|
| |
|
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 included |
*The metalloid status of Al, Po and At is disputed. |
|
|
|
|
Less commonly included |
|
|
|
Uncommonly included |
|
|
|
Rarely included |
|
|
Indicative (relative) frequency with which some elements appear in metalloid lists. Frequencies are from the list of metalloid lists and occur in a more or less geometric progression of clusters. The common elements (B, Si, Ge, As, Sb, Te) have appearance frequencies clustering around the low 90s. The 'less common' elements (Po, At) appear half as often (clustering around 45%). The single 'uncommon' representative (Se) and the following cluster of 'rare' elements (C, Al) have appearance frequencies each around half that of their immediate precursors. The series continues with the still less frequently appearing elements. This is not shown above because of the relatively small sample size.[n 1]
The grey staircase shaped line, which passes between B-Al, Al-Si, Si-Ge, Ge-As, As-Sb, Sb-Te, Te-Po and Po-At, is a typical example of the arbitrary metal-nonmetal dividing line that can be found on some periodic tables. Germanium, if classified as a nonmetal, then appears to fall on the wrong side of the line. This is a result of the publicity this form of the line received in the late 1920s and early 30s. Germanium was also thought to be a poorly conducting metal, up to at least the late 1930s.[1] |
|
A metalloid is a chemical element with properties that are in-between or a mixture of those of metals and nonmetals, and which is considered to be difficult to classify unambiguously as either a metal or a nonmetal. There is no standard definition of a metalloid nor is there agreement as to which elements are appropriately classified as such. Despite this lack of specificity the term continues to be used in the chemistry literature.
The six elements commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. They are metallic-looking brittle solids, with intermediate to relatively good electrical conductivities, and each having the electronic band structure of either a semiconductor or a semimetal. Chemically, they mostly behave as (weak) nonmetals, have intermediate ionization energy and electronegativity values, and form amphoteric or weakly acidic oxides. Being too brittle to have any structural uses, the metalloids and their compounds instead find common use in glasses, alloys and semiconductors. The electrical properties of silicon and germanium, in particular, enabled the establishment of the semiconductor industry in the 1950s and the development of solid state electronics from the early 60s onwards.[2][3]
The term metalloid 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. Metalloids are sometimes called semimetals, a practice which has been discouraged. This is because the term semimetal has a different meaning in physics, one which more specifically refers to the electronic band structure of a substance rather than the overall classification of a chemical element.
There is no universally agreed or rigorous definition of a metalloid.[4][5] The feasibility of establishing a specific definition has also been questioned, noting anomalies can be found in several such attempted constructs.[6] Classifying any particular element as a metalloid has been described as 'arbitrary'.[7]
The generic definition set out at the start of this article is based on metalloid attributes consistently cited in the literature. Illustrative definitions and extracts include:
- 'In chemistry a metalloid is an element with properties intermediate between those of metals and nonmetals.' [8]
- 'Between the metals and nonmetals in the periodic table we find elements…[that] share some of the characteristic properties of both the metals and nonmetals, making it difficult to place them in either of these two main categories.' [9]
- 'Chemists sometimes use the name metalloid…for these elements which are difficult to classify one way or the other.' [10]
- 'Because the traits distinguishing metals and nonmetals are qualitative in nature, some elements do not fall unambiguously in either category. These elements…are called metalloids…'. [11]
The criterion that metalloids are difficult to unambiguously classify one way or the other is a key tenet. In contrast, elements such as 'sodium and potassium have metallic properties to a high degree, and fluorine, chlorine and oxygen are almost exclusively nonmetallic.' [12] Although most other elements have a mixture of metallic and nonmetallic properties[12] most such elements can also be classified as either metals or nonmetals according to which set of properties are regarded as being more pronounced in them.[13][n 2] It is only the elements at or near the margins, ordinarily those that are regarded as lacking a sufficiently clear preponderance of metallic or nonmetallic properties, which are classified 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, boron, silicon, germanium, arsenic, antimony and tellurium are commonly classified as metalloids.[4][5][17][18][19][20][n 3] One or more from among selenium, polonium or astatine are sometimes added to the list.[5][22][23] Boron is sometimes excluded from the list, by itself or together with silicon.[24][25] Tellurium is sometimes not regarded as a metalloid.[26] The inclusion of antimony, polonium and astatine as metalloids has also been questioned.[5][27][28]
In its most stable state, pure boron appears as a shiny, silver-grey crystalline solid.[29][30][n 4] It is about ten percent lighter than aluminium but, unlike the latter,[34] is hard and brittle. It is barely reactive under normal conditions, except for attack by fluorine,[35] and has a melting point several hundred degrees higher than that of steel. Boron is a semiconductor,[36] with a room temperature electrical conductivity of 1.5 × 10−6 S•cm−1 [37] and a band gap of about 1.56 eV.[38] It becomes superconducting at a pressure of 250 Gpa and a temperature of 11.2 K.[39]
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.[40] 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,[41][42] 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.[43][44][45] 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).[46] The energy required to transform B, C, N, Si and P from nonmetallic to metallic states has been estimated as 30, 100, 240, 33 and 50 kJ/mol, respectively. This indicates how close boron is to the metal-nonmetal borderline.[47]
Most of the chemistry of boron is nonmetallic in nature.[47] The small size of the boron atom, however, enables the preparation of many interstitial alloy-type borides.[48] Analogies between boron and transition metals have also been noted in the formation of complexes,[49] and adducts (for example, BH3 + CO →BH3CO and, similarly Fe(CO)4 + CO →Fe(CO)5), as well in the geometric and electronic structures of cluster species such as [B6H6]2– and [Ru6(CO)18]2–.[50][n 5] The aqueous chemistry of boron, more conventionally, is characterised by the formation of many different polyborate anions.[52][53][54][55] Given its high charge-to-size ratio nearly all compounds of boron are covalent, barring some complexed anionic and cationic species.[56][57] Boron has a strong affinity for oxygen, a characteristic manifested in the extensive chemistry of the borates.[48] The oxide B2O3 is polymeric in structure,[58] weakly acidic,[59] and a glass former.[60] Organometallic compounds of boron have been known since the 19th century (see organoboron chemistry).[61]
Silicon appears as a shiny crystalline solid, with a blue-grey metallic lustre.[62] As with boron it is about ten per cent lighter than aluminium, hard and brittle.[63] It is a relatively unreactive element.[62] Although it is oxidized by nitric acid, the resulting thin surface layer of SiO2 prevents further corrosion.[64][65] It also dissolves in hot aqueous alkalis with the evolution of hydrogen, behaving in this way like metals[66] such as beryllium, aluminium, zinc, gallium and indium.[67][68] It melts at about the same temperature as steel. Silicon is a semiconductor with an electrical conductivity of 10−4 S•cm−1 [69] and a band gap of about 1.11 eV.[70] When it melts, silicon becomes a reasonable metal[71] with an electrical conductivity of 1.0–1.3 × 104S•cm−1, a value similar to that of liquid mercury.[72][73] At a pressure of 12 Gpa and a temperature of 8.5 K silicon becomes superconducting.[39]
The chemistry of silicon is generally nonmetallic (covalent) in nature.[74] It does, however, form alloys with metals such as iron and copper.[75] Silicon shows fewer tendencies to anionic behaviour than ordinary nonmetals.[76] Its solution chemistry is characterised by the formation of oxyanions.[55] The high bond strength of the silicon-oxygen bond dominates the chemical behaviour of silicon.[77] Polymeric silicates, built up by tetrahedral SiO4 units sharing their oxygen atoms, represent the most abundant and important compounds of silicon.[78] The polymeric borates, comprising linked trigonal and tetrahedral BO3 or BO4 units, are built on similar structural principles.[79] The oxide SiO2 is polymeric in structure,[58] weakly acidic,[80][n 6] and a glass former.[60] Traditional organometallic chemistry includes the carbon compounds of silicon (see organosilicon).[83]
Germanium appears as a shiny grey-white solid.[84] It is about one-third lighter than iron, hard and brittle.[85] It is mostly unreactive at room temperature[n 7] but is slowly attacked by hot concentrated sulphuric or nitric acid.[87] Germanium also reacts with molten caustic soda to yield sodium germanate Na2GeO3, together with the evolution of hydrogen.[88] 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 [87] and a band gap of 0.67 eV.[89] Liquid germanium is a metallic conductor, with an electrical conductivity on par with that of liquid mercury.[90] At a pressure of 5.4 Gpa and a temperature of 11.5 K germanium becomes superconducting.[39]
Most of the chemistry of germanium is characteristic of a nonmetal.[91] It does however form alloys with, for example, aluminium and gold.[92] Germanium shows fewer tendencies to anionic behaviour than ordinary nonmetals.[76] Its solution chemistry is characterised by the formation of oxyanions.[55] 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.[93][94] Germanium analogues of all of the major types of silicates have been prepared.[95] 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.[96][97] The oxide GeO2 is polymeric,[58] amphoteric,[98] and a glass former. [60] Germanium has an established organometallic chemistry (see organogermanium chemistry).[99]
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).[100] It is stable in dry air but develops a golden bronze patina 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.[101] 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.[102] Arsenic only melts under a pressure of 38.6 atm, at around half the melting point of steel.[103] Arsenic is a semimetal with an electrical conductivity of around 3.9 × 104 S•cm−1 [104] and a band overlap of 0.5 eV.[105][n 8] Liquid arsenic is a semiconductor with a band gap of 0.15 eV.[108][109] At a pressure of 24 Gpa and a temperature of 2.7 K arsenic becomes superconducting.[39]
The chemistry of arsenic is predominately nonmetallic in character.[110] It does however form alloys with many metals, most of these being brittle.[111] Arsenic shows fewer tendencies to anionic behaviour than ordinary nonmetals.[76] Its solution chemistry is characterised by the formation of oxyanions.[55] Arsenic generally forms compounds in which it has an oxidation state of +3 or +5.[112] The halides, and the oxides and their derivatives are illustrative examples.[113] In the trivalent state, arsenic shows some incipient metallic properties.[114] Thus, the halides are hydrolysed by water but these reactions, particularly those of the chloride, are reversible with the addition of a hydrohalic acid.[115][116] As well, and as noted below, the oxide is acidic but weakly amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties.[117][118] 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.[119][120][121][122] The oxide As2O3 is polymeric,[58] amphoteric,[123][124][125][n 9] and a glass former.[60] Arsenic has an extensive organometallic chemistry (see organoarsenic chemistry).[130]
Antimony appears as a silver-white solid with a blue tint and a brilliant lustre.[131] 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).[132] 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.[133] It is not affected by molten alkali.[134] Antimony is capable of displacing hydrogen from water, when heated: 2Sb + 3H2O → Sb2O3 + 3H2.[135] 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 [136] and a band overlap of 0.16 eV.[137][n 10] Liquid antimony is a metallic conductor with an electrical conductivity of around 5.3 × 104 S•cm−1.[139][140] At a pressure of 8.5 Gpa and a temperature of 3.6 K antimony becomes superconducting.[39]
Most of the chemistry of antimony is characteristic of a nonmetal.[141] It does however form alloys with one or more metals such as aluminium,[142] iron, nickel, copper, zinc, tin, lead and bismuth.[143] Antimony shows fewer tendencies to anionic behaviour than ordinary nonmetals.[76] Its solution chemistry is characterised by the formation of oxyanions.[55] Like arsenic, antimony generally forms compounds in which it has an oxidation state of +3 or +5.[112] The halides, and the oxides and their derivatives are illustrative examples.[113] 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.[144][145] Tripositive antimony is amphoteric; quinquepositive antimony is (predominately) acidic.[146] 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.[147] 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.[148] The oxide Sb2O3 is a polymeric,[58] amphoteric,[149] and a glass former.[60] Antimony has an extensive organometallic chemistry (see organoantimony chemistry).[150]
Tellurium appears as a silvery-white solid with a shiny lustre.[151] It is about 15 per cent lighter than iron, brittle, and the softest of the commonly recognised metalloids, being marginally harder than sulphur.[152] 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.[153] 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,[154] the basic nitrate (Te2O4H)+(NO3)–,[155][156] or the oxide sulphate Te2O3(SO4).[157] 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.[158] At higher temperatures tellurium is sufficiently plastic to be extrudable.[159] It melts at a temperature of around thirty per cent that of steel. 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.[160][161][162] Tellurium is a semiconductor with an (intrinsic) electrical conductivity of around 1.0 S•cm−1 [163] and a band gap of 0.32 to 0.38 eV.[164] Liquid tellurium is a semiconductor, with an electrical conductivity, on melting, of around 1.9 × 103 S•cm−1 [164] Superheated liquid tellurium is a metallic conductor.[165] At a pressure of 35 Gpa and a temperature of 7.4 K tellurium becomes superconducting.[39]
Most of the chemistry of tellurium is characteristic of a nonmetal.[166] It does however form alloys with, for example, aluminium, silver and tin.[167][168] Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals.[76] Its solution chemistry is characterised by the formation of oxyanions.[55] Tellurium generally forms compounds in which it has an oxidation state of –2, +4 or +6, with the tetrapositive state being the most stable.[153] 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.[169] 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.[170][171] Tellurium forms a polymeric,[58] amphoteric,[172] glass-forming oxide[60] TeO2. The latter is also a 'conditional' glass-forming oxide—it will form a glass with a very small amount of additive.[60] Tellurium has an extensive organometallic chemistry (see organotellurium chemistry).[173]
- 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.[174][175] Typical applications of these and the other elements commonly recognized as metalloids have instead encompassed:
- Use of their oxides as glass-formers.
- Their inclusion as alloying components or additives.
- Their employment as semiconductors, dopants or semiconductor constituents.[n 11]
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.[177] These compounds have found or continue to find practical uses in chemical, domestic and industrial glassware[178][179] and optics.[180][181] Boron trioxide is used as a glass fibre additive;[182] it is also a component of borosilicate glass, which is widely used for laboratory glassware, as well as in home ovenware[183]. Silicon dioxide forms the basis of ordinary domestic glassware.[184] Germanium dioxide is used as glass fibre additive, as well as in infrared optical systems.[185] Arsenic trioxide is used in the glass industry as a decolorizing and fining agent, as is antimony trioxide.[186][187]Tellurium dioxide finds application in laser and nonlinear optics.[188][189]
In 1914 Desch[190] 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[191] later noted that compounds of silicon, germanium, arsenic and antimony with the poor metals, 'are probably best classed as alloys'.
In terms of individual elements:
- Boron can form intermetallic compounds and alloys with transition metals, of the composition MnB, if n > 2.[192]
- Sanderson[193] 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.[194]
- Arsenic can form alloys with metals, including platinum and copper.[195]
- 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).[196]
- 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.[197]
All the elements commonly recognized as metalloids (or their compounds) have found application in the semiconductor or solid-state electronic industries.[198][199] 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.[200][201] Silicon is the foremost commercial semiconductor; it forms the basis of modern electronics and information and communication technologies.[202][203] This has occurred despite the study of semiconductors, early in the 20th century, being regarded as the 'physics of dirt' and not deserving of close attention.[204][205] Silcon has largely replaced germanium in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process.[206][207] Semiconducting silicon-germanium 'alloys' have however been growing in use, particularly for wireless communication devices; these alloys exploit the higher carrier mobility of germanium.[207] Arsenic and antimony are not semiconductors in their standard states. On the other hand, both form type III-V semiconductors (such as GaAs, AlSb or GaInAsSb) in which the average number of valence electrons per atom is the same as that of Group 14 elements; these compounds are preferred for some special applications.[208] Tellurium, which is a semiconductor in its standard state, is used mainly as a component in a very large group of type II/VI semiconducting-chalcogenides; these compounds have applications in electro-optics and electronics.[209]
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,[210] for example, recognized only four: germanium, arsenic, antimony and tellurium. James et al.,[211] on the other hand, listed twelve: boron, carbon, silicon, germanium, arsenic, selenium, antimony, tellurium, bismuth, polonium, ununpentium and livermorium.
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. A specified subset of this continuum can potentially serve its particular purpose as well as any other.[212] In any event, individual metalloid classification arrangements tend to share common ground (as described above) with most variations occurring around the indistinct[213][214] margins, as surveyed below.[n 12]
Carbon is ordinarily classified as a nonmetal[216] although it has some metallic properties and is occasionally classified as a metalloid.[217][218][219] Where applicable, properties listed below are for hexagonal graphitic carbon, the most thermodynamically stable form of carbon under ambient conditions.[220][221]
In terms of the metallic character of carbon:
- It has a lustrous appearance.[222]
- It shows good electrical conductivity.[223]
- It has a positive temperature coefficient of electrical resistivity, in the direction of its planes, that is, its conductivity decreases with increasing temperature (behaving in this way as a metal).[224][n 13]
- It has the electronic band structure of a semimetal.[224]
- The various allotropes of carbon, including graphite, are capable of accepting foreign atoms or compounds into their structures via substitution, intercalation or doping (interstitial or intrastitial) with the resulting materials being referred to as 'carbon alloys'.[228][229]
- It can form ionic salts, including a sulphate, perchlorate, nitrate, hydrogen selenate, and hydrogen phosphate.[230]
- In organic chemistry, carbon can form complex cations—termed carbocations—in which the positive charge is on the carbon atom; exemplars are CH3+ and CH5+, and their derivatives.[231][232]
In terms of the nonmetallic character of carbon:
- It is brittle.[233]
- It behaves as a semiconductor, perpendicular to the direction of its planes.[224]
- Most of its chemistry is nonmetallic.[234]
- It has a relatively high ionization energy.[235]
- It has a relatively high electronegativity, compared to most metals.[236]
- Its oxide CO2 forms a medium-strength carbonic acid H2CO3.[237][n 14]
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,[239] these properties are sometimes used as a basis to classify aluminium as a metalloid:[240][241]
- Its crystalline structure shows some evidence of directional bonding.[242][243][244]
- Although it forms an Al3+ cation in some compounds, it bonds covalently in most others.[245][246][247]
- Its oxide is amphoteric, and a conditional glass-former.[60]
- it forms anionic aluminates,[239] such behaviour being considered nonmetallic in character.[248]
Stott[249] labels aluminium as weak metal. It has the physical properties of a good metal but some of the chemical properties of a nonmetal. Steele[250] 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[251][252][253] 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.[28][n 15]
Selenium shows borderline metalloid or nonmetal behaviour.[255][256][n 16]
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.[259]
The metallic character of selenium is further shown by the following properties:
- Its lustre.[260]
- Its crystalline structure, which is thought to include weakly 'metallic' interchain bonding.[261]
- Its capacity, when molten, to be drawn into thin threads.[262]
- Its reluctance to acquire 'the high positive oxidation numbers characteristic of nonmetals'.[263]
- Its capacity to form cyclic polycations (such as Se2+
8) when dissolved in oleums[264] (an attribute it shares with sulfur and tellurium).
- The existence of a hydrolysed cationic salt in the form of trihydroxoselenium (IV) perchlorate [Se(OH)3]+.ClO–
4.[265][266]
The nonmetallic character of selenium is shown by:
- Its brittleness.[260]
- Its electronic band structure, which is that of a semiconductor.[267]
- The low electrical conductivity (~10−9 to 10−12 S·cm−1) of its highly purified form.[268][269][270] This is comparable to or less than that of bromine (7.95×10–12 S·cm−1),[271] a nonmetal.
- Its relatively high[272] electronegativity (2.55 revised Pauling).
- The retention of its semiconducting properties in liquid form.[267]
- Its reaction chemistry, which is mainly that of its nonmetallic anionic forms Se2–, SeO2−
3 and SeO2−
4.[273]
Polonium is 'distinctly metallic' in some ways,[274] or shows metallic character by way of:
- The metallic conductivity of both of its allotropic forms.[274]
- The presence of the rose-coloured Po2+ cation in aqueous solution.[275]
- The many salts it forms.[276][277]
- The predominating basicity of polonium dioxide.[278][279]
- The highly reducing conditions required for the formation of the Po2– anion in aqueous solution.[280][281][282]
However, polonium shows nonmetallic character in that:
- Its halides have properties generally characteristic of nonmetal halides (being volatile, easily hydrolyzed, and soluble in organic solvents).[283][284]
- Many metal polonides, obtained by heating the elements together at 500–1,000 °C, and containing the Po2– anion, are also known.[285][286]
Astatine may be a nonmetal or a metalloid.[287][n 17] It is ordinarily classified as a nonmetal,[27][28][289][290] but has some 'marked' metallic properties.[291] Immediately following its production in 1940, early investigators considered it to be a metal.[292] In 1949 it was called the most noble (difficult to reduce) nonmetal as well as being a relatively noble (difficult to oxidize) metal.[293] In 1950 astatine was described as a halogen and (therefore) a reactive nonmetal.[294]
In terms of metallic indicators:
- Samsonov[295] observes that, '[L]ike typical metals, it is precipitated by hydrogen sulfide even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the cathode on electrolysis'.
- Rossler[296] cites further indications of a tendency for astatine to behave like a (heavy) metal as: '...the formation of pseudohalide compounds...complexes of astatine cations...complex anions of trivalent astatine...as well as complexes with a variety of organic solvents'.
- Rao and Ganguly[297] note that elements with an enthalpy of vaporization (EoV) greater than ~42 kJ/mol are metallic when liquid. Such elements include boron,[n 18] silicon, germanium, antimony, selenium and tellurium. Vásaros & Berei[301] give estimated values for the EoV of diatomic astatine, the lowest of these being 50 kJ/mol. On this basis astatine may also be metallic in the liquid state. Diatomic iodine, with an EoV of 41.71,[302] falls just short of the threshold figure.
- Siekierski and Burgess[303] contend or presume that astatine would be a metal if it could form a condensed phase.[n 19]
- Champion et al.[305] argue that astatine demonstrates cationic behaviour, by way of stable At+ and AtO+ forms, in strongly acidic aqueous solutions.
For nonmetallic indicators:
- Batsanov gives a calculated band gap energy for astatine of 0.7 eV.[306] This is consistent with nonmetals (in physics) having separated valence and conduction bands and thereby being either semiconductors or insulators.[307][308]
- It has the narrow liquid range ordinarily associated with nonmetals (mp 575 K, bp 610).[309]
- Its chemistry in aqueous solution is predominately characterised by the formation of various anionic species.[310]
- Most of its known compounds resemble those of iodine,[289] which is halogen and a nonmetal.[311][312] Such compounds include astatides (XAt), astatates (XAtO3), and monovalent interhalogen compounds.
Restrepo et al.[313][314] 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.
Given there is no agreed definition of a metalloid, some other elements are occasionally classified as such. These elements include[315] hydrogen,[316][317][318] beryllium,[319] nitrogen,[320] phosphorus,[219][321] sulfur,[219][322][323] zinc,[324] gallium,[325] tin, iodine,[320][326] lead,[327] bismuth[26] and radon.[328][329][330] The term metalloid has also been used to refer to:
- Elements that exhibit metallic lustre and electrical conductivity, and that are also amphoteric. Arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead and aluminium are examples.[331]
- Elements that are otherwise sometimes referred to as poor metals.[332]
- Nonmetallic elements (for example, nitrogen; carbon) that can form alloys with,[333][334][335] or modify the properties of,[336] metals.
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[337] 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 (astatine), five years ahead of its synthesis in 1940. They excluded germanium from their considerations as it was still then regarded as a poorly conducting metal.[1] In 1954, Szabó & Lakatos[338] counted beryllium and aluminium in their list of metalloids, as well as boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine. In 1957, Sanderson[339][n 20] recognized carbon, phosphorus, selenium, and iodine as part of an intermediary class of elements with 'certain metallic properties', together with boron, silicon, arsenic, tellurium, and astatine. Germanium, antimony and polonium were classifed by him as metals. More recently, in 2007, Petty[343] included carbon, phosphorus, selenium, tin and bismuth in his list of metalloids, as well as boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine.
Elements such as these are occasionally called, or described as, near-metalloids,[344][345] 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,[346] marked covalent chemistry (molecular or polymeric),[347] and amphoterism.[348][349] Aluminium, tin and bismuth are examples. They are also referred to as (chemically) weak metals,[350][351] poor metals,[352][353] post-transition metals,[354][355][n 21] 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 the 'near-metalloid' category include carbon,[356][357] phosphorus,[358][359][360][361][362] selenium[256][363][364] and iodine.[365][366][367] They exhibit metallic lustre, semiconducting properties[n 22] 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 23] 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.
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[374] more characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth also have allotropes that display borderline or either metallic or nonmetallic behaviour.[375][376][377]
Metalloids are generally regarded as a third category of chemical elements, alongside metals and nonmetals.[378] 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 24] Metalloids are sometimes grouped instead with metals,[388][389] regarded as nonmetals[390] or treated as a sub-category of same.[391][392][393][394][395][n 25]
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H
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He
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Li
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Be
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B
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C
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N
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O
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F
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Ne
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Na
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Mg
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Al
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Si
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P
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S
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Cl
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Ar
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K
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Ca
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Zn
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Ga
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Ge
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As
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Se
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Br
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Kr
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Rb
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Sr
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Cd
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In
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Sn
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Sb
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Te
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I
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Xe
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Cs
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Ba
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Hg
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Tl
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Pb
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Bi
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Po
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At
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Rn
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Fr
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Ra
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Cn
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Uut
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Fl
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Uup
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Lv
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Uus
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Uuo
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Condensed periodic table showing distribution of elements that have sometimes[n 26] 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, Fl, Uup, Lv, 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). |
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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.[248] 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.[397]
Some authors do not classify elements bordering the metal-nonmetal dividing line as metalloids noting that a binary classification can facilitate the establishment of some simple rules for determining bond types between metals and/or nonmetals.[378] Other authors, in contrast, 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'.[398] Alternatively, 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[399] or diffuse region,[400] running from upper left to lower right, centred around arsenic.
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.[401] 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.[402][403][404] 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.[403][404] 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.[401] 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.[405]
The following two subsections summarize and tabulate selected physical and chemical properties of metalloids. Properties of metals and nonmetals are also shown, for comparative purposes.[406]
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. Relevant properties are set out in the following table, in loose order of ease of determination:
| Property |
Metals |
Metalloids |
Nonmetals |
| Form |
solid; a few liquid at or near room temperature (Ga, Hg, Cs, Fr)[407][408][n 27] |
solid[413] |
mostly gases[414] |
| Appearance |
lustrous (at least when freshly fractured) |
lustrous[413] |
colourless, red, yellow, green, black, or intermediate shades[415] |
| Elasticity |
typically elastic, ductile, malleable (when solid) |
brittle[416] |
brittle, if solid |
| Electrical conductivity |
good to high[n 28] |
intermediate[419] to good[n 29] |
poor to good[n 30] |
| Band structure |
metallic (Bi = semimetallic) |
are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms[393][425] |
semiconductor or insulator[307] |
Metalloids generally behave chemically as (weak) nonmetals, have intermediate ionization energies and electronegativities, and have amphoteric or weakly acidic oxides. They can also form alloys with metals. Relevant properties—general, specific and descriptive—are set out in the following table:
| Property |
Metals |
Metalloids |
Nonmetals |
| General behaviour |
metallic |
nonmetallic[426] |
nonmetallic |
| Ionization energy |
relatively low |
intermediate ionization energies,[427] usually falling between those of metals and nonmetals[428] |
relatively high |
| Electronegativity |
usually low |
have electronegativity values close to 2[429] (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)[21][n 31] |
high |
When mixed
with metals |
give alloys |
can form alloys[432][433][434] |
ionic or interstitial compounds formed |
| Oxides |
lower oxides basic; higher oxides increasingly acidic |
amphoteric or weakly acidic[413][435] |
acidic |
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Element |
IE
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EN
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Band structure |
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Boron |
191 |
2.04 |
semiconductor |
|
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Silicon |
187 |
1.90 |
same |
|
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Germanium |
182 |
2.01 |
same |
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Arsenic |
225 |
2.18 |
semimetal |
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Antimony |
198 |
2.05 |
same |
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Tellurium |
207 |
2.10 |
semiconductor |
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average |
198 |
2.05 |
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The elements commonly recognized as metalloids, and their ionization energies (kcal/mol);[436] electronegativities (revised Pauling); and electronic band structures[437][438] (most thermodynamically stable forms under ambient conditions). |
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Metalloids tend to be collectively characterized in terms of generalities or a few broadly indicative physical or chemical properties.[4] A single quantitative criterion is also occasionally mentioned.[n 32][n 33]
Masterton and Slowinski[444] 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 34]
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.[448][449][450] These values are lower than the values of most metals (at least 80% of which have a packing efficiency of at least 68%)[451][n 35] but higher than those of elements usually classified as nonmetals. Packing efficiencies for nonmetals are: graphite 17%,[454] sulphur 19.2,[455] iodine 23.9,[455] selenium 24.2,[455] and black phosphorus 28.5.[450]
- Goldhammer-Herzfeld criterion[n 36] ratios of between ~0.85 to 1.1 (average 1.0).[459][460]
The word metalloid comes from the Latin metallum = "metal" and the Greek oeides = "resembling in form or appearance".[461][462] Although the terms amphoteric element,[463][464] half-metal,[465][466] half-way element,[467] near metal,[388] meta-metal,[468] semiconductor[469] and semimetal[470] 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. The other meanings in question are:
- 'Amphoteric element', which is sometimes used more broadly to include transition metals capable of forming oxyanions, such as chromium and manganese.[471]
- 'Half-metal', which is sometimes instead used to refer to the poor metals.[472] It also has another meaning, in physics, as a compound (such as chromium dioxide) or alloy that can act as a conductor and an insulator.
- 'Meta-metal', which 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 table layouts.[465] These metals are mostly diamagnetic[473] and tend to have distorted crystalline structures, electrical conductivity values at the lower end of those of metals, and amphoteric (weakly basic) oxides.[474][475]
- 'Semimetal', which sometimes refers, loosely or explicitly, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure. Examples include gallium,[476][477][478] ytterbium,[479][480][481] bismuth[482] and neptunium.[483][484]
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. Only recently, since the mid-20th century, has it been widely used to refer to intermediate or borderline elements.[4] The International Union of Pure and Applied Chemistry (IUPAC) has previously recommended abandoning the term metalloid, and suggested using the term semimetal instead.[485][326][486] However, use of this latter term has recently been discouraged as it has a quite distinct and different meaning in physics, one which more specifically refers to the electronic band structure of a substance rather than the overall classification of a chemical element.[487] The most recent IUPAC publications on nomenclature and terminology do not include any recommendations on the usage or non-usage of the terms metalloid or semimetal.[488][489]
- ^ 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.
- ^ 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.[15] who conclude that in the aurides MAu (M = Li–Cs) gold 'behaves as a halogen, intermediate between Br and I'. On aurophilicity, see also.[16]
- ^ Mann et al.[21] refer to these elements as 'the recognized metalloids'.
- ^ 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.[31][32][33] Boron can also be prepared in an amorphous form, having the appearance of a brown powder.[29]
- ^ On the analogy between boron and metals, Greenwood[51] commented that: 'The extent to which metallic elements mimic boron (in having fewer electrons than orbitals available for bonding) has been a fruitful cohering concept in the development of metalloborane chemistry…Indeed, metals have been referred to as ‘honorary boron atoms’ or even as ‘flexiboron atoms’. The converse of this relationship is clearly also valid…'.
- ^ 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,[81] and with hydrofluoric acid to give hexafluorosilicic acid H2SiF6.[82]
- ^ Temperatures above 400 ºC are required to form a noticeable surface oxide layer.[86]
- ^ Arsenic also exists as a naturally occurring (but rare) allotrope (arsenolamprite), this being a semiconductor with a band gap of around 0.3 eV or 0.4 eV. Arsenic can also be prepared in a semiconducting amorphous form, with a band gap of around 1.2–1.4 eV.[106][107]
- ^ Whilst As2O3 is usually regarded as being amphoteric a few sources instead say it is (weakly)[126][127] 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)[128][129]
- ^ Antimony can also be prepared in an amorphous semiconducting black form, with an estimated (temperature-dependent) band gap of 0.06–0.18 eV.[138]
- ^ Olmsted and Williams[176] 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'.
- ^ Jones[215] 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'.
- ^ Liquid carbon may[225] or may not[226] be a metallic conductor, depending on pressure and temperature; see also.[227]
- ^ 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.[238]
- ^ A mnemonic which captures the elements commonly recognized as metalloids goes: Up, up-down, up-down, up...are the metalloids! [254]
- ^ Rochow,[257] who would later write his 1966 monograph The metalloids,[258] commented that, 'In some respects selenium acts like a metalloid and tellurium certainly does'.
- ^ A third option is to include astatine both as a nonmetal and as a metalloid.[288]
- ^ The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.[298] found that liquid boron behaved like a metal. Glorieux et al [299] characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity. Millot et al.[300] reported that the emissivity of liquid boron was not consistent with that of a liquid metal.
- ^ A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.[304]
- ^ 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;[340] Pitzer 1975;[341] Schrobilgen 2011[342]).
- ^ Aluminium sometimes is[354] or is not[355] counted as a post-transition metal.
- ^ For example: intermediate electrical conductivity;[368] a relatively narrow band gap;[369][370] light sensitivity.[368]
- ^ White phosphorus is the most common, industrially important,[371] and easily reproducible allotrope. For those reasons it is the standard state of the element.[372] Paradoxically, it is also thermodynamically the least stable, as well as the most volatile and reactive form.[373]
- ^ On the fuzziness of metalloids see, for example: Rouvray;[379] Cobb & Fetterolf;[380] and Fellet.[381] For the 'buffer zone' terminology see Rochow.[382] For examples of the application of a single criterion to classify metalloids see:
- Mahan and Myers,[383] who use electrical conductivity.
- Miessler and Tarr,[384] who use electronegativity.
- Hutton and Dickerson,[385] who rely on the acid-base behaviour of group oxides.
Kneen, Rogers & Simpson[386] 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.[387] They characterize metalloids on the concurrent basis of ionization energy, electronegativity and electrical behaviour.
- ^ Oderberg[396] argues on ontological grounds that anything that is not a metal, is a nonmetal and that this includes semi-metals (i.e. metalloids).
- ^ Some authors only recognize elements as either metals or nonmetals.
- ^ Copernicium is reported to be the only metal known to be a gas at room temperature.[409][410][411][412]
- ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[417][418]
- ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[420][421] If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.[422][268][269]
- ^ Nonmetals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.[423][424]
- ^ Chedd[430] 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[431] 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'.
- ^ Rochow[439] 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,[440] 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,[441] 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.,[442] 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).'
- ^ In contrast, Jones[443] (writing on the role of classification in science) observes that, 'Classes are usually defined by more than two attributes.'
- ^ 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.[445][446] Astatine has an estimated IE of ~210±10 kcal/mol[447] and an EN of 2.2. However its electronic band structure is not known with any great degree of certainty.
- ^ Gallium is unusual (for a metal) in having a packing efficiency of just 39%.[452] Other notable values are 42.9 for bismuth[450] and 58.5 for liquid mercury.[453]
- ^ 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.[456][457] Otherwise nonmetallic behaviour is anticipated. The Goldhammer-Herzfeld criterion is based on classical arguments.[458] It nevertheless offers a relatively simple first order rationalization for the occurrence of metallic character amongst the elements.[459]
- ^ a b Haller 2006, p. 3
- ^ Chedd 1969, pp. 58, 78
- ^ National Research Council 1984, p. 43
- ^ a b c d Goldsmith 1982, p. 526
- ^ a b c d Hawkes 2001, p. 1686
- ^ Hawkes 2001, p. 1687
- ^ Sharp 1981, p. 299
- ^ Cusack 1987, p. 360
- ^ Kelter, Mosher & Scott 2009, p. 268
- ^ Hill & Holman 2000, p. 41
- ^ King 1979, p. 13
- ^ a b Hopkins & Bailar 1956, p. 458
- ^ Glinka 1959, p. 77
- ^ Wiberg 2001, p. 1279
- ^ Belpassi et al. 2006, pp. 4543–4554
- ^ Schmidbaur & Schier 2008, pp. 1931–1951
- ^ Boylan 1962, p. 493
- ^ Sherman & Weston 1966, p. 64
- ^ Wulfsberg 1991, p. 201
- ^ Kotz, Treichel & Weaver 2009, p. 62
- ^ a b Mann et al. 2000, p. 2783
- ^ Segal 1989, p. 965
- ^ McMurray & Fay 2009, p. 767
- ^ Bucat 1983, p. 26
- ^ Brown c. 2007
- ^ a b Swift & Schaefer 1962, p. 100
- ^ a b Hawkes 2010
- ^ a b c Holt, Rinehart & Wilson c. 2007
- ^ a b Housecroft & Constable 2006, p. 331
- ^ Oganov 2010, p. 212
- ^ Donohue 1982, p. 48
- ^ Housecroft & Constable 2006, p. 332
- ^ Oganov et al. 2009, pp. 863–864
- ^ Russell & Lee 2005, pp. 358–360
- ^ Housecroft & Constable 2006, p. 333
- ^ Berger 1997, p. 37
- ^ Greenwood & Earnshaw 2002, p. 144
- ^ Prudenziati 1977, p. 242
- ^ a b c d e f Buzea & Robbie 2005
- ^ Rayner-Canham & Overton 2006, p. 291
- ^ Bowser 1993, p. 393
- ^ Grimes 2011, pp. 17–18
- ^ Greenwood & Earnshaw 2002, p. 141
- ^ Henderson 2000, p. 58
- ^ Housecroft & Constable 2006, pp. 360–372
- ^ Parry et al. 1970, pp. 438, 448–451
- ^ a b Fehlner 1990, p. 202
- ^ a b Greenwood & Earnshaw 2002, p. 145
- ^ Houghton 1979, p. 59
- ^ Fehlner 1990, pp. 204, 207
- ^ Greenwood 2001, p. 2057
- ^ Salentine 1987, pp. 128–132
- ^ MacKay, MacKay & Henderson 2002, pp. 439–440
- ^ Kneen, Rogers & Simpson 1972, p. 394
- ^ a b c d e f Hiller & Herber 1960, inside front cover; p. 225
- ^ Watt 1958, p. 387
- ^ Sharp 1983
- ^ a b c d e f Puddephatt & Monaghan 1989, p. 59
- ^ Mahan 1965, p. 485
- ^ a b c d e f g h Rao 2002, p. 22
- ^ Haiduc & Zuckerman 1985, p. 82
- ^ a b Greenwood & Earnshaw 2002, p. 331
- ^ Wiberg 2001, p. 824
- ^ Hamm 1969, p. 641
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- ^ Allen & Ordway 1968, p. 152
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- ^ Geckeler 1987, p. 20
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- ^ Haller 2006, p. 4: 'The study and understanding of the physics of semiconductors progressed slowly in the 19th and early 20th centuries…Impurities and defects…could not be controlled to the degree necessary to obtain reproducible results. This led influential physicists, including W. Pauli and I. Rabi, to comment derogatorily on the 'Physics of Dirt' '
- ^ Hoddeson 2007, pp. 25–34 (29)
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- ^ Burrows et al. 2009, p. 1192: 'Although the elements are conveniently described as metals, metalloids, and nonmetals, the transitions are not exact...'.
- ^ Jones 2010, p. 170
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- ^ a b c Warren & Geballe 1981
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- ^ IUPAC 2006–, rhombohedral graphite entry
- ^ Mingos 1998, p. 171
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- ^ Savvatimskiy 2005, p. 1138
- ^ Togaya 2000
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- ^ Wiberg 2001, p. 795
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- ^ Bailar et al. 1989, p. 743
- ^ Moore et al. 1985
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- ^ a b Metcalfe et al. 1974, p. 539
- ^ Cobb & Fetterolf 2005, p. 64
- ^ Metcalfe, Williams & Castka 1982, p. 585
- ^ Ogata, Li & Yip 2002
- ^ Boyer et al. 2004, p. 1023
- ^ Russell & Lee 2005, p. 359
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- ^ a b Hamm 1969, p. 653
- ^ Stott 1956, p. 100
- ^ Steele 1966, p. 60
- ^ Daub & Seese 1996, pp. 70, 109: 'Aluminum is not a metalloid but a metal because it has mostly metallic properties.'
- ^ Denniston, Topping & Caret 2004, p. 57: 'Note that aluminum (Al) is classified as a metal, not a metalloid.'
- ^ Hasan 2009, p. 16: 'Aluminum does not have the characteristics of a metalloid but rather those of a metal.'
- ^ Tuthill 2011
- ^ Young et al. 2010, p. 9
- ^ a b Craig 2003, p. 391. Selenium is included in this work on account of its 'near metalloidal' status.
- ^ Rochow 1957
- ^ Rochow 1966
- ^ Moss 1952, p. 192
- ^ a b Glinka 1965, p. 356
- ^ Evans 1966, pp. 124–5
- ^ Regnault 1853, p. 208
- ^ Scott & Kanda 1962, p. 311
- ^ Cotton et al. 1999, pp. 496, 503–504
- ^ Arlman 1939
- ^ Bagnall 1966, pp. 135, 142–143
- ^ a b Berger 1997, pp. 86–87
- ^ a b Kozyrev 1959, p. 104
- ^ a b Chizhikov & Shchastlivyi 1968, p. 25
- ^ Glazov, Chizhevskaya & Glagoleva 1969, p. 86
- ^ Chao & Stenger 1964
- ^ Snyder 1966, p. 242
- ^ Fritz & Gjerde 2008, p. 235
- ^ a b Cotton et al. 1999, p. 502
- ^ Wiberg 2001, p. 594
- ^ Greenwood & Earnshaw 2002, p. 786
- ^ Schwietzer & Pesterfield 2010, pp. 242–243
- ^ Bagnall 1966, p. 41
- ^ Nickless 1968, p. 79
- ^ Bagnall 1990, pp. 313–314
- ^ Lehto & Hou 2011, p. 220
- ^ Siekierski & Burgess 2002, p. 117: 'The tendency to form X2– anions decreases down the Group [16 elements]...'
- ^ Bagnall 1957, p. 62
- ^ Fernelius 1982, p. 741
- ^ Bagnall 1966, p. 41
- ^ Barrett 2003, p. 119
- ^ Harding, Johnson & Janes 2002, p. 61
- ^ Long & Hentz 1986, p. 58
- ^ a b Hawkes 1999
- ^ Roza 2009, p. 12
- ^ Keller 1985
- ^ Vasáros & Berei 1985, p. 109
- ^ Haissinsky & Coche 1949, p. 400
- ^ Brownlee et al. 1950, p. 173
- ^ Samsonov 1968, p. 590
- ^ Rossler 1985, pp. 143–144
- ^ Rao & Ganguly 1986
- ^ Krishnan et al. 1998
- ^ Glorieux, Saboungi & Enderby 2001
- ^ Millot et al. 2002
- ^ Vasáros & Berei 1985, p. 117
- ^ Kaye & Laby 1973, p. 228
- ^ Siekierski & Burgess 2002, pp. 65, 122
- ^ Emsley 2001, p. 48
- ^ Champion et al. 2010
- ^ Batsanov 1971, p. 811
- ^ a b Swalin 1962, p. 216
- ^ Feng & Lin 2005, p. 157
- ^ Borst 1982, pp. 465, 473
- ^ Schwietzer & Pesterfield 2010, pp. 258–260
- ^ Olmsted & Williams 1997, p. 328
- ^ Daintith 2004, p. 277
- ^ Restrepo et al. 2004, p. 69
- ^ Restrepo et al. 2006, p. 411
- ^ Dunstan 1968, pp. 310, 409. Dunstan lists Be, Al, Ge (maybe), As, Se (maybe), Sn, Sb, Te, Pb, Bi and Po as metalloids (pp. 310, 323, 409, 419).
- ^ Tilden 1876, pp. 172, 198–201
- ^ Smith 1994, p. 252
- ^ Bodner & Pardue 1993, p. 354
- ^ Bassett et al. 1966, p. 127
- ^ a b Rausch 1960
- ^ Thayer 1977, p. 604
- ^ Chalmers 1959, p. 72
- ^ United States Bureau of Naval Personnel 1965, p. 26
- ^ Siebring 1967, p. 513
- ^ Wiberg 2001, p. 282
- ^ a b Friend 1953, p. 68
- ^ Murray 1928, p. 1295
- ^ Hampel & Hawley 1966, p. 950
- ^ Stein 1985
- ^ Stein 1987, pp. 240, 247–248
- ^ Hatcher 1949, p. 223
- ^ Taylor 1960, p. 614
- ^ Considine & Considine 1984, p. 568
- ^ Cegielski 1998, p. 147
- ^ The American heritage science dictionary 2005 p. 397
- ^ Woodward 1948, p. 1
- ^ Fernelius & Robey 1935, p. 54
- ^ Szabó & Lakatos 1954, p. 133
- ^ Sanderson 1957
- ^ Stein 1969
- ^ Pitzer 1975
- ^ Schrobilgen 2011: 'The chemical behaviour of radon is similar to that of a metal fluoride and is consistent with its position in the periodic table as a metalloid element.'
- ^ Petty 2007, p. 25
- ^ Reid 2002. Reid refers to near metalloids as Al, C or P.
- ^ Carr 2011. Carr refers to near metalloids as C, P, Se, Sn and Bi.
- ^ Russell & Lee 2005, p. 5
- ^ Parish 1977, pp. 178, 192–3
- ^ Eggins 1972, p. 66
- ^ Rayner-Canham & Overton 2006, pp. 29–30
- ^ Stott 1956, pp. 99–106; 107
- ^ Rayner-Canham & Overton 2006, pp. 29–30: 'There is a subgroup of metals, those closest to the borderline, that exhibit some chemical behaviour that is more typical of the semimetals, particularly formation of anionic species. These nine chemically weak metals are beryllium, aluminium, zinc, gallium, tin, lead, antimony, bismuth, and polonium.'
- ^ Hill & Holman 2000, p. 40
- ^ Farrell & Van Sicien 2007, p. 1442: 'For simplicity, we will use the term poor metals to denote one with a significant covalent, or directional character.'
- ^ a b Whitten et al. 2007, p. 868
- ^ a b Cox 2004, p. 185
- ^ Bailar et al. 1989, p. 742–3
- ^ Atkins et al. 2006, pp. 320–21
- ^ Rochow 1966, p. 7
- ^ Taniguchi et al. 1984, p. 867: '...black phosphorus...[is] characterized by the wide valence bands with rather delocalized nature.'
- ^ Morita 1986, p. 230
- ^ Carmalt & Norman 1998, pp. 1–38: 'Phosphorus...should therefore be expected to have some metalloid properties'.
- ^ Du et al. 2010. Interlayer interactions in black phosphorus, which are attributed to van der Waals-Keesom forces, are thought to contribute to the smaller band gap of the bulk material (calculated 0.19 eV; observed 0.3 eV) as opposed to the larger band gap of a single layer (calculated ~0.75 eV).
- ^ Stuke 1974, p. 178
- ^ Cotton et al. 1999, p. 501
- ^ Steudel 1977, p. 240: '...considerable orbital overlap must exist, to form intermolecular, many-center...[sigma] bonds, spread through the layer and populated with delocalized electrons, reflected in the properties of iodine (lustre, color, moderate electrical conductivity).'
- ^ Segal 1989, p. 481: 'Iodine exhibits some metallic properties...'.
- ^ Jain 2005, p. 1458
- ^ a b Lutz 2011, p. 16
- ^ Yacobi & Holt 1990, p. 10
- ^ Wiberg 2001, p. 160
- ^ Eagleson 1994, p. 820
- ^ Oxtoby, Gillis & Campion 2008, p. 508
- ^ Greenwood & Earnshaw 2002, pp. 479, 482
- ^ Deprez & McLachan 1988
- ^ Addison 1964 (P, Se, Sn}
- ^ Marković, Christiansen & Goldman 1998 (Bi)
- ^ Nagao et al. 2004
- ^ a b Roher 2001, pp. 4–6
- ^ Rouvray 1995, p. 546. Rouvray submits that classifying the electrical conductivity of the elements using the overlapping domains of metals, metalloids, and nonmetals better reflects reality than a strictly black or white paradigm.
- ^ Cobb & Fetterolf 2005, p. 64: 'The division between metals and nonmetals is rather fuzzy, so the elements in the immediate vicinity of the zigzag staircase line are called metalloids, which means they don't fit either definition exactly.'
- ^ Fellet 2011: 'Chemistry has all sorts of fuzzy definitions'.
- ^ Rochow 1977, p. 14
- ^ Mahan & Myers 1987, p. 682
- ^ Miessler & Tarr 2004, p. 243
- ^ Hutton & Dickerson 1970, p. 162
- ^ Kneen, Rogers and Simpson 1972, p. 219
- ^ Masterton & Slowinski 1977, p. 160, as discussed in the Semi-quantitative characterization section of this article
- ^ a b Tyler 1948, p. 105
- ^ Reilly 2002, pp. 5–6
- ^ Hampel & Hawley 1976, p. 174
- ^ Goodrich 1844, p. 264
- ^ The Chemical News 1897, p. 189
- ^ a b Hampel & Hawley 1976, p. 191
- ^ Lewis 1993, p. 835
- ^ Hérold 2006, pp. 149–150
- ^ Oderberg 2007, p. 97
- ^ Horvath 1973, p. 336
- ^ Brown & Holme 2006, p. 57
- ^ Simple Memory Art c. 2005
- ^ Chedd 1969, pp. 12–13
- ^ a b Gray 2009, p. 9
- ^ Booth & Bloom 1972, p. 426
- ^ a b Cox 2004, pp. 17, 18, 27–28
- ^ a b Silberberg 2006, p. 305–313
- ^ Rayner-Canham 2011
- ^ Kneen, Rogers & Simpson, 1972, p. 263. Columns 2 and 4 are sourced from this reference unless otherwise indicated.
- ^ Stoker 2010, p. 62
- ^ Chang 2002, p. 304. Chang speculates that the melting point of francium would be about 23 °C.
- ^ New Scientist 1975
- ^ Soverna 2004
- ^ Eichler, Aksenov & Belozeroz et al. 2007
- ^ Austen 2010
- ^ a b c Rochow 1966, p. 4
- ^ Hunt 2000, p. 256
- ^ Pottenger & Bowes 1976, p. 138
- ^ McQuarrie & Rock 1987, p. 85
- ^ Desai, James & Ho 1984, p. 1160
- ^ Matula 1979, p. 1260
- ^ Choppin & Johnsen 1972, p. 351
- ^ Schaefer 1968, p. 76
- ^ Carapella 1968, p. 30
- ^ Glazov, Chizhevskaya & Glagoleva 1969 p. 86
- ^ Bogoroditskii & Pasynkov 1967, p. 77
- ^ Jenkins & Kawamura 1976, p. 88
- ^ Wulfsberg 2000, p. 620
- ^ Bailar et al. 1989, p. 742
- ^ Metcalfe, Williams & Castka 1966, p. 72
- ^ Chang 1994, p. 311
- ^ Pauling 1988, p. 183
- ^ Chedd 1969, pp. 24–25
- ^ Adler 1969, pp. 18–19
- ^ Hultgren 1966, p. 648
- ^ Young & Sessine 2000, p. 849
- ^ Bassett et al. 1966, p. 602
- ^ Atkins et al. 2006, pp. 8, 122–23
- ^ NIST 2010. Values shown in the above table have been converted from the NIST values, which are given in eV.
- ^ Berger 1997
- ^ Lovett 1977, p. 3
- ^ Rochow 1966, pp. 4–7
- ^ Hill & Holman 2000, p. 41
- ^ Bond 2005, p. 3
- ^ Edwards et al. 2010, p. 958
- ^ Jones 2010, p. 169
- ^ 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.'
- ^ Kraig, Roundy & Cohen 2004, p. 412
- ^ Alloul 2010, p. 83
- ^ NIST 2011. They cite Finkelnburg & Humbach (1955) who give a figure of 9.2±0.4 eV = 212.2±9.224 kcal/mol.
- ^ Van Setten et al. 2007, pp. 2460–61 (B)
- ^ Russell & Lee 2005, p. 7 (Si, Ge)
- ^ a b c Pearson 1972, p. 264 (As, Sb, Te; also black P)
- ^ Russell & Lee 2005, p. 1
- ^ Russell & Lee 2005, pp. 6–7, 387
- ^ Okakjima & Shomoji 1972, p. 258
- ^ Kitaĭgorodskiĭ 1961, p. 108
- ^ a b c Neuburger 1936
- ^ Herzfeld 1927
- ^ Edwards 2000, pp. 100–103
- ^ Edwards 1999, p. 416
- ^ a b Edwards & Sienko 1983, p. 695
- ^ Edwards et al. 2010
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- ^ Gordh, Gordh & Headrick 2003, p. 753
- ^ Foster 1936, pp. 212–13
- ^ Brownlee et al. 1943, p. 293
- ^ a b Klemm 1950, pp. 133–142
- ^ Reilly 2004, p. 4
- ^ Walters 1982, pp. 32–33
- ^ 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.'
- ^ Slade 2006, p. 16
- ^ Corwin 2005, p. 80
- ^ Bradbury et al. 1957, pp. 157, 659
- ^ Hoppe 2011
- ^ Miller, Lee & Choe 2002, p. 21
- ^ King 2004, pp. 196–198
- ^ Ferro & Saccone 2008, p. 233
- ^ Pashaey & Seleznev 1973, p. 565
- ^ Gladyshev & Kovaleva 1998, p. 1445
- ^ Eason 2007, p. 294
- ^ Johansen & Mackintosh 1970, pp. 121–124
- ^ Divakar, Mohan & Singh 1984, p. 2337
- ^ Dávila et al. 2002, p. 035411-3
- ^ Jezequel & Thomas 1997, pp. 6620–6626
- ^ 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.'
- ^ 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...'
- ^ IUPAC 1959, p. 10
- ^ IUPAC 1971, p. 11
- ^ Atkins et al. 2010, p. 20
- ^ IUPAC 2005
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