Metalloid

		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
			Common	*The metalloid status of Al, Po and At is disputed.
			Less common
			Uncommon
			Rare
	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. The grey staircase 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.

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 universally agreed or rigorous definition of a metalloid. Classifying any particular element as such has been described as 'arbitrary'. The term was first popularly used to refer to nonmetals. Its more recent meaning as a category of intermediate or hybrid elements did not become widespread until the period 1940‒1960. The six elements commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. They or their compounds find uses in glasses, alloys or semiconductors.

Although the terms amphoteric element, half-metal, half-way element, near metal, meta-metal, semiconductor and semimetal are sometimes used synonymously, most of these have other meanings, which may not be interchangeable:

• 'Amphoteric element' is sometimes used more broadly to include transition metals capable of forming oxyanions, such as chromium and manganese.
• 'Half-metal' 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' 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' 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.

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

Metalloids are generally regarded as a third classification 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 the metals, regarded as nonmetals or treated as a sub-category of same.

Properties

The following two subsections summarise and tabulate the physical and chemical properties of metalloids. The tables include the properties of metals and nonmetals, for comparative purposes. Shading to either side of the metalloid column denotes immediately apparent commonalities.

Physical

Metalloids are metallic looking solids that have a brittle comportment, show intermediate to relatively good electrical conductivity, and have the band structure of a semimetal or semiconductor. Most of their other physical properties are intermediate in nature. The following table sets out a range of physical properties of metals, metalloids and nonmetals, in descending order of superficiality.

Property	Metals	Metalloids	Nonmetals
Form	solid; a few liquid at or near room temperature (Ga, Hg, Cs, Fr)	solid	mostly gases
Appearance	characteristic lustre	metallic lustre	colourless; red, yellow, green, black, or intermediate shades
Allotropy	many show metallic allotropes; Bi, Sn have semiconducting allotropes	tend to exist in several (conspicuously) 'metallic' and nonmetallic allotropic forms	show nonmetallic allotropy (O, S), with elements close to the metal-nonmetal line (C, P, Se) showing more 'metallic' allotropes
Density	generally high, with some exceptions such as the alkali metals	densities lower than neighbouring poor metals but higher than those of neighbouring nonmetals	often low
Elasticity	typically elastic, ductile, malleable (when solid)	brittle	brittle (when solid)
Electrical conductivity	good to high	intermediate to good	poor to intermediate
Temperature coefficient of resistance	nearly all positive (Pu is negative)	negative (B, Si, Ge, Te) or positive (As, Sb)	nearly all negative (C, as graphite, is positive in the direction of its planes)
Thermal conductivity	medium to high	mostly intermediate; Si is high	almost negligible to very high
Packing	close-packed crystal structures; high coordination numbers	have relatively open crystal structures, with medium coordination numbers, in contrast to the close-packed crystal structures of metals	low coordination numbers
Melting behaviour	volume generally expands	some contract, unlike (most) metals	volume generally expands
Enthalpy of fusion	may be high	often have abnormally high enthalpy of fusion values (compared to other close-packed metals)	often low
Liquid electrical conductivity	metallic	most exhibit metallic conductivity in liquid form	nonmetallic
Periodic table block	s, p, d, f	p	s, p
Band structure	metallic (Bi = semimetallic)	are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms	semiconductor or insulator
Electron behaviour	"free" electrons	• valence electrons not as freely delocalized as in metals; considerable covalent bonding present • have Goldhammer-Herzfeld criterion ratios straddling unity	no "free" electrons

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

Chemical

Metalloids generally behave chemically as (weak) nonmetals, and have intermediate ionization energies and electronegativities, and amphoteric or weakly acidic oxides. Most of their other chemical properties are intermediate in nature. The following table sets out some general, specific and descriptive chemical properties of metals, metalloids and nonmetals.

Property	Metals	Metalloids	Nonmetals
General behaviour	metallic	nonmetallic	nonmetallic
Ionization energy	relatively low	intermediate ionization energies, usually falling between those of metals and nonmetals	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
Ion formation	tend to form cations	• have a reduced tendency to form anions in water, when compared to ordinary nonmetals • solution chemistry is dominated by the formation and reactions of oxyanions	tend to form anions
Bonds	seldom form covalent	can form salts as well as covalent compounds	form many covalent
Oxidation number	nearly always positive	positive or negative	positive or negative
+Metals	give alloys	can form alloys	ionic or interstitial compounds formed
Oxides	• lower oxides are ionic and basic • higher oxides are increasingly covalent and acidic • very few glass formers	• polymeric in structure; tend to be amphoteric or weakly acidic • are glass formers (B, Si, Ge, As, Sb, Te)	• covalent, acidic • few glass formers (P, S, Se)
Halides, esp. chlorides (see also)	• ionic, involatile • mostly water soluble (not hydrolysed) • higher halides and those of weaker metals have greater covalency and volatility, and are more or less prone to hydrolysis (layer-lattice types often reversibly so) and dissolution in organic solvents	• covalent, volatile • some partly reversibly hydrolysed • usually dissolve in organic solvents	• covalent, volatile • most irreversibly hydrolysed by water • usually dissolve in organic solvents
Hydrides	• active metals (alkali and alkaline earth metals) form ionic, solid hydrides with high melting points; • transition metals form metallic hydrides; • poor metals form covalent hydrides	covalent, volatile hydrides	covalent, gaseous or liquid hydrides
Sulfates	do form	most form	some form
Organometallic compounds	many form such	can form	not formed

Of the twelve chemical properties listed in the table, two (+metals; organometallic compounds) are shared with metals and three (general behaviour; ion formation; and oxidation number) with nonmetals. The other seven properties are characteristic of metalloids. However, as noted at the end of the introduction of this article, some authors count metalloids as nonmetals or a sub-category of same. In this case most of the chemical properties of metalloids would be regarded as non-metallic in nature, albeit weakly so.

Distinctive

Of the above physical and chemical properties, brittleness or semiconductivity or both have been cited or used as distinguishing indicators of metalloid status. Metallic lustre together with very marked dualistic chemical behaviour—by way of, for example, amphoteric oxides—has also been cited as a benchmark.

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

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

Applicable elements

Variability

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. Selwood, on the other hand, listed twelve: boron, aluminium, silicon, gallium, germanium, arsenic, tin, antimony, tellurium, bismuth, polonium, and astatine.

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

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

Common metalloids

Consistent with the list of metalloid lists, the following elements are commonly classified as metalloids:

• Boron
• Silicon
• Germanium
• Arsenic
• Antimony
• Tellurium

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

Boron

In its most stable state, pure boron appears as a shiny, silver-grey crystalline solid. It is about ten per cent lighter than aluminium but, unlike the former, 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.

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 electons, rather than metallic bonding. The associated structural unit which pervades the various allotropes of boron is the icosahedral B₁₂ moiety. 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 non-metallic to metallic states has been estimated as 30, 100, 240, 33 and 50 kJ/mol, respectively. This gives an idea of how close boron is to the metal-nonmetal borderline.

The small size of the boron atom enables the preparation of many interstitial alloy-type borides. Boron also has a strong affinity for oxygen, a characteristic manifested in the extensive chemistry of the borates.

Given its high charge-to-size ratio nearly all compounds of boron are covalent, barring some complexed anionic and cationic species. The aqueous chemistry of boron is characterised by the formation of oxyanions. The oxide B₂O₃ is polymeric in structure, weakly acidic, and a glass former. Organometallic compounds of boron have been known since the 19th century.

Silicon

Silicon

Silicon appears as a shiny crystalline solid, with a blue-grey metallic lustre. As with boron it is about ten per cent lighter than aluminium, hard and brittle. It is a relatively unreactive element and 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 × 10⁴S•cm⁻¹, a value similar to that of liquid mercury.

Although silicon is oxidized by nitric acid, the resulting thin surface layer of SiO₂ prevents further corrosion. Silicon dissolves in hot aqueous alkalis with the evolution of hydrogen, behaving in this way like metals such as beryllium, aluminium, zinc and gallium.

The chemistry of silicon is generally non-metallic (covalent) in nature and dominated by the high bond strength of the silicon-oxygen bond. 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. Silicon shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. It forms alloys with metals such as iron and copper. The oxide SiO₂ is polymeric in structure, weakly acidic, and a glass former. Traditional organometallic chemistry includes the carbon compounds of silicon.

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.

Most of the chemistry of germanium is characteristic of a non-metal. 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. 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. Germanium analogues of all of the major types of silicates have been prepared. The oxide GeO₂ is polymeric, amphoteric, and a glass former. Germanium has an established organometallic chemistry.

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 platina in moist air, which blackens on further exposure. Arsenic is attacked by nitric acid and concentrated sulphuric acid. It reacts with fused caustic soda to give the arsenate Na₃AsO₃, together with the evolution of hydrogen. 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.

The chemistry of arsenic is predominately non-metallic 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 hydrohalic acid. As well, and as noted below, the oxide is acidic but weakly amphoteric. The higher, less stable, pentavalent state has strongly acidic (non-metallic) 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 a long organometallic chemistry.

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.

Most of the chemistry of antimony is characteristic of a non-metal. 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 form 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.

Selenium, polonium and astatine

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 the following properties:

• Its lustre.
• Its crystalline structure, which is thought to include weakly 'metallic' interchain bonding.
• Its capacity, when molten, to be drawn into thin threads.
• Its reluctance to acquire 'the high positive oxidation numbers characteristic of nonmetals'.
• Its capacity to form cyclic polycations (such as Se2+
  8) when dissolved in oleums (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.

The nonmetallic character of selenium is shown by:

• Its brittleness.
• Its electronic band structure, which is that of a semiconductor.
• 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.
• Its relatively high electronegativity (2.55 revised Pauling).
• The retention of its semiconducting properties in liquid form.
• Its reaction chemistry, which is mainly that of its nonmetallic anionic forms Se^2–, SeO2−
  3 and SeO2−
  4.

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 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.

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.

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.
• Champion et al. argue that astatine demonstrates cationic behaviour, by way of stable At⁺ and AtO⁺ forms, in strongly acidic aqueous solutions.

Siekierski and Burgess contend or presume that astatine would be a metal if it could form a condensed phase. A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.

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.

Semi-quantitative characterization

	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 common metalloids, and their ionization energies (kcal/mol); electronegativities (revised Pauling); 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 common 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 common 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%, sulphur 19.2, iodine 23.9, selenium 24.2, and black phosphorus 28.5.

The common metalloids also have Goldhammer-Herzfeld criterion ratios of between ~0.85 to 1.1 (average 1.0).

Other metalloids

Some other elements are occasionally classified as metalloids, given there is no agreed definition of same. These elements include hydrogen, beryllium, carbon, nitrogen, aluminium, 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.

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, it bonds covalently in most others.
• Its oxide is amphoteric, and a conditional glass-former.
• it 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.

Near metalloids

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

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

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

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

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

Elements such as these are occasionally called, or described as, near-metalloids, or the like. They are located near the common 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 this 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

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 more characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth also have allotropes that display borderline or either metallic or nonmetallic behaviour.

Location and identification

	H									He
	Li	Be			B	C	N	O	F	Ne
	Na	Mg			Al	Si	P	S	Cl	Ar
	K	Ca		Zn	Ga	Ge	As	Se	Br	Kr
	Rb	Sr		Cd	In	Sn	Sb	Te	I	Xe
	Cs	Ba		Hg	Tl	Pb	Bi	Po	At	Rn
	Fr	Ra		Cn	Uut	Uuq	Uup	Uuh	Uus	Uuo
	Condensed periodic table showing distribution of elements that have sometimes been classified as metalloids. Elements with grey shading (B, C, Al, Si, Ge, As, Se, Sb, Te, Po, At) appear commonly to rarely in the list of metalloid lists. Elements with light tan shading (H, Be, P, S, Ga, Sn, Pb, Bi, Uuq, Uup, Uuh, Uus) appear still less frequently. Elements with pale blue shading (N, Zn, Rn) are outliers that show that the metalloid net is sometimes cast very widely. Although they do not appear in the list of metalloid lists, isolated references to their designation as metalloids can be found in the literature (as cited in this article).

Metalloids cluster on either side of the dividing line between metals and nonmetals. This can be found, in varying configurations, on some periodic tables (see mini-example, right). Elements to the lower left of the line generally display increasing metallic behaviour; elements to the upper right display increasing nonmetallic behaviour. When presented as a regular stair-step, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.

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

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

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

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

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

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

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

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

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.

Mendeleev wrote that, 'It is...impossible to draw a strict line of demarcation between metals and non-metals, there being many intermediate substances.'

Several other sources note confusion or ambiguity as to the location of the dividing line; suggest its apparent arbitrariness provides grounds for refuting its validity; and comment as to its misleading, contentious or approximate nature.

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

Typical applications

Tellurium

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

Common metalloids, such as arsenic and antimony, are too brittle to have any structural uses in their pure forms.

Typical applications of the common 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.

Glass formation

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

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

Alloys

In 1914 Desch 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 later noted that compounds of silicon, germanium, arsenic and antimony with the poor metals, 'are probably best classed as alloys'.

Boron can form intermetallic compounds and alloys with transition metals, of the composition M_nB, if n > 2.

Sanderson 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.

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

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).

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.

Semiconductors and electronics

All the common 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.

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

Pre-1800

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

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

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

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

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

This idea of a semimetal, as a brittle (and thereby imperfect) metal, was gradually discarded after 1789 with the publication of Lavoisier's 'revolutionary' Elementary Treatise on Chemistry.

1800–1950s

In 1807 Erman and Simon suggested using the term metalloid to refer to the newly discovered elements sodium and potassium. These elements were lighter than water and many chemists did not regard them as proper metals. Erman and Simon's proposal may have been made '[in] an attempt to revive this old distinction between metals and substances resembling metals'. The word metalloid in fact comes from the Latin metallum = "metal" and the Greek oeides = "resembling in form or appearance". Their suggestion was ignored by the chemical community.

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

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

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

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

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

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

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

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

Beach, writing in 1911, explained it this way:

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

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

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

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

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

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

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

1960–

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

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

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

Use of the term semimetal, rather than metalloid, has recently been discouraged. This is because the former term 'has a well defined and quite distinct meaning in physics'. References to the term 'metalloid' as being outdated have also been described as 'nonsense' noting that 'it accurately describes these weird in-between elements'.

In physics, a semimetal is an element or a compound in which the valence band marginally (rather than substantially) overlaps the conduction band. This results in only a small number of effective charge carriers. Thus, the densities of charge carriers in the elemental semimetals carbon (as graphite, in the direction of its planes), arsenic, antimony and bismuth are 3×10¹⁸ cm⁻³, 2 ×10²⁰ cm⁻³, 5×10¹⁹ cm⁻³ and 3×10¹⁷ cm⁻³ respectively. In contrast, the room-temperature concentration of electrons in metals usually exceeds 10²² cm⁻³.

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