Metalloid

Elements recognized as metalloids

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

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.

Potatoas are metalloids '

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 has 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 1960s onward. All six elements have toxic and medicinal properties to varying degrees.

Other elements less commonly recognized as metalloids include carbon, aluminium, selenium, polonium and astatine. On a standard periodic table these elements, as well as the elements commonly recognized as metalloids, occur in or near a diagonal region of the p-block, having its main axis anchored by boron at one end and astatine at the other. Some periodic tables include a dividing line between metals and nonmetals and it is generally the elements adjacent to this line or, less frequently, one or more of the elements adjacent to those elements, that are identified as metalloids.

The term metalloid was first popularly used to refer to nonmetals. Its more recent meaning as a category of elements with intermediate or hybrid properties 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 that more specifically refers to the electronic band structure of a substance rather than the overall classification of a chemical element.

Definitions

There is no universally agreed or rigorous definition of a metalloid. The feasibility of establishing a specific definition has also been questioned, noting anomalies that can be found in several such attempted constructs. Classifying any particular element as a metalloid has been described as 'arbitrary'.

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.'
• '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.'
• 'Chemists sometimes use the name metalloid…for these elements which are difficult to classify one way or the other.'
• '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…'.

More broadly, metalloids have also been referred to as:

• 'elements that…are somewhat of a cross between metals and nonmetals' or
• 'weird in-between elements.'

The criterion that metalloids be 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.' Although most other elements have a mixture of metallic and nonmetallic properties, 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. 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.

Elements commonly recognized as metalloids

This section presents brief sketches of the physical and chemical properties of the applicable elements—in their most thermodynamIcally stable forms under ambient conditions—followed by information about their typical (shared) uses as metalloids, their toxic and medicinal properties, and their abundance. For complete profiles, including history, production, specific uses, and biological roles and precautions, see the main article for each element.

Boron, silicon, germanium, arsenic, antimony and tellurium are commonly classified as metalloids. One or more from among selenium, polonium or astatine are sometimes added to the list. Boron is sometimes excluded from the list, by itself or together with silicon. Tellurium is sometimes not regarded as a metalloid. The inclusion of antimony, polonium and astatine as metalloids has also been questioned.

Boron

Main article: Boron

Boron

Pure boron appears as a shiny, silver-grey crystalline solid. It is about ten percent lighter than aluminium but, unlike the latter, is hard and brittle. It is barely reactive under normal conditions, except for attack by fluorine, and has a melting point several hundred degrees higher than that of steel. Boron is a semiconductor, with a room temperature electrical conductivity of 1.5 × 10⁻⁶ S•cm⁻¹ and a band gap of about 1.56 eV. It becomes superconducting at a pressure of 250 GPa and a temperature of 11.2 K.

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. 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, 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. 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). 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.

Most of the chemistry of boron is nonmetallic in nature. The small size of the boron atom, however, enables the preparation of many interstitial alloy-type borides. Analogies between boron and transition metals have also been noted in the formation of complexes, 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–. The aqueous chemistry of boron, more conventionally, is characterised by the formation of many different polyborate anions. Given its high charge-to-size ratio nearly all compounds of boron are covalent, barring some complexed anionic and cationic species. Boron has a strong affinity for oxygen, a characteristic manifested in the extensive chemistry of the borates. The oxide B₂O₃ is polymeric in structure, weakly acidic, and a glass former. Organometallic compounds of boron have been known since the 19th century (see organoboron chemistry).

Silicon

Main article: Silicon

Silicon

Silicon appears as a shiny crystalline solid, with a blue-grey metallic lustre. As with boron it is about ten per cent less dense than aluminium, hard and brittle. It is a relatively unreactive element, the massive, crystalline form (especially if pure) being 'remarkably inert to all acids, including hydrofluoric'. Less pure silicon, and the powdered form, are variously susceptible to attack by strong or heated acids, as well as by steam and fluorine. Silicon also dissolves in hot aqueous alkalis with the evolution of hydrogen, behaving in this way like metals such as beryllium, aluminium, zinc, gallium and indium. It melts at about the same temperature as steel. Silicon is a semiconductor with an electrical conductivity of 10⁻⁴ S•cm⁻¹ and a band gap of about 1.11 eV. When it melts, silicon becomes a reasonable metal with an electrical conductivity of 1.0–1.3 × 10⁴ S•cm⁻¹, a value similar to that of liquid mercury. At a pressure of 12 GPa and a temperature of 8.5 K silicon becomes superconducting.

The chemistry of silicon is generally nonmetallic (covalent) in nature. It does, however, form alloys with metals such as iron and copper. Silicon shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. The high bond strength of the silicon-oxygen bond dominates the chemical behaviour of silicon. Polymeric silicates, built up by tetrahedral SiO₄ units sharing their oxygen atoms, represent the most abundant and important compounds of silicon. The polymeric borates, comprising linked trigonal and tetrahedral BO₃ or BO₄ units, are built on similar structural principles. The oxide SiO₂ is polymeric in structure, weakly acidic, and a glass former. Traditional organometallic chemistry includes the carbon compounds of silicon (see organosilicon).

Germanium

Main article: Germanium

Germanium

Germanium appears as a shiny grey-white solid. It is about one-third lighter than iron, hard and brittle. It is mostly unreactive at room temperature but is slowly attacked by hot concentrated sulphuric or nitric acid. Germanium also reacts with molten caustic soda to yield sodium germanate Na₂GeO₃, together with the evolution of hydrogen. 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 and a band gap of 0.67 eV. Liquid germanium is a metallic conductor, with an electrical conductivity on par with that of liquid mercury. At a pressure of 5.4 GPa and a temperature of 11.5 K germanium becomes superconducting.

Most of the chemistry of germanium is characteristic of a nonmetal. It does however form alloys with, for example, aluminium and gold. Germanium shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. 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. Germanium analogues of all of the major types of silicates have been prepared. 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. The oxide GeO₂ is polymeric, amphoteric, and a glass former. Germanium has an established organometallic chemistry (see organogermanium chemistry).

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). 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. 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. Arsenic only melts under a pressure of 38.6 atm, at around half the melting point of steel. Arsenic is a semimetal with an electrical conductivity of around 3.9 × 10⁴ S•cm⁻¹ and a band overlap of 0.5 eV. Liquid arsenic is a semiconductor with a band gap of 0.15 eV. At a pressure of 24 GPa and a temperature of 2.7 K arsenic becomes superconducting.

The chemistry of arsenic is predominately nonmetallic in character. It does however form alloys with many metals, most of these being brittle. Arsenic shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. Arsenic generally forms compounds in which it has an oxidation state of +3 or +5. The halides, and the oxides and their derivatives are illustrative examples. In the trivalent state, arsenic shows some incipient metallic properties. Thus, the halides are hydrolysed by water but these reactions, particularly those of the chloride, are reversible with the addition of a hydrohalic acid. As well, and as noted below, the oxide is acidic but weakly amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties. 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. The oxide As₂O₃ is polymeric, amphoteric, and a glass former. Arsenic has an extensive organometallic chemistry (see organoarsenic chemistry).

Antimony

Main article: Antimony

Antimony

Antimony appears as a silver-white solid with a blue tint and a brilliant lustre. 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). 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. It is not affected by molten alkali. Antimony is capable of displacing hydrogen from water, when heated: 2Sb + 3H₂O → Sb₂O₃ + 3H₂. 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 and a band overlap of 0.16 eV. Liquid antimony is a metallic conductor with an electrical conductivity of around 5.3 × 10⁴ S•cm⁻¹. At a pressure of 8.5 GPa and a temperature of 3.6 K antimony becomes superconducting.

Most of the chemistry of antimony is characteristic of a nonmetal. It does however form alloys with one or more metals such as aluminium, iron, nickel, copper, zinc, tin, lead and bismuth. Antimony shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. Like arsenic, antimony generally forms compounds in which it has an oxidation state of +3 or +5. The halides, and the oxides and their derivatives are illustrative examples. 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. Tripositive antimony is amphoteric; quinquepositive antimony is (predominately) acidic. 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. 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. The oxide Sb₂O₃ is a polymeric, amphoteric, and a glass former. Antimony has an extensive organometallic chemistry (see organoantimony chemistry).

Tellurium

Main article: Tellurium

Tellurium

Tellurium appears as a silvery-white solid with a shiny lustre. It is about 15 per cent less dense than iron, brittle, and the softest of the commonly recognised metalloids, being marginally harder than sulphur. 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. 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, the basic nitrate (Te₂O₄H)⁺(NO₃)^–, or the oxide sulphate Te₂O₃(SO₄). 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. At higher temperatures tellurium is sufficiently plastic to be extrudable. 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. Tellurium is a semiconductor with an (intrinsic) electrical conductivity of around 1.0 S•cm⁻¹ and a band gap of 0.32 to 0.38 eV. Liquid tellurium is a semiconductor, with an electrical conductivity, on melting, of around 1.9 × 10³ S•cm⁻¹ Superheated liquid tellurium is a metallic conductor. At a pressure of 35 GPa and a temperature of 7.4 K tellurium becomes superconducting.

Most of the chemistry of tellurium is characteristic of a nonmetal. It does however form alloys with, for example, aluminium, silver and tin. Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. Tellurium generally forms compounds in which it has an oxidation state of −2, +4 or +6, with the tetrapositive state being the most stable. It combines easily with most other elements to form binary tellurides X_xTe_y 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. 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. Tellurium forms a polymeric, amphoteric, glass-forming oxide TeO₂. The latter is also a 'conditional' glass-forming oxide—it will form a glass with a very small amount of additive. Tellurium has an extensive organometallic chemistry (see organotellurium chemistry).

Typical applications

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

Metalloids are too brittle to have any structural uses in their pure forms. Typical applications have instead encompassed their presence in, or specific uses as, glasses (oxide and metallic); fire retardants; alloying components; semiconductors and electronics; and optical storage media.

Glass formation

Fibre-optic strands, usually made of pure silicon dioxide glass, but with additives such as boron trioxide or germanium dioxide for increased sensitivity.

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. These compounds have found or continue to find practical uses in chemical, domestic and industrial glassware and optics. Boron trioxide is used as a glass fibre additive; it is also a component of borosilicate glass, which is widely used for laboratory glassware, as well as in home ovenware. Silicon dioxide forms the basis of ordinary domestic glassware. Germanium dioxide is used as glass fibre additive, as well as in infrared optical systems. Arsenic trioxide is used in the glass industry as a decolorizing and fining agent, as is antimony trioxide. Tellurium dioxide finds application in laser and nonlinear optics.

Amorphous metallic glasses are generally most easily prepared if one of the components is a metalloid or 'near metalloid' such as boron, carbon, silicon, phosphorus or germanium. Aside from thin films deposited at very low temperatures, the first known metallic glass was a metal-metalloid alloy of composition Au₇₅Si₂₅ reported in 1960. A metallic glass having a strength and toughness not previously seen in any other material, of composition Pd_82.5P₆Si_9.5Ge₂, was reported in 2011.

Fire retardants

Compounds of boron, silicon, arsenic and antimony have found or continue to find uses as fire retardants. Boron, in the form of borax, has been used as a textile fire retardant since at least the 18th century. Silicon compound additives such as silicones, silanes, silsesquioxane, silica and silicates, some of which were developed as alternatives to more toxic halogenated products, can considerably improve the flame retardancy of plastic materials. Arsenic compounds in the form of sodium arsenite or sodium arsenate are effective fire retardants for wood but were less frequently used due to their toxicity. Antimony, as antimony trioxide, finds its greatest use as a flame retardant additive.

Alloys

Copper-germanium master alloy pellets, likely ~84% Cu; 16% Ge. When combined with silver the result is a tarnish resistant sterling silver. Also present are two silver pellets.

All the elements commonly recognised as metalloids can form alloys. Writing early in the history of intermetallic compounds, the British metallurgist Cecil Desch observed 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. More specifically, compounds of silicon, germanium, arsenic and antimony with the poor metals, it has also been suggested, 'are probably best classed as alloys.'

In terms of individual alloy types, those with transition metals are well-represented. Boron can form intermetallic compounds and alloys with such metals, of the composition M_nB, if n > 2. Ferroboron (15% boron) is used to introduce boron into steel; nickel-boron alloys are ingredients in welding alloys and face-hardening compositions for the engineering industry. Alloys of silicon with iron, and with aluminium, are widely used by the steel and automotive industries, respectively. Germanium forms many alloys, most importantly with the coinage metals. Arsenic can form alloys with metals, including platinum and copper; it is also employed as an additive for copper and copper alloys in order to improve corrosion resistance. Antimony is well known as an alloy former, including with the coinage metals. Its alloys are exemplified by pewter (a tin alloy with up to 20% antimony) and type metal (a lead alloy with up to 25% antimony). Tellurium readily forms alloys with iron, in the form of ferrotellurium (50–58% tellurium), and with copper, in the form of copper tellurium (40–50% tellurium). Ferrotellurium, in particular, is used as a stabilizer for carbon in steel casting.

Semiconductors and electronics

A 52 megawatt (peak capacity) photovoltaic power station, using type II/VI cadium-telluride solar cells, located in the Waldpolenz Solar Park, Germany.

All the elements commonly recognized as metalloids (or their compounds) have found application in the semiconductor or solid-state electronic industries. 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. Silicon is the foremost commercial semiconductor; it forms the basis of modern electronics and information and communication technologies. 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. Silicon has largely replaced germanium in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process. Semiconducting silicon-germanium 'alloys' have however been growing in use, particularly for wireless communication devices; these alloys exploit the higher carrier mobility of germanium. 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. 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. In the form of bismuth telluride (alloyed with selenium and antimony), tellurium is also a component of thermoelectric devices used for refrigeration or portable power generation. More ubiquitously, five metalloids—boron, silicon, germanium, arsenic and antimony—can be found in cell phones (along with at least 39 other metals and nonmetals). Tellurium, the last of the commonly recognized metalloids, is a component of phase change memory (see also below) and, as such, either has achieved cell phone incorporation, or is expected to find such use.

Optical storage

Varying compositions of GeSbTe ('GST alloys') and Ag- and In- doped Sb₂Te ('AIST alloys'), being examples of phase-change materials, are widely used in rewritable optical disks and phase-change memory devices. By applying heat, they can be switched between amorphous (glassy) and crystalline states, thereby changing their optical and electrical properties and allowing the storage of information.

Biological interactions

Arsenic trioxide or white arsenic, one of the most toxic and prevalent forms of arsenic. The antileukemic properties of white arsenic were first reported in 1878.

Of the elements commonly recognized as metalloids: two (arsenic and antimony) are notably toxic; two (boron and silicon) or three (arsenic) are essential trace elements; and four (boron, silicon, arsenic and antimony) have medical applications (with germanium and tellurium also being thought to have potential). All six elements have toxic and medicinal properties to greater or lesser degrees.

B. Boron is used in insecticides and herbicides. Notwithstanding, it is an essential trace element. As boric acid, it also has antiseptic, antifungal, and antiviral properties. Si. Silicon is not toxic however it makes up the central component of silatrane, a highly toxic rodenticide. Long-term inhalation of silica dust also causes silicosis, a fatal disease of the lungs. Silicon is, however, an essential trace element. It can also be applied to badly burned patients, in the form a silcone gel, to reduce scarring. Ge. Salts of germanium are potentially harmful to humans and animals if ingested on a prolonged basis. It is not an essential trace element. Although interest in the pharmacological actions of germanium compounds is ongoing there is (as yet) no licensed medicine. As. Arsenic is notoriously poisonous. It may possibly be, however, an essential element in ultratrace amounts. Arsenic has been used as a pharmaceutical agent since antiquity and notably for the treatment of syphilis prior to the development of antibiotics. Arsenic is also a component of melarsoprol, a medicinal drug used in the treatment of human African trypanosomiasis or sleeping sickness. In 2003, arsenic trioxide (Trisenox®) was re-introduced for the treatment of acute promyelocytic leukemia, a cancer of the blood and bone marrow. Sb. Unlike metallic antimony, most antimony compounds are poisonous. It is not an essential element. Compounds of antimony are however used as as antiprotozoan drugs, and in some veterinary preparations. Te. Tellurium is not particulary noted for its toxicity although as little as two grams of sodium tellurate, if administered, can be lethal. As well, people exposed to small amounts of airborne tellurium will exude a foul and persistent garlic-like odour. It is not an essential element. Tellurium dioxide has been used to treat seborrhoeic dermatitis; other tellurium compounds were used as antimicrobial agents before the development of antibiotics. Ironically, such compounds may have the potential to act as substitutes for antibiotics that have become ineffective due to increasing bacterial resistance.

Abundance

Various estimates of elemental abundances have been published and these often disagree to some extent. Even so, useful order of magnitude comparisons can be made. Silicon is the second most abundant element (after oxygen) in the Earth's crust and therefore the most prevalent of the recognised metalloids. Boron is about abundant as lead. Germanium and arsenic are on par with tin. Antimony is comparable to silver or mercury. Tellurium is the scarcest of the metalloids; it is roughly as abundant as gold and the platinum group metals.

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, for example, recognized only four: germanium, arsenic, antimony and tellurium. James et al., on the other hand, listed twelve: boron, carbon, silicon, germanium, arsenic, selenium, antimony, tellurium, bismuth, polonium, ununpentium and livermorium. As of 2011 the list of metalloid lists recorded an average of just over seven elements classified as metalloids, per list of metalloids, based on a sample size of 194 lists.

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

Carbon

Main article: Carbon

Carbon (as graphite)

Carbon is ordinarily classified as a nonmetal although it has some metallic properties and is occasionally classified as a metalloid. As applicable, the properties summarised in the following paragraphs are for hexagonal graphitic carbon, the most thermodynamically stable form of carbon under ambient conditions.

In terms of metallic character, carbon has a lustrous appearance and is a fairly good electrical conductor. Its conductivity in the direction of its planes decreases as the temperature is raised, behaving in this way as a metal; it actually has the electronic band structure of a semimetal. 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'. Carbon can form ionic salts, including a sulphate, perchlorate, nitrate, hydrogen selenate, and hydrogen phosphate; in organic chemistry, carbon can also form complex cations—termed carbocations—in which the positive charge is on the carbon atom; examples are CH⁺
₃ and CH⁺
₅, and their derivatives.

In terms of nonmetallic character, carbon is brittle and behaves as a semiconductor perpendicular to the direction of its planes. Most of its chemistry is nonmetallic; it has a relatively high ionization energy and, compared to most metals, a relatively high electronegativity. Its oxide CO₂ forms a medium-strength carbonic acid H₂CO₃.

Aluminium

Main article: Aluminium

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, these properties are sometimes used as a basis to classify aluminium as a metalloid. Its crystalline structure shows some evidence of directional bonding. Although it forms an Al³⁺ cation in some compounds, aluminium bonds covalently in most others. Its oxide is amphoteric, and a conditional glass-former. Aluminium also forms anionic aluminates, such behaviour being considered nonmetallic in character.

Stott labels aluminium as weak metal. It has the physical properties of a good metal but some of the chemical properties of a nonmetal. Steele 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 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.

Selenium

Main article: Selenium

Selenium

Selenium shows borderline metalloid or nonmetal behaviour.

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. The metallic character of selenium is further shown by its lustre and its crystalline structure, the latter of which is thought to include weakly 'metallic' interchain bonding. Selenium can be drawn into thin threads, when molten. It exhibits a reluctance to acquire 'the high positive oxidation numbers characteristic of nonmetals'. It can form cyclic polycations (such as Se²⁺
₈) when dissolved in oleums (an attribute it shares with sulfur and tellurium); and a hydrolysed cationic salt in the form of trihydroxoselenium (IV) perchlorate [Se(OH)₃]⁺.ClO^–
₄.

The nonmetallic character of selenium is shown by its brittleness and the low electrical conductivity (~10⁻⁹ to 10⁻¹² S·cm⁻¹) of its highly purified form. This is comparable to or less than that of bromine (7.95×10^–12 S·cm⁻¹), a nonmetal. Selenium has the electronic band structure of a semiconductor and retains its semiconducting properties in liquid form. It has a relatively high electronegativity (2.55 revised Pauling scale). Its reaction chemistry is mainly that of its nonmetallic anionic forms Se^2–, SeO²⁻
₃ and SeO²⁻
₄.

Selenium is commonly described as a metalloid in the environmental chemistry literature. Processes and reactions affecting its fate in the aquatic environment are similar to those found for arsenic and antimony. Moreover, while trace amounts of selenium are essential to human health, its water soluble salts (in higher concentrations) have a toxicological profile similar to that of arsenic.

Polonium

Main article: Polonium

Polonium, in the form of a thin film on a stainless-steel disc

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

• The metallic conductivity of both of its allotropic forms.
• The presence of the rose-coloured Po²⁺ cation in aqueous solution.
• The many salts it forms.
• The predominating basicity of polonium dioxide.
• The highly reducing conditions required for the formation of the Po^2– anion in aqueous solution.

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).
• Many metal polonides, obtained by heating the elements together at 500–1,000 °C, and containing the Po^2– anion, are also known.

Astatine

Main article: Astatine

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

In terms of metallic indicators:

• Samsonov 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 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 note that elements with an enthalpy of vaporization (EoV) greater than ~42 kJ/mol are metallic when liquid. Such elements include boron, silicon, germanium, antimony, selenium and tellurium. Vásaros & Berei 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, falls just short of the threshold figure.
• Siekierski and Burgess contend or presume that astatine would be a metal if it could form a condensed phase.
• Champion et al. 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. This is consistent with nonmetals (in physics) having separated valence and conduction bands and thereby being either semiconductors or insulators.
• It has the narrow liquid range ordinarily associated with nonmetals (mp 575 K, bp 610).
• Its chemistry in aqueous solution is predominately characterised by the formation of various anionic species.
• Most of its known compounds resemble those of iodine, which is halogen and a nonmetal. Such compounds include astatides (XAt), astatates (XAtO₃), and monovalent interhalogen compounds.

Restrepo et al. 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

Given there is no agreed definition of a metalloid, some other elements are occasionally classified as such. These elements include hydrogen, beryllium, nitrogen, phosphorus, sulfur, zinc, gallium, tin, iodine, lead, bismuth and radon. 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.
• Elements that are otherwise sometimes referred to as poor metals.
• Nonmetallic elements (for example, nitrogen; carbon) that can form alloys with, or modify the properties of, metals.

Near metalloids

Iodine crystals, showing a metallic lustre. Iodine is a semiconductor in the direction of its planes, with a band gap of ~1.3 eV, and an electrical conductivity of 1.7 × 10⁻⁸ S•cm⁻¹ at room temperature. The latter value is higher than that of selenium but lower than that of boron, the least electrically conducting of the recognized 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 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. In 1954, Szabó & Lakatos counted beryllium and aluminium in their list of metalloids, as well as boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine. In 1957, Sanderson 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 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, 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, marked covalent chemistry (molecular or polymeric), and amphoterism. Aluminium, tin and bismuth are examples. They are also referred to as (chemically) weak metals, poor metals, post-transition metals, 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, phosphorus, selenium and iodine. They exhibit metallic lustre, semiconducting properties 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; 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

White tin (left) and grey tin (right)

When an element exists in more than one crystalline form, the different forms are called allotropes. Some allotropes, particulary those of elements located (in periodic table terms) alongside or near the notional dividing line between metals and nonmetals, exhibit more pronounced metallic, metalloidal or nonmetallic behaviour than others.

Tin, for example, has two allotropes: tetragonal 'white' β-tin and cubic 'grey' α-tin, as shown in the picture to the right. White tin is a silvery-white, very shiny, ductile and malleable metal. It is the stable form of tin at or above room temperature and has an electrical conductivity of 9.17 × 10⁴ ohm-cm⁻¹ (~1/6th that of copper). Grey tin, in contrast, usually has the appearance of a grey micro-crystalline powder however it can also be prepared in ordinary crystalline or polycrystalline forms having a semi-lustrous appearance and a brittle comportment. It is the stable form of tin below 13.2 °C (56 °F) and has an electrical conductivity of between 2–5 × 10² ohm-cm⁻¹ (~1/250th that of white tin). Grey tin has the same crystalline structure as that of the diamond allotrope of carbon. It behaves as if it was a semiconductor (with a band gap of 0.08 eV) but has the electronic band structure of a semimetal. It is sometimes referred to as a metalloid.

The diamond allotrope of carbon, as another example, is clearly nonmetallic, being translucent and having a relatively poor electical conductivity of 10⁻¹⁴ to 10⁻¹⁶ ohm-cm⁻¹. The semi-lustrous graphite allotrope, however, has an electrical conductivity of 3 × 10⁴ ohm-cm⁻¹, a figure more characteristic of a metal. Phosphorus, arsenic, selenium, antimony and bismuth also have allotropes that display borderline or either metallic or nonmetallic behaviour.

Categorization and periodic table territory

Metalloids are generally regarded as a third category of chemical elements, alongside metals and nonmetals. 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. Metalloids are sometimes grouped instead with metals, regarded as nonmetals or treated as a sub-category of same. Template:Periodic table (metalloid border)

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.

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. 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'. 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 or diffuse region, 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. 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. 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. 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. 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.

Comparison of properties with those of metals and nonmetals

Main article: Metalloid (comparison of properties with those of metals and nonmetals)

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.

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. 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)	solid	mostly gases
Appearance	lustrous (at least when freshly fractured)	lustrous	colourless, red, yellow, green, black, or intermediate shades
Elasticity	typically elastic, ductile, malleable (when solid)	brittle	brittle, if solid
Electrical conductivity	good to high	intermediate to good	poor to good
Band structure	metallic (Bi = semimetallic)	are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms	semiconductor or insulator

Chemical properties

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	nonmetallic
Ionization energy	relatively low	intermediate ionization energies, usually falling between those of metals and nonmetals	relatively high
Electronegativity	usually low	have electronegativity values close to 2 (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)	high
When mixed with metals	give alloys	can form alloys	ionic or interstitial compounds formed
Oxides	lower oxides basic; higher oxides increasingly acidic	amphoteric or weakly acidic	acidic

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); electronegativities (revised Pauling scale); and electronic band structures (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. A single quantitative criterion is also occasionally mentioned.

Masterton and Slowinski 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.

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. These values are lower than the values of most metals (at least 80% of which have a packing efficiency of at least 68%) but higher than those of elements usually classified as nonmetals. Packing efficiencies for nonmetals are: graphite 17%, sulfur 19.2, iodine 23.9, selenium 24.2, and black phosphorus 28.5.
• Goldhammer-Herzfeld criterion ratios of between ~0.85 to 1.1 (average 1.0).

Nomenclature and history

Derivation and other names

The word metalloid comes from the Latin metallum = "metal" and the Greek oeides = "resembling in form or appearance". Although the terms amphoteric element, boundary element, half-metal, half-way element, near metal, meta-metal, semiconductor, semimetal and submetal are sometimes used synonymously, some 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.
• 'Half-metal', which is sometimes instead used to refer to the poor metals. 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. These metals are mostly diamagnetic and tend to have distorted crystalline structures, electrical conductivity values at the lower end of those of metals, and amphoteric (weakly basic) oxides.
• 'Semimetal', which sometimes refers, loosely or explicitly, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure. Examples include gallium, ytterbium, bismuth and neptunium.

Origin and usage

Main article: Metalloid (nomenclature 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 in the early 19th century to metals that floated on water (sodium and potassium), and then more popularly to nonmetals. Earlier usage in mineralogy, to describe a mineral having a metallic appearance, can be sourced to at least as early as 1800. Only recently, since the mid-20th century, has it been widely used to refer to intermediate or borderline chemical elements. The International Union of Pure and Applied Chemistry (IUPAC) has previously recommended abandoning the term metalloid, and suggested using the term semimetal instead. 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. 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.

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