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Metalloid Information

A metalloid is a chemical element with properties that are in-between[2] or a mixture[3] of those of metals and nonmetals, and which is considered to be difficult to classify unambiguously[4] as either a metal or a nonmetal.[5][6][n 2] There is no universally agreed or rigorous definition of a metalloid.[10][11] Classifying any particular element as such has been described as 'arbitrary'.[12] The term was first popularly used to refer to nonmetals. Its more recent meaning as a category of intermediate or hybrid elements did not become widespread until the period 1940–1960. The six elements commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. They or their compounds find uses in glasses, alloys or semiconductors.

Contents

Elements commonly recognized as metalloids

This section includes brief sketches of the physical and chemical properties of the applicable elements. For complete profiles, including occurrence, production, history, biological role and precautions, see the main article for each element.

Consistent with the list of metalloid lists, the following elements are commonly classified as metalloids:[10][11][13][14][15][16][n 3]

One or more from among selenium, polonium or astatine are sometimes added to the list.[11][18][19] Boron is sometimes excluded from the list, by itself or together with silicon.[20][21] Tellurium is sometimes not regarded as a metalloid.[22] The inclusion of antimony, polonium and astatine as metalloids has also been questioned.[11][23][24]

Boron

Main article: Boron Boron

In its most stable state, pure boron appears as a shiny, silver-grey crystalline solid.[25][26][n 4] It is about ten percent lighter than aluminium but, unlike the latter,[30] is hard and brittle. It is barely reactive under normal conditions, except for attack by fluorine,[31] and has a melting point several hundred degrees higher than that of steel.

Boron is a semiconductor,[32] with a room temperature electrical conductivity of 1.5 × 10−6 S•cm−1 [33] and a band gap of about 1.56 eV.[34]

The chemistry of boron is dominated by its small size, relatively high ionization energy, and having fewer valence electrons (three) than atomic orbitals (four) available for bonding. With only three valence electrons, simple covalent bonding will be electron deficient with respect to the octet rule.[35] Elements in this situation usually adopt metallic bonding. However, the small size and high ionization energies of boron tends to result in delocalized covalent bonding,[36][37] in which three atoms share two electrons, rather than metallic bonding. The associated structural component which pervades the various allotropes of boron is the icosahedral B12 unit. This also occurs, as do deltahedral variants or fragments, in several metal borides, certain hydrides, and some halides.[38][39][40]

The bonding in boron has been described as being characteristic of behaviour intermediate between metals and nonmetallic covalent network solids (a classic example of the latter being diamond).[41] 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 gives an idea of how close boron is to the metal-nonmetal borderline.[42]

The small size of the boron atom enables the preparation of many interstitial alloy-type borides.[43]

The aqueous chemistry of boron is characterised by the formation of many different polyborate anions.[44][45][46][47]

Given its high charge-to-size ratio nearly all compounds of boron are covalent, barring some complexed anionic and cationic species.[48][49] Boron has a strong affinity for oxygen, a characteristic manifested in the extensive chemistry of the borates.[43]

The oxide B2O3 is polymeric in structure,[50] weakly acidic,[51] and a glass former.[52] Organometallic compounds of boron have been known since the 19th century (see organoboron chemistry).[53]

Silicon

Main article: Silicon Silicon

Silicon appears as a shiny crystalline solid, with a blue-grey metallic lustre.[54] As with boron it is about ten per cent lighter than aluminium, hard and brittle.[55] It is a relatively unreactive element.[54] Although it is oxidized by nitric acid, the resulting thin surface layer of SiO2 prevents further corrosion.[56][57] It also dissolves in hot aqueous alkalis with the evolution of hydrogen, behaving in this way like metals[58] such as beryllium, aluminium, zinc and gallium.[59] It melts at about the same temperature as steel.

Silicon is a semiconductor with an electrical conductivity of 10−4 S•cm−1 [60] and a band gap of about 1.11 eV.[61] When it melts, silicon becomes a reasonable metal[62] with an electrical conductivity of 1.0–1.3 × 104S•cm−1, a value similar to that of liquid mercury.[63][64]

Silicon forms alloys with metals such as iron and copper.[65]

Silicon shows fewer tendencies to anionic behaviour than ordinary nonmetals.[66] Its solution chemistry is characterised by the formation of oxyanions.[47] The chemistry of silicon is generally nonmetallic (covalent) in nature[67] and dominated by the high bond strength of the silicon-oxygen bond.[68] Polymeric silicates, built up by tetrahedral SiO4 units sharing their oxygen atoms, represent the most abundant and important compounds of silicon.[69] The polymeric borates, comprising linked trigonal and tetrahedral BO3 or BO4 units, are built on similar structural principles.[70]

The oxide SiO2 is polymeric in structure,[50] weakly acidic,[71][n 5] and a glass former.[52] Traditional organometallic chemistry includes the carbon compounds of silicon (see organosilicon).[74]

Germanium

Main article: Germanium Germanium

Germanium appears as a shiny grey-white solid.[75] It is about one-third lighter than iron, hard and brittle.[76] It is mostly unreactive at room temperature[n 6] but is slowly attacked by hot concentrated sulphuric or nitric acid.[78] Germanium also reacts with molten caustic soda to yield sodium germanate Na2GeO3, together with the evolution of hydrogen.[79] It melts at a temperature around one-third less than that of steel.

Germanium is a semiconductor with an electrical conductivity of around 2 × 10−2 S•cm−1 [78] and a band gap of 0.67 eV.[80] Liquid germanium is a metallic conductor, with an electrical conductivity on par with that of liquid mercury.[81]

Most of the chemistry of germanium is characteristic of a nonmetal.[82] It does however form alloys with, for example, aluminium and gold.[83] Germanium shows fewer tendencies to anionic behaviour than ordinary nonmetals.[66] Its solution chemistry is characterised by the formation of oxyanions.[47]

Germanium generally forms tetravalent (IV) compounds, although it can also form a smaller number of less stable divalent (II) compounds, in which it behaves more like a metal.[84][85] Germanium analogues of all of the major types of silicates have been prepared.[86]

The metallic character of germanium is also suggested by the formation of various oxoacid salts. A phosphate [(HPO4)2Ge.H2O] and highly stable trifluoroacetate Ge(OCOCF3)4 have been described, as have Ge2(SO4)2, Ge(ClO4)4 and GeH2(C2O4)3.[87][88]

The oxide GeO2 is polymeric,[50] amphoteric,[89] and a glass former. [52] Germanium has an established organometallic chemistry (see organogermanium chemistry).[90]

Arsenic

Main article: Arsenic Arsenic

Arsenic is a grey, metallic looking solid. It is about one-third lighter than iron, brittle, and moderately hard (more than aluminium; less than iron).[91] It is stable in dry air but develops a golden bronze platina in moist air, which blackens on further exposure. Arsenic is attacked by nitric acid and concentrated sulphuric acid. It reacts with fused caustic soda to give the arsenate Na3AsO3, together with the evolution of hydrogen.[92] Arsenic sublimes, rather than melts, at around forty per cent of the melting point of steel. The vapour is lemon-yellow and smells like garlic.[93] Arsenic only melts under a pressure of 38.6 atm, at around half the melting point of steel.[94]

Arsenic is a semimetal with an electrical conductivity of around 3.9 × 104 S•cm−1 [95] and a band overlap of 0.5 eV.[96] Liquid arsenic is a semiconductor with a band gap of 0.15 eV.[97][98]

The chemistry of arsenic is predominately nonmetallic in character.[99] It does however form alloys with many metals, most of these being brittle.[100] Arsenic shows fewer tendencies to anionic behaviour than ordinary nonmetals.[66] Its solution chemistry is characterised by the formation of oxyanions.[47]

Arsenic generally forms compounds in which it has an oxidation state of +3 or +5.[101] The halides, and the oxides and their derivatives are illustrative examples.[102] In the trivalent state, arsenic shows some incipient metallic properties.[103] Thus, the halides are hydrolysed by water but these reactions, particularly those of the chloride, are reversible with the addition of a hydrohalic acid.[104][105] As well, and as noted below, the oxide is acidic but weakly amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties.[106][107] More generally, and compared to phosphorus, the stronger metallic character of arsenic is indicated by the formation of oxoacid salts such as AsPO4, As2(SO4)3 and arsenic acetate As(CH3COO)3.[108][109][110][111]

The oxide As2O3 is polymeric,[50] amphoteric,[112][113][114][n 7] and a glass former.[52] Arsenic has an extensive organometallic chemistry (see organoarsenic chemistry).[119]

Antimony

Main article: Antimony Antimony

Antimony appears as a silver-white solid with a blue tint and a brilliant lustre.[120] It is about 15 per cent lighter than iron, brittle, and moderately hard (more so than arsenic; less so than iron; about the same as copper).[121]

It is stable in air, and moisture, at room temperature. It is attacked by: concentrated nitric acid, yielding the hydrated pentoxide Sb2O5; aqua regia, giving the pentachloride SbCl5; and (hot) concentrated sulphuric acid, resulting in the sulphate Sb2(SO4)3.[122] It is not affected by molten alkali.[123] Antimony is capable of displacing hydrogen from water, when heated: 2Sb + 3H2O → Sb2O3 + 3H2.[124] It melts at a temperature around half that of steel.

Antimony is a semimetal with an electrical conductivity of around 3.1 × 104 S•cm−1 [125] and a band overlap of 0.16 eV.[126] Liquid antimony is a metallic conductor with an electrical conductivity of around 5.3 × 104 S•cm−1.[127][128]

Most of the chemistry of antimony is characteristic of a nonmetal.[129] It does however form alloys with one or more metals such as aluminium,[130] iron, nickel, copper, zinc, tin, lead and bismuth.[131] Antimony shows fewer tendencies to anionic behaviour than ordinary nonmetals.[66] Its solution chemistry is characterised by the formation of oxyanions.[47]

Like arsenic, antimony generally forms compounds in which it has an oxidation state of +3 or +5.[101] The halides, and the oxides and their derivatives are illustrative examples.[102] The +5 state is less stable than the +3, but relatively easier to attain than is the case with arsenic. This is on account of the poor shielding afforded the arsenic nucleus by its 3d10 electrons. In comparison, the tendency of antimony to be oxidized more easily partially offsets the effect of its 4d10 shell.[132][133] Tripositive antimony is amphoteric; quinquepositive antimony is (predominately) acidic.[134]

Consistent with an increase in metallic character down Group 15, antimony forms salts or salt-like compounds including a nitrate Sb(NO3)3, phosphate SbPO4, sulfate Sb2(SO4)3 and perchlorate Sb(ClO4)3.[135] The otherwise acidic pentoxide Sb2O5 also shows some basic (metallic) behaviour in that it can be dissolved in very acidic solutions, with the formation of the oxycation SbO+ 2.[136]

The oxide Sb2O3 is a polymeric,[50] amphoteric,[137] and a glass former.[52] Antimony has an extensive organometallic chemistry (see organoantimony chemistry).[138]

Tellurium

Main article: Tellurium Tellurium

Tellurium appears as a silvery-white solid with a shiny lustre.[139] It is about 15 per cent lighter than iron, brittle, and the softest of the commonly recognised metalloids, being marginally harder than sulphur.[140] Massive tellurium is 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 + 2H2O → TeO2 + 2H2.[141] It reacts (to varying degrees) with, or combinations of, nitric, sulphuric and hydrochloric acids to give compounds such as the sulphoxide TeSO3 or tellurous acid H2TeO3,[142] the basic nitrate (Te2O4H)+(NO3)–,[143][144] or the oxide sulphate Te2O3(SO4).[145] It dissolves in boiling alkalis, with the formation of the tellurite and telluride: 3Te + 6KOH = K2TeO3 + 2K2Te +3H2O, a reaction which proceeds or is reversible with increasing or decreasing temperature.[146] At higher temperatures tellurium is sufficiently plastic to be extrudable.[147] It melts at a temperature of around thirty per cent that of steel.

Tellurium is a semiconductor with an (intrinsic) electrical conductivity of around 1.0 S•cm−1 [148] and a band gap of 0.32 to 0.38 eV.[149] Liquid tellurium is a semiconductor, with an electrical conductivity, on melting, of around 1.9 × 103 S•cm−1 [149] Superheated liquid tellurium is a metallic conductor.[150]

Crystalline tellurium has a structure consisting of parallel infinite spiral chains. Whereas the bonding between adjacent atoms in a chain is covalent, there is evidence of a weak metallic interaction between the neighbouring atoms of different chains.[151][152][153]

Mendeleev said of tellurium that:

In a free state [it] has a perfectly metallic appearance; it is of a silver-white colour, crystallises very easily in long brilliant needles; is very brittle, so that it can be easily reduced to powder; but it is a bad conductor of heat and electricity, and in this respect, as in many others, it forms a transition from the metals to the nonmetals.[154]

Most of the chemistry of tellurium is characteristic of a nonmetal.[155] It does however form alloys with, for example, aluminium, silver and tin.[156][157] Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals.[66] Its solution chemistry is characterised by the formation of oxyanions.[47]

Tellurium generally forms compounds in which it has an oxidation state of –2, +4 or +6, with the tetrapositive state being the most stable.[141] It combines easily with most other elements to form binary tellurides XxTey these representing the most common mineral form. Non-stoichiometry is frequently encountered. This is particularly so with the transition metals, where electronegativity differences are small and irregular valency is favoured. Many of the associated tellurides can be treated as metallic alloys.[158]

The increase in metallic character evident in tellurium, as compared to the lighter chalcogens, is further reflected in the reported formation of various other oxyacid salts, such as a basic selenate 2TeO2.SeO3 and an analogous perchlorate and periodate 2TeO2.HXO4.[159][160]

Tellurium forms a polymeric,[50] amphoteric,[161] glass-forming oxide[52] TeO2. The latter is also a 'conditional' glass-forming oxide—it will form a glass with a very small amount of additive.[52] Tellurium has an extensive organometallic chemistry (see organotellurium chemistry).[162]

Typical applications

For prevalent and speciality applications of individual metalloids see the main article for each element.

Metalloids such as boron, arsenic and antimony are too brittle to have any structural uses in their pure forms.[163][164]

Typical applications of the elements commonly recognized as metalloids have instead encompassed:

Glass formation

The oxides B2O3, SiO2, GeO2, As2O3 and Sb2O3 readily form glasses. TeO2 will also form a glass but this requires a 'heroic quench rate' or the addition of an impurity. Otherwise the crystalline form results.[166]

These compounds have found or continue to find practical uses in chemical, domestic and industrial glassware[167][168] and optics (especially Ge and Te).[169][170]

Alloys

In 1914 Desch[171] wrote that 'certain non-metallic elements are capable of forming compounds of distinctly metallic character with metals, and these elements may therefore enter into the composition of alloys'. He associated silicon, arsenic and tellurium—in particular—with the alloy-forming elements. Phillips and Williams[172] later noted that compounds of silicon, germanium, arsenic and antimony with the poor metals, 'are probably best classed as alloys'.

Boron can form intermetallic compounds and alloys with transition metals, of the composition MnB, if n > 2.[173]

Sanderson[174] commented that silicon 'is metalloid in nature, appearing quite metallic in its ability to alloy with metals.'

Germanium forms many alloys, most importantly with the coinage metals.[175]

Arsenic can form alloys with metals, including platinum and copper.[176]

Antimony is well known as an alloy former. This is exemplified by type metal (a lead alloy with up to 25%, by weight, antimony) and pewter (a tin alloy with up to 20% antimony).[177]

In 1973 the US Geological Survey reported that about 18% of tellurium production was sold in alloy form. Copper tellurium (40–50% tellurium) was one type; ferrotellurium (50–58% tellurium) the other.[178]

Semiconductors and electronics

All the elements commonly recognized as metalloids or their compounds have found application in the semiconductor or solid-state electronic industries.[179][180] Some properties of boron have retarded its use as a semiconductor. It has a high melting point and single crystals are relatively hard to obtain. Introducing and retaining controlled impurities is also difficult.[181][182]

Elements less commonly recognized as metalloids

There is no universally agreed or rigorous definition of the term metalloid. So the answer to the question "Which elements are metalloids?" can vary, depending on the author and their inclusion criteria. Emsley,[183] for example, recognized only four: germanium, arsenic, antimony and tellurium. Selwood,[184] on the other hand, listed twelve: boron, aluminium, silicon, gallium, germanium, arsenic, tin, antimony, tellurium, bismuth, polonium, and astatine.

The absence of a standardized division of the elements into metals, metalloids and nonmetals is not necessarily an issue. There is a more or less continuous progression from the metallic to the nonmetallic. Any subset of this continuum can potentially serve its particular purpose as well as any other.[185]

In any event, individual metalloid classification arrangements tend to share common ground (as described above) with most variations occurring around the indistinct[186][187] margins, as surveyed below.[n 9]

Carbon

Main article: Carbon

Carbon is ordinarily classified as a nonmetal[189] although it has some metallic properties and is occasionally classified as a metalloid.[190][191][192] Where applicable, properties listed below are for hexagonal graphitic carbon, the most thermodynamically stable form of carbon under ambient conditions.[193][194]

In terms of the metallic character of carbon:

In terms of the nonmetallic character of carbon:

Aluminium

Main article: Aluminium

Aluminium is ordinarily classified as a metal, given its lustre, malleability and ductility, high electrical and thermal conductivity and close-packed crystalline structure.

It does however have some properties that are unusual for a metal. Taken together,[212] these properties are sometimes used as a basis to classify aluminium as a metalloid:[213][214]

Stott[222] labels aluminium as weak metal. It has the physical properties of a good metal but some of the chemical properties of a nonmetal. Steele[223] notes the somewhat paradoxical chemical behaviour of aluminium. It resembles a weak metal with its amphoteric oxide and the covalent character of many of its compounds. Yet it is also a strongly electropositive metal, with a high negative electrode potential.

The notion of aluminium as a metalloid is sometimes disputed[224][225][226] given it has many metallic properties. Aluminium is therefore argued to be an exception to the mnemonic that elements adjacent to the metal-nonmetal dividing line are metalloids.[24][n 12]

Selenium

Main article: Selenium

Selenium shows borderline metalloid or nonmetal behaviour.[228][229][n 13]

Its most stable form, the grey trigonal allotrope, is sometimes called 'metallic' selenium. This is because its electrical conductivity is several orders of magnitude greater than that of the red monoclinic form.[232]

The metallic character of selenium is further shown by the following properties:

The nonmetallic character of selenium is shown by:

Polonium

Main article: Polonium

Polonium is 'distinctly metallic' in some ways,[247] or shows metallic character by way of:

However, polonium shows nonmetallic character in that:

Astatine

Main article: Astatine

Astatine may be a nonmetal or a metalloid.[260] It is ordinarily classified as a nonmetal,[23][24][261][262] but has some 'marked' metallic properties.[263] Immediately following its production in 1940, early investigators considered it to be a metal.[264] In 1949 it was called the most noble (difficult to reduce) nonmetal as well as being a relatively noble (difficult to oxidize) metal.[265] In 1950 astatine was described as a halogen and (therefore) a reactive nonmetal.[266]

In terms of metallic indicators:

For nonmetallic indicators:

Restrepo et al.[285][286] reported that astatine appeared to share more in common with polonium than it did with the established halogens. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.

Other metalloids

Some other elements are occasionally classified as metalloids, given there is no agreed definition of same. These elements include[287] hydrogen,[288][289][290] beryllium,[291] nitrogen,[292] phosphorus,[192][293] sulfur,[192][294][295] zinc,[296] gallium,[297] tin, iodine,[292][298] lead,[299] bismuth[22] and radon.[300][301][302]

The term metalloid has also been used to refer to:

Near metalloids

The concept of a class of elements intermediate between metals and nonmetals is sometimes extended to include elements that most chemists, and related science professionals, would not ordinarily recognize as metalloids.

In 1935, Fernelius and Robey[309] allocated carbon, phosphorus, selenium, and iodine to such an intermediary class of elements, together with boron, silicon, arsenic, antimony, tellurium and polonium. They also included a placeholder for the missing element 85 (five years ahead of its production in 1940, as astatine). Germanium was excluded as it was still then regarded as a poorly conducting metal.[1]

In 1954, Szabó & Lakatos[310] counted beryllium and aluminium in their list of metalloids. This was in addition to boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine.

In 1957, Sanderson[311][n 16] recognized carbon, phosphorus, selenium, and iodine as part of an intermediary class of elements with 'certain metallic properties'. Boron, silicon, arsenic, tellurium, and astatine also belonged to this class. Germanium, antimony and polonium were classified as metals.

More recently, in 2007, Petty[315] included carbon, phosphorus, selenium, tin and bismuth in his list of metalloids. Boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine were treated similarly.

Elements such as these are occasionally called, or described as, near-metalloids,[316][317] or the like. They are located near the elements commonly recognized as metalloids, and usually classified as either metals or nonmetals.

Metals falling into this loose category tend to show 'odd' packing structures,[318] marked covalent chemistry (molecular or polymeric),[319] and amphoterism.[320][321] Aluminium, tin and bismuth are examples. They are also referred to as (chemically) weak metals,[322][323] poor metals,[324][325] post-transition metals,[326][327][n 17] or semimetals (in the aforementioned sense of metals with incomplete metallic character). These classification groupings generally cohabit the same periodic table territory but are not necessarily mutually inclusive.

Nonmetals in this category include carbon,[328][329] phosphorus,[330][331][332][333][334] selenium[229][335][336][337] and iodine.[338][339][340] They exhibit metallic lustre, semiconducting properties[n 18] 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 19] and selenium as grey selenium. These elements are alternatively described as being 'near metalloidal', showing metalloidal character, or having metalloid-like or some metalloid(al) or metallic properties.

Allotropes

Some allotropes of the elements exhibit more pronounced metallic, metalloidal or nonmetallic behaviour than others. For example, the diamond allotrope of carbon is clearly nonmetallic. The graphite allotrope however displays limited electrical conductivity[347] more characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth also have allotropes that display borderline or either metallic or nonmetallic behaviour.[348][349][350]

Categorization and periodic table territory

Metalloids are generally regarded as a third category of chemical elements, alongside metals and nonmetals.[351] They have been described as forming a (fuzzy) buffer zone between metals and nonmetals. The make-up and size of this zone depends on the classification criteria being used.[n 20] Metalloids are sometimes grouped instead with metals,[361][362] regarded as nonmetals[363] or treated as a sub-category of same.[364][365][366][367][368][n 21]

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 Uut Uuq Uup Uuh Uus Uuo
Condensed periodic table showing distribution of elements that have sometimes[n 22] been classified as metalloids. Elements with grey shading (B, C, Al, Si, Ge, As, Se, Sb, Te, Po, At) appear commonly to rarely in the list of metalloid lists. Elements with light tan shading (H, Be, P, S, Ga, Sn, Pb, Bi, Uuq, Uup, Uuh, Uus) appear still less frequently. Elements with pale blue shading (N, Zn, Rn) are outliers that 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).

Metalloids cluster on either side of the dividing line between metals and nonmetals. This can be found, in varying configurations, on some periodic tables (see mini-example, right). 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 stair-step, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.[370]

The diagonal positioning of the metalloids represents somewhat of an exception to the phenomenon that elements with similar properties tend to occur in vertical columns.[371] Going across a periodic table row, the nuclear charge increases with atomic number just as there is as a corresponding increase in electrons. The additional 'pull' on outer electrons with increasing nuclear charge generally outweighs the screening efficacy 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.[372][373][374] Going down a main group periodic table column, the effect of increasing nuclear charge is generally outweighed by the effect of additional electrons being further away from the nucleus. With some irregularities, atoms therefore become larger, ionization energy falls, and metallic character increases.[373][374] The combined effect of these competing horizontal and vertical trends is that the location of the metal-nonmetal transition zone shifts to the right in going down a period.[371] A related effect can be seen in other diagonal similarities that occur between some elements and their lower right neighbours, such as lithium-magnesium, beryllium-aluminum, carbon-phosphorus, and nitrogen-sulfur.[375]

Metal-nonmetal dividing line

The dividing line between metals and nonmetals has been called the metal-nonmetal line,[376] the metalloid line,[377][378] the semimetal line,[379] the Zintl border [380] or the Zintl line.[381][382][n 23] The last two terms also refer to a vertical line sometimes drawn between groups 13 and 14. This line was christened by Laves in 1941.[384] It differentiates group 13 boron elements from those in and to the right of group 14 (the carbon elements). The former generally combine with electropositive metals to make intermetallic compounds whereas the latter usually form salt-like compounds.[385]

References to a dividing line between metals and nonmetals appear in the literature as far back as at least 1869.[386]

In 1891, Walker published a periodic 'tabulation' with a diagonal straight line drawn between the metals and the nonmetals.[387]

In 1906, Alexander Smith published a periodic table with a zigzag line separating the nonmetals from the rest of elements, in his highly influential[388] textbook Introduction to General Inorganic Chemistry.[389]

In 1923, Deming, an American chemist, published short (Mendeleev style) and medium (18-column) form periodic tables.[390] Each one had a regular stepped line separating metals from nonmetals. Merck and Company prepared a handout form of Deming's 18-column table, in 1928, which was widely circulated in American schools. By the 1930s Deming's table was appearing in handbooks and encyclopaedias of chemistry. It was also distributed for many years by the Sargent-Welch Scientific Company.[391][392][393]

Relevance

Some authors do not classify elements bordering the metal-nonmetal dividing line as metalloids. They instead note, for example, that such elements to the left of the line 'show some nonmetallic character'. Those on the right, in contrast, 'show some metallic character'.[221] A binary classification can also facilitate the establishment of some simple rules for determining bond types between metals and/or nonmetals.[351]

Other authors have suggested that classifying some elements as metalloids 'emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table'.[394]

A dividing line between metals and nonmetals is sometimes replaced by two dividing lines. One line separates metals and metalloids; the other metalloids and nonmetals.[394][395]

Some periodic tables distinguish elements that are metalloids in the absence of any formal dividing line between metals and nonmetals. Metalloids are instead shown as occurring in a diagonal fixed band[396] or diffuse region,[397] running from upper left to lower right, centred around arsenic.

Concerns

Mendeleev wrote that, 'It is...impossible to draw a strict line of demarcation between metals and nonmetals, there being many intermediate substances'. [398][n 24]

Several other sources note confusion or ambiguity as to the location of the dividing line;[400][401] suggest its apparent arbitrariness[402] provides grounds for refuting its validity;[351] and comment as to its misleading, contentious or approximate nature.[11][403][404]

Deming himself noted that the line could not be drawn very accurately.[405]

Comparison of properties with those of metals and nonmetals

The following two subsections summarize and tabulate the physical and chemical properties of metalloids. Properties of metals and nonmetals are also shown, for comparative purposes.[406] Shading to either side of the metalloid column denotes immediately apparent commonalities, as compiled after the end of each table.

Physical properties

Metalloids are metallic looking solids that have a brittle comportment, show intermediate to relatively good electrical conductivity, and have the band structure of a semimetal or semiconductor. Most of their other physical properties are intermediate in nature. Applicable properties are set out in the following table, in loose order of ease of determination:

Physical properties of metals, metalloids, and nonmetals
Property Metals Metalloids Nonmetals
Form solid; a few liquid at or near room temperature (Ga, Hg, Cs, Fr)[407][408] solid[409] mostly gases[410]
Appearance characteristic lustre metallic lustre[409] colourless, red, yellow, green, black, or intermediate shades[411]
Allotropy many show metallic allotropes; Sn, Bi have semiconducting allotropes tend to exist in several (conspicuously)[412] 'metallic' and nonmetallic allotropic forms[413] show nonmetallic allotropy (O, S), with elements close to the metal-nonmetal line (C, P, Se) showing more 'metallic' allotropes
Density generally high, with some exceptions such as the alkali metals[414] densities lower than neighbouring poor metals but higher than those of neighbouring nonmetals[368] often low
Elasticity typically elastic, ductile, malleable (when solid) brittle[415] brittle (when solid)
Electrical conductivity good to high[n 25] intermediate[418] to good[n 26] poor to good[n 27]
Temperature coefficient of resistance[n 28] nearly all positive (Pu is negative)[424] negative (B, Si, Ge, Te)[425] or positive (As, Sb)[426] nearly all negative (C, as graphite, is positive in the direction of its planes)[427][428]
Thermal conductivity medium to high[429] mostly intermediate;[415][430] Si is high almost negligible[431] to very high[432]
Packing close-packed crystal structures; high coordination numbers have relatively open crystal structures, with medium coordination numbers,[433] in contrast to the close-packed crystal structures of metals[434] low coordination numbers
Melting behaviour volume generally expands[435] some contract, unlike (most)[436] metals[437] volume generally expands[435]
Enthalpy of fusion may be high often have abnormally high enthalpy of fusion values[438] (compared to other close-packed metals)[439] often low
Liquid electrical conductivity[269] metallic most exhibit metallic conductivity in liquid form[440][441] nonmetallic
Periodic table block s, p, d, f [442] p [443] s, p [443]
Band structure metallic (Bi = semimetallic) are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms[366][444] semiconductor or insulator[279]
Electron behaviour "free" electrons • valence electrons not as freely delocalized as in metals; considerable covalent bonding present[445] • have Goldhammer-Herzfeld criterion[n 29] ratios straddling unity[440][450] no "free" electrons

Of the fifteen physical properties listed in the above table, four (form; appearance; enthalpy of fusion; and liquid electrical conductivity) are shared with metals and one (elasticity) with nonmetals. The other ten properties are characteristic, by and large, of metalloids.

Chemical properties

Metalloids generally behave chemically as (weak) nonmetals, and have intermediate ionization energies and electronegativities, and amphoteric or weakly acidic oxides. Most of their other chemical properties are intermediate in nature. Applicable properties—general, specific and descriptive—are set out in the following table:

Chemical properties of metals, metalloids, and nonmetals
Property Metals Metalloids Nonmetals
General behaviour metallic nonmetallic[451] nonmetallic
Ionization energy relatively low intermediate ionization energies,[452] usually falling between those of metals and nonmetals[453] high
Electronegativity usually low have electronegativity values close to 2[454] (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)[17][n 30] high
Ion formation tend to form cations • have a reduced tendency to form anions in water, when compared to ordinary nonmetals[66] • solution chemistry is dominated by the formation and reactions of oxyanions[47][457] tend to form anions
Bonds seldom form covalent can form salts as well as covalent compounds[458] form many covalent
Oxidation number nearly always positive positive or negative[459] positive or negative
When mixed with metals give alloys can form alloys[413][458][460] ionic or interstitial compounds formed
Oxides • lower oxides are ionic and basic • higher oxides are increasingly covalent and acidic • very few glass formers[461] • polymeric in structure;[462] tend to be amphoteric or weakly acidic[409][463] • are glass formers (B, Si, Ge, As, Sb, Te)[464] • covalent, acidic • few glass formers (P, S, Se)[52]
Halides, esp. chlorides (see also[465][466]) • ionic, involatile • mostly water soluble (not hydrolysed) • higher halides and those of weaker metals[467] have greater covalency and volatility, and are more or less prone to hydrolysis (layer-lattice types often reversibly so)[468] and dissolution in organic solvents • covalent, volatile[469] • some partly reversibly hydrolysed[470][471][472] • usually dissolve in organic solvents[473][474] • covalent, volatile • most irreversibly[475] hydrolysed by water • usually dissolve in organic solvents
Hydrides • active metals (alkali and alkaline earth metals) form ionic, solid hydrides with high melting points; • transition metals form metallic hydrides; • poor metals form covalent hydrides covalent, volatile hydrides[476] covalent, gaseous or liquid hydrides
Sulfates do form[n 31][n 32] most form[n 33] some form[n 34]
Organometallic compounds many form such can form[500] not formed

Of the twelve chemical properties listed in the above table, two (when mixed with metals; organometallic compounds) are shared with metals and three (general behaviour; ion formation; and oxidation number) with nonmetals. The other seven properties are characteristic of metalloids. However, as noted at the start of the categorization section, some authors count metalloids as nonmetals or a sub-category of same. In this case most of the chemical properties of metalloids would be regarded as nonmetallic in nature, albeit weakly so.[n 35]

Distinctive properties

Some of the above physical and chemical properties have been cited as benchmark or stand-out indicators of metal, metalloid or nonmetal status.

Metals. Distinctive and invariable properties of (bulk) metals are their lustrous appearance (at least when freshly fractured), their relatively high conductivity of heat and electricity,[505] and their capacity to form mixtures (alloys) with other metals.[506]

Metalloids. Brittleness[507][508] or semiconductivity[509] or both[510] have been cited or used as distinguishing indicators of metalloid status. Metallic lustre together with very marked dualistic chemical behaviour—by way of, for example, amphoteric oxides—has also been cited as a benchmark.[511]

The concepts of metalloid and semiconductor should not be confused. 'Metalloid' is chemistry-based concept referring to the physical (including electronic) and chemical properties of certain periodic table elements. 'Semiconductor' is a physics-based concept referring to the electronic properties of materials (including elements and compounds).[512] Not all elements classified in the literature as metalloids display semiconductivity, although most do.[513]

Although metalloids are all reckoned to be solid[514] and have metallic lustre, their other properties are said to vary.[515] Hawkes[11] suggests judging metalloid status separately for each element, given metallic character is a combination of several properties. This could be done based on the extent to which they exhibit the properties relevant to such status.

Nonmetals. More notable properties of (unambiguous) nonmetals are that they have a dull, non-lustrous, or colourless appearance; are poor conductors of electricity (compared to the metallic elements); and that (if solid) they are brittle.[516] As well, no nonmetal forms a decidedly basic oxide.[517]

Semi-quantitative description

Element IE EN Band structure
Boron 191 2.04 semiconductor
Silicon 187 1.90 same
Germanium 182 2.01 same
Arsenic 225 2.18 semimetal
Antimony 198 2.05 same
Tellurium 207 2.10 semiconductor
average 198 2.05
The elements commonly recognized as metalloids, and their ionization energies (kcal/mol);[518] electronegativities (revised Pauling); and electronic band structures[519][520] (most thermodynamically stable forms under ambient conditions).

Metalloids tend to be collectively characterized in terms of generalities or a few broadly indicative physical or chemical properties.[10] A single quantitative criterion is also occasionally mentioned.[n 36][n 37]

Masterton and Slowinski[525] give a more specific treatment. They wrote that metalloids have ionization energies clustering around 200 kcal/mol, and electronegativity values close to 2.0. They also said that metalloids are typically semiconductors, 'although antimony and arsenic [being semimetals in the physics-based sense] have electrical conductivities which approach those of metals'.

Their description, using these three more or less clearly defined properties, encompasses the six elements commonly recognized as metalloids (see table, right).

Selenium and polonium are probably excluded from this scheme; astatine may or may not be included.[n 38]

In other quantitative terms, the elements commonly recognized as metalloids have packing efficiencies of between 34% to 41%. That of boron is 38%; silicon and germanium 34; arsenic 38.5; antimony 41; and tellurium 36.4.[529][530][531] These values are lower than the values of most metals (at least 80% of which have a packing efficiency of at least 68%)[532][n 39] but higher than those of elements usually classified as nonmetals. Packing efficiencies for nonmetals are: graphite 17%,[535] sulphur 19.2,[536] iodine 23.9,[536] selenium 24.2,[536] and black phosphorus 28.5.[531]

The elements commonly recognized as metalloids also have Goldhammer-Herzfeld criterion ratios of between ~0.85 to 1.1 (average 1.0).[449][450]

Nomenclature and history

Derivation

The word metalloid comes from the Latin metallum = "metal" and the Greek oeides = "resembling in form or appearance".[537][538]

Other names

Although the terms amphoteric element,[539][540] half-metal,[541][542] half-way element,[543] near metal,[361] meta-metal,[544] semiconductor[545] and semimetal[546] are sometimes used synonymously, most of these have other meanings, which may not be interchangeable:

As well, some elements referred to as metalloids do not show marked amphoteric behaviour or semiconductivity in their most stable forms.

Origin and usage

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 to metals that floated on water (sodium and potassium), and then more popularly to nonmetals. Only recently, since the mid-20th century, has it been widely used to refer to intermediate or borderline elements.

Pre-1800

Ancient conceptions of metals as solid, fusible and malleable substances can be found in Plato's Timaeus (c. 360 BCE) and Aristotle’s Meteorology.[561][562]

More sophisticated classification arrangements were proposed by Pseudo-Geber (c. 1310), Basil Valentine[n 40] (Conclusiones), Paracelsus (1539?), and Boerhaave (Elementa Chemiæ, 1733). They attempted to separate the more characteristic metals from substances having those characteristics to a lesser degree. Such substances included zinc, antimony, bismuth, stibnite, pyrite and galena. These were all then called semimetals or bastard metals.[564][565]

In 1735 Brandt proposed to make the presence or absence of malleability the principle of this classification. On that basis he separated mercury from the metals. The same view was adopted by Vogel (1755, Institutiones Chemiæ) and Buffon (1785, Histoire Naturelle des Minéraux). In the interim, Braun had observed the solidification of mercury by cold in 1759–60. This was confirmed by Hutchins and Cavendish in 1783.[566] The malleability of mercury then became known, and it was included amongst the metals.[564]

In 1789 Fourcroy highlighted the weakness of this distinction between metals and semimetals (Eleméns d’Histoire Naturalle et de Chemie, ii. 380). He said it was evident from the fact that

between the extreme malleability of gold and the singular fragility of arsenic, other metals presented only imperceptible gradations of this character, and because there was probably no greater difference between the malleability of gold and that of lead, which was considered to be a metal, than there was between lead and zinc, which was classed among semi-metals, while in the substances intermediate between zinc and arsenic the differences were slight.

This idea of a semimetal, as a brittle (and thereby imperfect)[567][568] metal, was gradually discarded after 1789 with the publication of Lavoisier's 'revolutionary' [569] Elementary Treatise on Chemistry.[570][n 41]

1800–1950s

In 1807 Erman and Simon suggested using the term metalloid to refer to the newly discovered elements sodium and potassium. These elements were lighter than water and many chemists did not regard them as proper metals. Erman and Simon's proposal may have been made '[in] an attempt to revive this old distinction between metals and substances resembling metals'.[572] Their suggestion was ignored by the chemical community.[10]

In 1811[10] or 1812, Berzelius referred to nonmetallic elements as metalloids, in reference to their ability to form oxyanions.[573][574] A common oxyanion of sulfur, for example, is the sulfate ion SO2− 4. Many metals can do the same. Chromium, for instance, can form the chromate ion CrO2− 4. Berzelius' terminology was widely adopted[10] although it was subsequently regarded by some commentators as counterintuitive,[574] misapplied,[570] incorrect[575] or invalid.[368]

In 1825, in a revised German edition of his Textbook of Chemistry,[576][577] Berzelius subdivided the metalloids into three classes. These were constantly gaseous 'gazolyta' (hydrogen, nitrogen, oxygen); real metalloids (sulfur, phosphorus, carbon, boron, silicon); and salt-forming 'halogenia' (fluorine, chlorine, bromine, iodine).[578]

In 1844, Jackson[579] gave the meaning of 'metalloid' as 'like metals, but wanting some of their properties.'

In 1845, in A dictionary of science, literature and art, Berzelius' classification of the elementary bodies was represented as: I. gazolytes; II. halogens; III. metalloids ('resemble the metals in certain aspects, but are in others widely different'); and IV. metals.[580]

In 1864, calling nonmetals 'metalloids' was still sanctioned 'by the best authorities' even though this did not always seem appropriate. The greater propriety of applying the word metalloid to other elements, such as arsenic, had been considered.[581]

By as early as 1866 some authors were instead using the term nonmetal, rather than metalloid, to refer to nonmetallic elements.[582]

In 1876, Tilden[583] protested against, 'the too common though illogical practice of giving the name metalloid to such bodies as oxygen, chlorine or fluorine'. He instead divided the elements into ('basigenic') true metals, metalloids ('imperfect metals') and ('oxigenic') nonmetals.

As late as 1888, classifying the elements into metals, metalloids, and nonmetals, rather than metals and metalloids, was still regarded as peculiar and potentially confusing.[584]

Beach, writing in 1911, explained it this way:[585]

Metalloid (Gr. "metal-like"), in chemistry, any nonmetallic element. There are 13, namely, sulfur, phosphorus, fluorin, chlorin, iodine, bromine, silicon, boron, carbon, nitrogen, hydrogen, oxygen, and selenium. The distinction between the metalloids and the metals is slight. The former, excepting selenium and phosphorus, do not have a "metallic" lustre; they are poorer conductors of heat and electricity, are generally not reflectors of light and not electropositive; that is, no metalloid fails of all these tests. The term seems to have been introduced into modern usage instead of nonmetals for the very reason that there is no hard and fast line between metals and nonmetals, so that "metal-like" or "resembling metals" is a better description of the class than the purely negative "nonmetals". Originally it was applied to the nonmetals which are solid at ordinary temperature.

In or around 1917, the Missouri Board of Pharmacy wrote[586] that:

A metal may be said to differ from a metalloid [that is, a nonmetal] in being an excellent conductor of heat and electricity, in reflecting light more or less powerfully and in being electropositive. A metalloid may possess one or more of these characters, but not all of them...Iodine is most commonly given as an example of a metalloid because of its metallic appearance.

During the 1920s the two meanings of the word metalloid appeared to be undergoing a transition in popularity. Writing in A Dictionary of Chemical Terms, Couch[587] defined 'metalloid' as an old, obsolescent term for 'nonmetal.' [n 42] In contrast, Webster's New International Dictionary[588] noted that use of the term metalloid to refer to nonmetals was the norm. Its application to elements resembling the typical metals in some way only, such as arsenic, antimony and tellurium, was recorded merely on a 'sometimes' basis.

Use of the term metalloid subsequently underwent a period of great flux up to 1940. Consensus as to its application to intermediate or borderline elements did not occur until the ensuing years, between 1940 and 1960.[10]

In 1947, Pauling included a reference to metalloids in his classic[589] and influential[590] textbook, General chemistry: An introduction to descriptive chemistry and modern chemical theory. He described them as 'elements with intermediate properties...occupy[ing] a diagonal region [on the periodic table], which includes boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.'[591]

In 1959 the International Union of Pure and Applied Chemistry (IUPAC) recommended that '[t]he word metalloid should not be used to denote nonmetals'[592] although it was still being used in this sense (around that time) by, for example, the French.[298]

1960–

In 1969 the classic[593] and authoritative[594] Hackh's Chemical Dictionary included entries for both 'metalloid' and 'semimetal'. The latter term was described as obsolete.[595]

In 1970 IUPAC recommended abandoning the term metalloid because of its continuing inconsistent use in different languages. They suggested using the terms metal, semimetal and nonmetal instead.[298][596] Despite this recommendation, use of the term 'metalloid' increased dramatically.[10] Google's Ngram viewer showed a fourfold increase in the use of the word 'metalloid' (as compared to 'semimetal') in the American English corpus from 1972–1983. There was a sixfold increase in the British English corpus from 1976–1983.[597] As at 2011, the difference in usage across the English corpus was around 4:1 in favour of 'metalloid'.

The most recent IUPAC publications on chemical nomenclature (2005) and terminology (2006–) do not include any recommendations as to the usage or non-usage of the terms metalloid or semimetal.[n 43]

Use of the term semimetal, rather than metalloid, has recently been discouraged. This is because the former term 'has a well defined and quite distinct meaning in physics'.[600] In physics, a semimetal is an element or a compound in which the valence band marginally (rather than substantially) overlaps the conduction band. This results in only a small number of effective charge carriers.[520][601] Thus, the densities of charge carriers in the elemental semimetals carbon (as graphite, in the direction of its planes), arsenic, antimony and bismuth are 3×1018 cm−3, 2 ×1020 cm−3, 5×1019 cm−3 and 3×1017 cm−3 respectively.[602] In contrast, the room-temperature concentration of electrons in metals usually exceeds 1022 cm−3.[603]

References to the term 'metalloid' as being outdated have also been described as 'nonsense' noting that 'it accurately describes these weird in-between elements'.[604]

Notes

  1. ^ Sample size = 194 lists of metalloid lists, as of August 23, 2011. Mean appearance frequencies were: Cluster 1 (93%) = B, Si, Ge, As, Sb, Sb, Te; cluster 2 (44.7%) = Po, At; cluster 3 (24%) = Se; cluster 4 (9%) = C, Al; cluster 5 (5%) = Be, P, Bi; cluster 6 (3%) = S, Sn, Uuh; and cluster 7 (1%) = H, Ga, I, Pb, Uuq, Uup, Uus. See also the location and identification section of this article.
  2. ^ Not all elements with mixed or intermediate properties are necessarily hard to characterize. Gold, for example, has mixed properties but is still recognized as 'king of metals.' Besides metallic behaviour (such as high electrical conductivity, and cation formation), gold also shows marked nonmetallic behaviour: On halogen character, see also Belpassi et al.[8] who conclude that in the aurides MAu (M = Li–Cs) gold 'behaves as a halogen, intermediate between Br and I'. On aurophilicity, see also.[9]
  3. ^ Mann et al.[17] refer to these elements as 'the recognized metalloids'.
  4. ^ Although up to 18 allotropes of boron have been reported, possibly only three of these represent the pure element: rhombohedral β-boron; tetragonal T-192 boron; and ionic γ-boron ('boron boride'). The other forms are based on tenuous evidence, or are stable only at elevated pressures, or are thought to represent boron frameworks stabilized by impurities.[27][28][29] Boron can also be prepared in an amorphous form, having the appearance of a brown powder.[25]
  5. ^ Although SiO2 is classified as an acidic oxide, and hence reacts with alkalis to give silicates, it also reacts with phosphoric acid, giving silicon orthophosphate Si5O(PO4)6,[72] and with hydrofluoric acid to give hexafluorosilicic acid H2SiF6.[73]
  6. ^ Temperatures above 400 ºC are required to form a noticeable surface oxide layer.[77]
  7. ^ Whilst As2O3 is usually regarded as being amphoteric a few sources instead say it is (weakly)[115][116] acidic. They describe its 'basic' properties (that is, its reaction with concentrated hydrochloric acid to form arsenic trichloride) as being alcoholic, by analogy with the formation of covalent alklyl chlorides by covalent alcohols (e.g. R-OH + HCl → RCl + H2O)[117][118]
  8. ^ Olmsted and Williams[165] commented that, 'Until quite recently, chemical interest in the metalloids consisted mainly of isolated curiosities, such as the poisonous nature of arsenic and the mildly therapeutic value of borax. With the development of metalloid semiconductors, however, these elements have become among the most intensely studied'.
  9. ^ Jones[188] 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'.
  10. ^ Liquid carbon may[198] or may not[199] be a metallic conductor, depending on pressure and temperature; see also.[200]
  11. ^ Only a very small fraction of dissolved CO2 is present in water as carbonic acid so, even though H2CO3 is actually a medium-strong acid, solutions of carbonic acid are only weakly acidic.[211]
  12. ^ A mnemonic which captures the elements commonly recognized as metalloids goes: Up, up-down, up-down, up...are the metalloids! [227]
  13. ^ Rochow,[230] who would later write his 1966 monograph The metalloids,[231] commented that, 'In some respects selenium acts like a metalloid and tellurium certainly does'.
  14. ^ The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.[270] found that liquid boron behaved like a metal. Glorieux et al [271] characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity. Millot et al.[272] reported that the emissivity of liquid boron was not consistent with that of a liquid metal.
  15. ^ A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.[276]
  16. ^ Sanderson proposed a simple rule for distinguishing between metals and nonmetals: 'With the single exception of hydrogen, all elements are metals if the number of electrons in the outermost shell of their atoms is equal to or less than the period number of the element (which is the same as the principal quantum number of that shell). Hydrogen and all other elements are nonmetals, but if the number of electrons in the outermost shell is one (or two) greater than their principal quantum number, they may show some metallic characteristics.' Radon was left out of his list of somewhat metallic elements despite its apparent eligibility (principle quantum number = 6; outermost shell electrons = 8). At that time, the noble gases were still considered to be incapable of forming compounds. Following the synthesis of the first noble gas compound in 1962, references to cationic behaviour by radon appear from as early as 1969 (Stein;[312] Pitzer 1975;[313] Schrobilgen 2011[314]).
  17. ^ Aluminium sometimes is[326] or is not[327] counted as a post-transition metal.
  18. ^ For example: intermediate electrical conductivity;[341] a relatively narrow band gap;[342][343] light sensitivity.[341]
  19. ^ White phosphorus is the most common, industrially important,[344] and easily reproducible allotrope. For those reasons it is the standard state of the element.[345] Paradoxically, it is also thermodynamically the least stable, as well as the most volatile and reactive form.[346]
  20. ^ On the fuzziness of metalloids see, for example: Rouvray;[352] Cobb & Fetterolf;[353] and Fellet.[354] For the 'buffer zone' terminology see Rochow.[355] For examples of the application of a single criterion to classify metalloids see:
    • Mahan and Myers,[356] who use electrical conductivity.
    • Miessler and Tarr,[357] who use electronegativity.
    • Hutton and Dickerson,[358] who rely on the acid-base behaviour of group oxides.
    Kneen, Rogers & Simpson[359] further suggest the use of such individual criteria as the structure of the elements, or their reactions with acids. For an example of the use of multiple criteria see Masterton and Slowinski.[360] They characterize metalloids on the concurrent basis of ionization energy, electronegativity and electrical behaviour.
  21. ^ Oderberg[369] argues on ontological grounds that anything that is not a metal, is a nonmetal and that this includes semi-metals (i.e. metalloids).
  22. ^ Some authors only recognize elements as either metals or nonmetals.
  23. ^ Sacks[383] described the dividing line as, 'A jagged line, like Hadrian's Wall...[separating] the metals from the rest, with a few "semimetals," metalloids—arsenic, selenium—straddling the wall.'
  24. ^ In the context of Mendeleev's observation, Glinka[399] adds that: 'In classing an element as a metal or a nonmetal we only indicate which of its properties—metallic or nonmetallic—are more pronounced in it'.
  25. ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[416][417]
  26. ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[419][420] If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.[421][241][242]
  27. ^ Nonmetals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.[422][423]
  28. ^ At or near room temperature
  29. ^ The Goldhammer-Herzfeld criterion is a ratio that compares the force holding an individual atom's valence electrons in place with the forces, acting on the same electrons, arising 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. Metallic behaviour is then predicted.[446][447] Otherwise nonmetallic behaviour is anticipated. The Goldhammer-Herzfeld criterion is based on classical arguments.[448] It nevertheless offers a relatively simple first order rationalization for the occurrence of metallic character amongst the elements.[449]
  30. ^ Chedd[455] defines metalloids as having electronegativity values of 1.8 to 2.2 (Allred-Rochow scale). He included boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine in this category. In reviewing Chedd's work, Adler[456] described this choice as arbitrary, given other elements have electronegativities in this range, including copper, silver, phosphorus, mercury and bismuth. He went on to suggest defining a metalloid simply as, 'a semiconductor or semimetal' and 'to have included the interesting materials bismuth and selenium in the book'.
  31. ^ See, for example, the sulfates of the transition metals,[477][478] the lanthanides[479] and the actinides.[480]
  32. ^ Sulfates of osmium have not been characterized with any great degree of certainty.[481]
  33. ^ Common metalloids: Boron is reported to be capable of forming an oxysulfate (BO)2SO4,[482] a bisulfate B(HSO4)3[483][484] and a sulfate B2(SO4)3.[485] The existence of a sulfate has been disputed.[486] In light of the existence of silicon phosphate, a silicon sulfate might also exist.[487] Germanium forms an unstable sulfate Ge2SO4 (d 200 °C).[488][489] Arsenic forms oxide sulfates As2O(SO4)2 (= As2O3.2SO3)[490] and As2(SO4)3 (= As2O3.3SO3).[491] Antimony forms a sulfate Sb2(SO4)3 and an oxysulfate (SbO)2SO4.[492] Tellurium forms an oxide sulfate Te2O3(SO)4.[493] Less common: Polonium forms a sulfate Po(SO4)2.[494] It has been suggested that the astatine cation forms a weak complex with sulfate ions in acidic solutions.[495]
  34. ^ Hydrogen forms hydrogen sulfate H2SO4. Carbon forms (a blue) graphite hydrogen sulfate C+ 24HSO– 4 • 2.4H2SO4.[496] Nitrogen forms nitrosyl hydrogen sulfate (NO)HSO4 and nitronium (or nitryl) hydrogen sulfate (NO2)HSO4.[497][498] There are indications of a basic sulfate of selenium SeO2.SO3 or SeO(SO4).[249] Iodine forms a polymeric yellow sulfate (IO)2SO4.[499]
  35. ^ See, for example:
    • Brinkley,[501] who writes that boron has weakly nonmetallic properties.
    • Glinka,[502] who describes silicon as a weak nonmetal.
    • Eby et al.,[503] who discuss the weak chemical behaviour of the elements close to the metal-nonmetal borderline.
    • Booth and Bloom,[504] who comment that, 'A period represents a stepwise change from elements strongly metallic to weakly metallic to weakly nonmetallic to strongly nonmetallic, and then, at the end, to an abrupt cessation of almost all chemical properties…'.
    • Cox,[66] who notes that 'nonmetallic elements close to the metallic borderline (Si, Ge, As, Sb, Se, Te) show less tendency to anionic behaviour and are sometimes called metalloids.'
  36. ^ Rochow[521] concluded there was no single measurement 'which will...indicate exactly which elements...are properly classified as metalloids' and that 'Present-day students and teachers [therefore] usually agree to use electronegativity as a compromise criterion'. He described metalloids as a collection of 'in between' elements, of electronegativity 1.8 to 2.2 (classical Pauling), which were neither metals nor nonmetals. See also, for example:
    • Hill and Hollman,[6] who 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'.
    • Bond,[522] who suggests that 'one criterion for distinguishing semi-metals from true metals under normal conditions is that the co-ordination number of the former is never greater than eight'.
    • Edwards et al.,[523] who state that, 'Using the Goldhammer-Herzfeld criterion with measured atomic electronic polarizabilities and condensed phase molar volumes allows one to readily predict which elements are metallic, which are nonmetallic, and which are borderline when in their condensed phases (solid or liquid).'
  37. ^ In contrast, Jones[524] (writing on the role of classification in science) observes that, 'Classes are usually defined by more than two attributes.'
  38. ^ Selenium has an IE of ~226 kcal/mol and is sometimes described as a semiconductor. However it has a relatively high 2.55 EN. Polonium has an IE of ~196 kcal/mol and a 2.0 EN, but has a metallic band structure.[526][527] Astatine has an estimated IE of ~210±10 kcal/mol[528] and an EN of 2.2. However its electronic band structure is not known with any great degree of certainty.
  39. ^ Gallium is unusual (for a metal) in having a packing efficiency of just 39%.[533] Other notable values are 42.9 for bismuth[531] and 58.5 for liquid mercury.[534]
  40. ^ Allegedly born c. 1394[563]
  41. ^ In its first seventeen years, Lavoisier's work was republished in twenty-three editions and six languages, and carried his 'new chemistry' across Europe and America.[571]
  42. ^ Couch also commented (p. 128) that there was, 'no sharp line of demarcation between metals and nonmetals as many of the latter class possess some metallic properties' [italics added].
  43. ^ IUPAC recommendations on the nomenclature of inorganic chemistry are set out in the "Red Book", 2005.[598] This does not make any direct reference to semimetals or metalloids. The complementary compendium of chemical terminology is known as the "Gold Book", 2006–.[599] This contains one reference to semimetals in the physics-based sense (see 'semiconductor-metal transition') and one reference in the chemistry based sense (see 'organometallic compounds'). The latter entry notes that 'traditional metals and semi-metals' can form such compounds, as can 'boron, silicon, arsenic and selenium'.

Citations

  1. ^ a b Haller 2006, p. 3
  2. ^ Deming & Hendricks 1942, p. 170
  3. ^ Butler 1930, p. 23
  4. ^ King 1979, p. 13
  5. ^ International Textbook Company 1908, p. 21
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  474. ^ Lidin 1996, passim
  475. ^ Dunstan 1968, pp. 312, 408
  476. ^ Rochow 1966, p. 34
  477. ^ Wickleder, Pley & Büchner 2006
  478. ^ Betke & Wickleder 2011
  479. ^ Cotton 1994, p. 3606
  480. ^ Keogh 2005, p. 16
  481. ^ Raub & Griffith 1980, p. 167
  482. ^ Nemodruk & Karalova 1969, p. 48
  483. ^ Sneed 1954, p. 472
  484. ^ Gillespie & Robinson 1959, p. 407
  485. ^ Zuckerman & Hagen 1991, p. 303
  486. ^ Sanderson 1967, p. 178
  487. ^ Iler 1979, p. 190
  488. ^ Sanderson 1960, p. 162
  489. ^ Greenwood & Earnshaw 2002, p. 387
  490. ^ Mercier & Douglade 1982
  491. ^ Douglade & Mercier 1982
  492. ^ Wiberg 2001, p. 764
  493. ^ Wickleder 2007, p. 350
  494. ^ Bagnall 1966, pp. 140−41
  495. ^ Berei & Vasáros 1985, pp. 221, 229
  496. ^ Wiberg 2001, p. 795
  497. ^ Lidin 1996, pp. 266, 270
  498. ^ Brescia et al. 1975, p. 453
  499. ^ Furuseth et al. 1974
  500. ^ Rock & Gerhold 1974, pp. 535, 537
  501. ^ Brinkley 1945, p. 378
  502. ^ Glinka 1965, p. 88
  503. ^ Eby et al. 1943, p. 404
  504. ^ Booth & Bloom 1972, p. 426
  505. ^ Blanchard & Wade 1914, p. 226
  506. ^ Kent 1950, p. 1–2
  507. ^ Nickelès 1861
  508. ^ United States Air Force Medical Service 1966, p. 3-3
  509. ^ Schaffter 2006, p. 46
  510. ^ Remy 1956, p. 1
  511. ^ Johnston 1992, p. 57
  512. ^ Malerba 1985, p. 13
  513. ^ Rochow 1966, p. 14
  514. ^ Boikess & Edelson 1985, p. 85
  515. ^ Aldridge 1998, p. 290
  516. ^ Hein et al. 2004, p. 222
  517. ^ Frankland & Japp 1885, p. 398
  518. ^ NIST 2010. Values shown in the above table have been converted from the NIST values, which are given in eV.
  519. ^ Berger 1997
  520. ^ a b Lovett 1977, p. 3
  521. ^ Rochow (1966, pp. 4–7)
  522. ^ Bond 2005, p. 3
  523. ^ Edwards et al. 2010, p. 958
  524. ^ Jones 2010, p. 169
  525. ^ Masterton & Slowinski 1977, p. 160. They list B, Si, Ge, As, Sb and Te as metalloids, and comment that Po and At are ordinarily classified as metalloids but add that, 'since very little is known about their chemical and physical properties, and such classification must be rather arbitrary.'
  526. ^ Kraig, Roundy & Cohen 2004, p. 412
  527. ^ Alloul 2010, p. 83
  528. ^ NIST 2011. They cite Finkelnburg & Humbach (1955) who give a figure of 9.2±0.4 eV = 212.2±9.224 kcal/mol.
  529. ^ Van Setten et al. 2007, pp. 2460–61 (B)
  530. ^ Russell & Lee 2005, p. 7 (Si, Ge)
  531. ^ a b c Pearson 1972, p. 264 (As, Sb, Te; also black P)
  532. ^ Russell & Lee 2005, p. 1
  533. ^ Russell & Lee 2005, pp. 6–7, 387
  534. ^ Okakjima & Shomoji 1972, p. 258
  535. ^ Kitaĭgorodskiĭ 1961, p. 108
  536. ^ a b c Neuburger 1936
  537. ^ Oxford English Dictionary 1989, 'metalloid'
  538. ^ Gordh, Gordh & Headrick 2003, p. 753
  539. ^ Foster 1936, pp. 212–13
  540. ^ Brownlee et al. 1943, p. 293
  541. ^ a b Klemm 1950, pp. 133–142
  542. ^ Reilly 2004, p. 4
  543. ^ Walters 1982, pp. 32–33
  544. ^ Foster & Wrigley 1958, p. 218: 'The elements may be grouped into two classes: those that are metals and those that are nonmetals. There is also an intermediate group known variously as metalloids, meta-metals, semiconductors, or semimetals.'
  545. ^ Slade 2006, p. 16
  546. ^ Corwin 2005, p. 80
  547. ^ Bradbury et al. 1957, pp. 157, 659
  548. ^ Hoppe 2011
  549. ^ Miller, Lee & Choe 2002, p. 21
  550. ^ King 2004, pp. 196–198
  551. ^ Ferro & Saccone 2008, p. 233
  552. ^ Pashaey & Seleznev 1973, p. 565
  553. ^ Gladyshev & Kovaleva 1998, p. 1445
  554. ^ Eason 2007, p. 294
  555. ^ Johansen & Mackintosh 1970, pp. 121–124
  556. ^ Divakar, Mohan & Singh 1984, p. 2337
  557. ^ Dávila et al. 2002, p. 035411-3
  558. ^ Jezequel & Thomas 1997, pp. 6620–6626
  559. ^ Hindman 1968, p. 434: 'The high values obtained for the [electrical] resistivity indicate that the metallic properties of neptunium are closer to the semimetals than the true metals. This is also true for other metals in the actinide series.'
  560. ^ Dunlap et al. 1970, pp. 44, 46: '...α-Np is a semimetal, in which covalency effects are believed to also be of importance...For a semimetal having strong covalent bonding, like α-Np...'
  561. ^ Cornford 1937, pp. 249–50
  562. ^ Obrist 1990, pp. 163–64
  563. ^ Thomson 1830, p. 44
  564. ^ a b Paul 1865, p. 933
  565. ^ Roscoe & Schorlemmer 1894, pp. 3–4
  566. ^ Jungnickel & McCormmach 1996, p. 279–281
  567. ^ Craig 1849
  568. ^ Roscoe & Schorlemmer 1894, pp. 1–2
  569. ^ Strathern 2000, p. 239
  570. ^ a b Roscoe & Schormlemmer 1894, p. 4
  571. ^ Salzberg 1991, p. 204
  572. ^ Tweney & Shirshov 1935
  573. ^ Partington 1964, p. 168
  574. ^ a b Bache 1832, p. 250
  575. ^ Glinka 1959, p. 76
  576. ^ Partington 1964, pp. 145, 168
  577. ^ Jorpes 1970, p. 95
  578. ^ Berzelius 1825, p. 168
  579. ^ Jackson 1844, p. 368
  580. ^ Brande & Cauvin 1945, p. 223
  581. ^ The Chemical News and Journal of Physical Science 1864
  582. ^ Oxford English Dictionary 1989, 'nonmetal'
  583. ^ Tilden 1876, p. 198
  584. ^ The Chemical News and Journal of Physical Science 1888
  585. ^ Beach 1911
  586. ^ Mayo 1917, p. 55
  587. ^ Couch 1920, p. 128
  588. ^ Webster's New International Dictionary 1926 p. 1359
  589. ^ Lundgren & Bensaude-Vincent 2000, p. 409
  590. ^ Greenberg 2007, p. 562
  591. ^ Pauling 1947, p. 65
  592. ^ IUPAC 1959, p. 10
  593. ^ American Institute of Chemists 1969, p. 485
  594. ^ American Chemical Society California section 1969, p. 55
  595. ^ Grant 1969, pp. 422, 604: 'metalloid.—(1) having the physical properties of metals and the chemical properties of nonmetals, e.g., As. (2) a nonmetal (incorrect usage)...semimetal.—an element midway in properties between metals and nonmetals, as arsenic (obsolete).'
  596. ^ IUPAC 1971, p. 11
  597. ^ Google Ngram, viewed 11 February 2011
  598. ^ IUPAC 2005
  599. ^ IUPAC 2006–
  600. ^ Atkins 2010, p. 20
  601. ^ Wilson 1939, pp. 21–22
  602. ^ Feng & Jin 2005, p. 324
  603. ^ Sólyom 2008, p. 91
  604. ^ Gray 2010

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