.67 eV.^[314] Liquid germanium is a metallic conductor, with an electrical conductivity similar to that of liquid mercury.^[315]

Most of the chemistry of germanium is characteristic of a nonmetal.

Arsenic is a grey, metallic looking solid. It has a density of 5.727 g/cm³ and is brittle, and moderately hard (more than aluminium; less than

Antimony is a silver-white solid with a blue tint and a brilliant lustre.

Tellurium is a silvery-white shiny solid.[386] It has a density of 6.24 g/cm³, is brittle, and is the softest of the commonly recognised metalloids, being marginally harder than sulfur.^[338] Large pieces of tellurium are stable in air. The finely powdered form is oxidized by air in the presence of moisture. Tellurium reacts with boiling water, or when freshly precipitated even at 50 °C, to give the dioxide and hydrogen: Te + 2 H₂O → TeO₂ + 2 H₂.^[387] It reacts (to varying degrees) with nitric, sulfuric, and hydrochloric acids to give compounds such as the sulfoxide TeSO₃ or tellurous acid H₂TeO₃,^[388] the basic nitrate (Te₂O₄H)⁺(NO₃)⁻,^[389] or the oxide sulfate Te₂O₃(SO₄).^[390] It dissolves in boiling alkalis, to give the tellurite and telluride: 3 Te + 6 KOH = K₂TeO₃ + 2 K₂Te + 3 H₂O, a reaction that proceeds or is reversible with increasing or decreasing temperature.^[391]

At higher temperatures tellurium is sufficiently plastic to extrude.

Most of the chemistry of tellurium is characteristic of a nonmetal.[397] It shows some cationic behaviour. The dioxide dissolves in acid to yield the trihydroxotellurium(IV) Te(OH)₃⁺ ion;

Carbon is ordinarily classified as a nonmetal

Like a metal, the conductivity of graphite in the direction of its planes decreases as the temperature is raised;[413]^{[n 43]} it has the electronic band structure of a semimetal.^[413] The allotropes of carbon, including graphite, can accept foreign atoms or compounds into their structures via substitution, intercalation, or doping. The resulting materials are referred to as "carbon alloys".^[417] Carbon can form ionic salts, including a hydrogen sulfate, perchlorate, and nitrate (C⁺
₂₄X⁻.2HX, where X = HSO₄, ClO₄; and C⁺
₂₄NO^–
₃.3HNO₃).^{[418][n 44]} In organic chemistry, carbon can form complex cations – termed carbocations – in which the positive charge is on the carbon atom; examples are CH⁺
₃ and CH⁺
₅, and their derivatives.^[419]

Carbon is brittle,

It has some properties that are unusual for a metal; taken together,[431] these are sometimes used as a basis to classify aluminium as a metalloid.^[432] Its crystalline structure shows some evidence of directional bonding.^[433] Aluminium bonds covalently in most compounds.^[434] The oxide Al₂O₃ is amphoteric^[435] and a conditional glass-former.^[281] Aluminium can form anionic aluminates,^[431] such behaviour being considered nonmetallic in character.^[69]

Classifying aluminium as a metalloid has been disputed^[436] given its many metallic properties. It is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal–nonmetal dividing line are metalloids.^{[437][n 46]}

Stott

Selenium shows borderline metalloid or nonmetal behaviour.[443]^{[n 47]}

Its most stable form, the grey

The nonmetallic character of selenium is shown by its brittleness[447] and the low electrical conductivity (~10⁻⁹ to 10⁻¹² S•cm⁻¹) of its highly purified form.^[93] This is comparable to or less than that of bromine (7.95×10^–12 S•cm⁻¹),^[453] a nonmetal. Selenium has the electronic band structure of a semiconductor^[454] and retains its semiconducting properties in liquid form.^[454] It has a relatively high^[455] electronegativity (2.55 revised Pauling scale). Its reaction chemistry is mainly that of its nonmetallic anionic forms Se²⁻, SeO²⁻
₃ and SeO²⁻
₄.^[456]

Selenium is commonly described as a metalloid in the environmental chemistry literature.^[457] It moves through the aquatic environment similarly to arsenic and antimony;^[458] its water-soluble salts, in higher concentrations, have a similar toxicological profile to that of arsenic.^[459]

Polonium

Polonium is "distinctly metallic" in some ways.^[240] Both of its allotropic forms are metallic conductors.^[240] It is soluble in acids, forming the rose-coloured Po²⁺ cation and displacing hydrogen: Po + 2 H⁺ → Po²⁺ + H₂.^[460] Many polonium salts are known.^[461] The oxide PoO₂ is predominantly basic in nature.^[462] Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly reducing conditions are required for the formation of the Po²⁻ anion in aqueous solution.^[463]

Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants.^[464] It has a simple cubic crystalline structure. Such a structure has few slip systems and "leads to very low ductility and hence low fracture resistance".^[465]

Polonium shows nonmetallic character in its halides, and by the existence of

As a

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.[476]^{[n 49]} Astatine may be metallic in the liquid state on the basis that elements with an enthalpy of vaporization (∆H_vap) greater than ~42 kJ/mol are metallic when liquid.^[478] Such elements include boron,^{[n 50]} silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆H_vap of diatomic astatine are 50 kJ/mol or higher;^[482] diatomic iodine, with a ∆H_vap of 41.71,^[483] falls just short of the threshold figure.

"Like typical metals, it [astatine] is precipitated by

Restrepo et al.[495] 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.

Related concepts

Near metalloids

In the periodic table, some of the elements adjacent to the commonly recognised metalloids, although usually classified as either metals or nonmetals, are occasionally referred to as near-metalloids[499] or noted for their metalloidal character. To the left of the metal–nonmetal dividing line, such elements include gallium,^[500] tin^[501] and bismuth.^[502] They show unusual packing structures,^[503] marked covalent chemistry (molecular or polymeric),^[504] and amphoterism.^[505] To the right of the dividing line are carbon,^[506] phosphorus,^[507] selenium^[508] and iodine.^[509] They exhibit metallic lustre, semiconducting properties^{[n 53]} and bonding or valence bands with delocalized character. This applies to their most thermodynamically stable forms under ambient conditions: carbon as graphite; phosphorus as black phosphorus;^{[n 54]} and selenium as grey selenium.

Allotropes

Different crystalline forms of an element are called allotropes. Some allotropes, particularly those of elements located (in periodic table terms) alongside or near the notional dividing line between metals and nonmetals, exhibit more pronounced metallic, metalloidal or nonmetallic behaviour than others.^[515] The existence of such allotropes can complicate the classification of the elements involved.^[516]

Tin, for example, has two allotropes:

The diamond allotrope of carbon is clearly nonmetallic, being translucent and having a low electrical conductivity of 10⁻¹⁴ to 10⁻¹⁶ S·cm⁻¹.[523] Graphite has an electrical conductivity of 3 × 10⁴ S·cm⁻¹,^[524] a figure more characteristic of a metal. Phosphorus, sulfur, arsenic, selenium, antimony, and bismuth also have less stable allotropes that display different behaviours.^[525]

Abundance, extraction, and cost

Abundance

The table gives

The recognised metalloids and their closer neighbours mostly cost less than silver; only polonium and astatine are more expensive than gold, on account of their significant radioactivity. As of 5 April 2014, prices for small samples (up to 100 g) of silicon, antimony and tellurium, and graphite, aluminium and selenium, average around one third the cost of silver (US$1.5 per gram or about $45 an ounce). Boron, germanium, and arsenic samples average about three-and-a-half times the cost of silver.[n 55] Polonium is available for about $100 per microgram.^[535] Zalutsky and Pruszynski^[536] estimate a similar cost for producing astatine. Prices for the applicable elements traded as commodities tend to range from two to three times cheaper than the sample price (Ge), to nearly three thousand times cheaper (As).^{[n 56]}

Notes