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 that has properties that are in between those of metals and nonmetals. 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 remains in use in chemistry literature.

The six elements commonly recognised as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. Other elements less commonly recognised as metalloids include carbon, aluminium, selenium, polonium and astatine. On a standard periodic table all of these elements can be found 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 often, one or more of the elements adjacent to those elements, which are identified as metalloids.

Physically, metalloids usually have a metallic appearance but they are brittle and only fair conductors of electricity; chemically, they mostly behave as (weak) nonmetals. They can form alloys with metals. Ordinarily, most of the other physical and chemical properties of metalloids are intermediate in nature.

Metalloids and their compounds are too brittle to have any structural uses. They are used in alloys, biological agents, flame retardants, glasses, optical storage and semiconductors. The electrical properties of silicon and germanium enabled the establishment of the semiconductor industry in the 1950s and the development of solid-state electronics from the early 1960s onward.

The term metalloid originally referred to nonmetals. Its more recent meaning, as a category of elements with intermediate or hybrid properties, did not become widespread until 1940–1960. Metalloids are sometimes called semimetals, a practice that 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.

Definition

Metalloids are usually regarded as a third category of chemical elements occupying a fuzzy 'buffer zone' between metals and nonmetals. There is no universally agreed, rigorous definition of a metalloid. The feasibility of establishing a specific definition has also been questioned, as anomalies can be found in such attempted constructs. Classifying an element as a metalloid has been described as 'arbitrary'.

Generic

A metalloid is a chemical element that has properties in between or a mixture of those of metals and nonmetals and is difficult to classify as a metal or a nonmetal. 'In chemistry a metalloid is an element with properties intermediate between those of metals and nonmetals.' These elements are characterised by their mixed properties, and categorisation difficulties: '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.' These difficulties can be accommodated by recognising another category of elements: 'Chemists sometimes use the name metalloid...for these elements difficult to classify one way or the other.' A few authors are more explicit: '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...'. 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 elements have a mixture of metallic and nonmetallic properties, they can be classified as either metals or nonmetals according to which set of properties is more pronounced. Only the elements at or near the margins, regarded as lacking a sufficiently clear preponderance of metallic or nonmetallic properties, are classified as metalloids.

Boron, silicon, germanium, arsenic, antimony and tellurium are commonly recognised as metalloids. Depending on the author, one or more from selenium, polonium or astatine are sometimes added to this list. Boron is sometimes excluded, 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.

Some other elements are occasionally classified as metalloids. 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 amphoteric; examples include arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead and aluminium. The poor metals, and nonmetals (such as carbon or nitrogen) that can form alloys with, or modify the properties of, metals have also occasionally been considered as metalloids.

Specific

	Element	IE	EN	Band structure
	Boron	191	2.04	semiconductor
	Silicon	187	1.90	semiconductor
	Germanium	182	2.01	semiconductor
	Arsenic	225	2.18	semimetal
	Antimony	198	2.05	semimetal
	Tellurium	207	2.10	semiconductor
	average	198	2.05
	The elements commonly recognised 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 characterised in terms of generalities or a few broadly indicative physical or chemical properties. A single quantitative criterion, such as electronegativity, is also occasionally mentioned. Jones (writing on the role of classification in science) observed that, 'Classes are usually defined by more than two attributes.'

Masterton and Slowinski offer a more specific treatment. They write that metalloids have ionization energies around 200 kcal/mol, and electronegativity values close to 2.0. They also say that metalloids are typically semiconductors, though antimony and arsenic (semimetals in the physics sense) have electrical conductivities that approach those of metals. Their description, using these three more or less clearly defined properties, encompasses the six elements commonly recognised as metalloids (see table, upper right). Selenium and polonium are probably excluded from this scheme; astatine may or may not be included.

The commonly recognised metalloids can also be quantitatively described in terms of their intermediate packing efficiencies (between 34% to 41%) and Goldhammer-Herzfeld criterion metallization ratios (between ~0.85 to 1.1; average 1.0). The packing efficiency 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. The Goldhammer-Herzfeld criterion ratio values of the recognised metalloids are lower than those of representative and transition metals and higher than those of nonmetals.

Periodic table territory

Location

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

The diagonal positioning of the metalloids represents an exception to the observation that elements with similar properties tend to occur in vertical columns. Going along a period, the nuclear charge increases with atomic number as there is as an 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, 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 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 group. A related effect can be seen in other diagonal similarities between some elements and their lower right neighbours, such as lithium-magnesium, beryllium-aluminium, carbon-phosphorus, and nitrogen-sulfur.

Number, composition and alternative treatments

How many and which elements are metalloids depends on the classification criteria being used. Emsley, for example, recognised only four: germanium, arsenic, antimony and tellurium; James et al., listed twelve: boron, carbon, silicon, germanium, arsenic, selenium, antimony, tellurium, bismuth, polonium, ununpentium and livermorium. On average, seven elements are included in such lists.

With no standardized division of the elements into metals, metalloids and nonmetals, a specified subset of the continuum from the metallic to the nonmetallic can potentially serve its particular purpose as well as any other. Individual metalloid classification arrangements tend to share common ground, with most variations occurring around the indistinct margins.

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 nonmetals. Metalloids are grouped instead with metals, regarded as nonmetals or treated as a sub-category of them. 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'. Some periodic tables distinguish elements that are metalloids in the absence of any formal dividing line between metals and nonmetals. Metalloids are shown as occurring in a diagonal band or diffuse region.

Properties of metalloids

Physical and chemical

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

Distinctive

Brittleness, semiconductivity or both have been used as distinguishing indicators of metalloid status. Metallic lustre along with dualistic chemical behaviour—for example, amphoterism—has also been cited as a benchmark.

A metalloid is a chemistry-based concept describing the physical (including electronic) and chemical properties of elements. Semiconductor is a physics-based concept referring to the electronic properties of materials (including both elements and compounds). Not all elements classified in the literature as metalloids display semiconductivity, although most do.

Metalloids are all solid, and have metallic lustre, but their other properties vary. Given that metallic character (for example) is a combination of several properties, it has been suggested that metalloid status be judged separately for each element. This could be done based on the extent to which an element exhibits properties relevant to such status.

Compared to those of metals and nonmetals

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

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

Properties of metals, metalloids and nonmetals

Physical property	Metals	Metalloids	Nonmetals
Form	solid; a few liquid at or near room temperature (Ga, Hg, Rb, Cs, Fr)	solid	mostly gases
Appearance	lustrous (at least when freshly fractured)	lustrous	several colourless; others coloured, or metallic grey to black
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 property	Metals	Metalloids	Nonmetals
General chemical 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 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

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

Typical or shared applications

Metalloids are too brittle to have any structural uses in their pure forms. Theyand their compounds are used as alloying components, biological agents (toxicological, nutritional and medicinal), flame retardants, glasses (oxide and metallic), optical storage media and electronics, and semiconductors.

Alloys

Copper-germanium 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.

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. Compounds of silicon, germanium, arsenic and antimony with the poor metals, it has been suggested, 'are probably best classed as alloys.'

Alloys 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 added to copper and its alloys to improve corrosion resistance and appears to confer the same benefit when added to magnesium. 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 alloys with iron, as ferrotellurium (50–58% tellurium), and with copper, in the form of copper tellurium (40–50% tellurium). Ferrotellurium is used as a stabilizer for carbon in steel casting. Of the non-metallic elements that are less often recognised as metalloids, selenium—in the form of ferroselenium (50–58% selenium)—is used to improve the workability of stainless steels.

Biological agents

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

All six of the elements commonly recognised as metalloids have toxic, dietary or medicinal properties to varying degrees. Arsenic and antimony are toxic; boron, silicon, and possibly arsenic, are essential trace elements. Boron, silicon, arsenic and antimony have medical applications (with germanium and tellurium being thought to have potential).

Boron is used in insecticides and herbicides. It is an essential trace element. As boric acid, it has antiseptic, antifungal, and antiviral properties.

Silicon is not toxic. It is present in silatrane, a highly toxic rodenticide. Long-term inhalation of silica dust causes silicosis, a fatal disease of the lungs. Silicon is an essential trace element. Silicone gel can be applied to badly burned patients to reduce scarring.

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.

Arsenic is notoriously poisonous and may possibly also be an essential element in ultratrace amounts. It has been used as a pharmaceutical agent since antiquity 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 (under the trade name Trisenox) was re-introduced for the treatment of acute promyelocytic leukaemia, a cancer of the blood and bone marrow.

Metallic antimony is relatively non-toxic; most antimony compounds are poisonous. It is not an essential element. Compounds of antimony are used as antiprotozoan drugs, and in some veterinary preparations.

Tellurium is not considered particularly toxic although as little as two grams of sodium tellurate, if administered, can be lethal. People exposed to small amounts of airborne tellurium 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. Such compounds may have the potential to act as substitutes for antibiotics that have become ineffective due to increasing bacterial resistance.

Of the elements less often recognised as metalloids, beryllium and lead are noted for their toxicity, with lead arsenate having been extensively used as an insecticide. Sulfur is one of the oldest fungicides and pesticides. Phosphorus, sulfur, zinc and iodine are essential nutrients, as are possibly aluminium, tin and lead. Sulfur, gallium, iodine and bismuth have medicinal applications. Sulfur is a constituent of sulfonamide drugs, still widely used for conditions such as acne and urinary tract infections. Gallium nitrate is used to treat the side effects of cancer; and gallium citrate, a radiopharmaceutical, is used to facilitate body imaging in areas of inflammation, such as infection, and areas of rapid cell division. Iodine is used as a disinfectant in various forms. Bismuth is an ingredient in some antibacterial pharmaceuticals.

Flame retardants

Compounds of boron, silicon, arsenic and antimony have been used as flame retardants. Boron, in the form of borax, has been used as a textile flame retardant since at least the 18th century. Silicon compounds 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 flame retardants for wood but were less frequently used due to their toxicity. Antimony, as antimony trioxide, is a flame retardant. Aluminium hydroxide, has been used as a wood-fibre, rubber, plastic and textile flame retardant since the 1890s. Barring aluminium hydroxide, use of phosphorus based flame-retardants in the form of, for example, organophosphates, now exceeds that of any of the other main retardant types, which employ boron, antimony or halogenated hydrocarbon compounds.

Glass formation

Optical fibres, usually made of pure silicon dioxide glass, with additives such as boron trioxide or germanium dioxide for increased sensitivity

The oxides B₂O₃, SiO₂, GeO₂, As₂O₃ and Sb₂O₃ readily form glasses. TeO₂ forms a glass but this requires a 'heroic quench rate' or the addition of an impurity; otherwise the crystalline form results. These compounds are used 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 and domestic ovenware. Silicon dioxide forms most ordinary glassware. Germanium dioxide is used as a glass fibre additive, as well as in infrared optical systems. Arsenic trioxide is used in the glass industry as a decolourizing 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, of composition Pd_82.5P₆Si_9.5Ge₂, was reported in 2011.

Phosphorus, selenium and lead, which are less often recognised as metalloids, are also used in glasses. Phosphate glass has a substrate of phosphorus pentoxide (P₂O₅), rather than the silica (SiO₂) of convention silicate glasses and is used, for example, to make sodium lamps. Selenium compounds can be used both as decolourising agents and to add a red colour to glass. Decorative glassware made of traditional lead glass contains at least 30% lead(II) oxide (PbO); lead glass used for radiation shielding may have up to 65% PbO. Lead-based glasses have also been extensively used in electronics components; enamelling; sealing and glazing materials; and solar cells. Bismuth based oxide glasses have emerged as a less toxic replacement for lead in many of these applications.

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, changing their optical and electrical properties and allowing the storage of information.

Semiconductors and electronics

Semiconductor-based electronic components. From left to right: a transistor, an integrated circuit and an LED. The elements commonly recognised as metalloids find widespread use in such devices, as elemental or compound semiconductor constituents (Si, Ge or GaAs, for example) or as doping agents (B, Sb, Te, for example).

All the elements commonly recognised 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, single crystals are relatively hard to obtain, and introducing and retaining controlled impurities is difficult. Silicon is the leading commercial semiconductor; it forms the basis of modern electronics (including standard solar cells) and information and communication technologies. This 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. Silicon-germanium 'alloys' have been growing in use, particularly for wireless communication devices; these exploit the higher carrier mobility of germanium. The synthesis of gram-scale quantities of semiconducting germanane was also reported in 2013. This comprises one-atom thick sheets of hydrogen-terminated germanium atoms. It conducts electrons more than ten times faster than silicon and five times faster than conventional germanium, and is thought to have potential for optoelectronic and sensing applications.

Arsenic and antimony are not semiconductors in their standard states; 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 large group of type II/VI semiconducting-chalcogenides; these compounds have applications in electro-optics and electronics. Cadmium telluride (CdTe) finds application in solar modules due its high conversion efficiency, low manufacturing costs, and large band gap of 1.44 eV, meaning it absorbs a wide range of solar wavelengths. Bismuth telluride (Bi₂Te₃) alloyed with selenium and antimony, is a component of thermoelectric devices used for refrigeration or portable power generation. Five metalloids—boron, silicon, germanium, arsenic and antimony—can be found in cell phones (along with at least 39 other metals and nonmetals). Tellurium is a component of phase change memory and is expected to find such use. Of the elements less often recognised as metalloids, phosphorus, gallium (in particular) and selenium have semiconductor applications. Phosphorus is used in trace amounts as a dopant for n-type semiconductors. The commercial use of gallium compounds is dominated by semiconductor applications—in integrated circuits; cell phones; laser diodes; light emitting diodes; photodetectors; and solar cells. Selenium is used in the production of solar cells and in high-energy surge protectors.

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 that may not be interchangeable. 'Amphoteric element' is sometimes used more broadly to include transition metals capable of forming oxyanions, such as chromium and manganese. Some elements referred to as metalloids do not show marked amphoteric behaviour or semiconductivity in their most stable forms. 'Half-metal' is used in physics to refer to 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 tables. 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.

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 as early as 1800. Only 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. Use of this latter term has more recently been discouraged as it has a different meaning in physics—one that more specifically refers to the electronic band structure of a substance rather than the overall classification of an element. The most recent IUPAC publications on nomenclature and terminology do not include any recommendations on the usage or non-usage of the terms metalloid or semimetal.

Elements commonly recognised as metalloids

Boron

Main article: Boron

Boron, shown here in the form of its β-rhombohedral phase (its most thermodynamically stable allotrope)

Pure boron is a shiny, silver-grey crystalline solid. It is less dense than aluminium, and 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 (about 200 times less than that of tap water) and a band gap of about 1.56 eV.

The chemistry of boron is dominated by its small atomic 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 is electron deficient with respect to the octet rule. Elements in this situation usually adopt metallic bonding, but 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 that 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 (such as 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 enables the preparation of many interstitial alloy-type borides. Analogies between boron and transition metals have 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 as in the geometric and electronic structures of cluster species such as [B₆H₆]^2– and [Ru₆(CO)₁₈]^2–. The aqueous chemistry of boron is characterised by the formation of many different polyborate anions. Given its high charge-to-size ratio, nearly all compounds of boron are covalent, with a few complexed anionic and cationic species. Boron has a strong affinity for oxygen, and has an extensive chemistry as 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 has a shiny blue-grey metallic lustre.

Silicon is a shiny crystalline solid, with a blue-grey metallic lustre. Like boron, it is less dense than aluminium, and is 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 reacts with hot aqueous alkalis with the evolution of hydrogen, like a metal 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, similar to that of liquid mercury.

The chemistry of silicon is generally nonmetallic (covalent) in nature. It can 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 strength of the silicon-oxygen bond dominates the chemical behaviour of silicon. Polymeric silicates, built up by tetrahedral SiO₄ units sharing their oxygen atoms, are 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 is sometimes described as a metal.

Germanium appears as a shiny grey-white solid. It is about one-third less dense than iron, hard and brittle. It is mostly unreactive at room temperature but is slowly attacked by hot concentrated sulfuric 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 lower temperature than steel, 938 °C vs. ~1400 °C. 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 nonmetal. It forms alloys with 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, and it can also form 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. The dioxide is soluble in acidic solutions (and the monoxide GeO, is even more so), and this is sometimes used as a basis to classify germanium as a metal. Up to the 1930s germanium was considered to be a poorly conducting metal rather than a nonmetal. As is the case with all the elements commonly recognised as metalloids, germanium has an established organometallic chemistry (see organogermanium chemistry).

Arsenic

Main article: Arsenic

Arsenic, sealed in a container to prevent tarnishing

Arsenic is a grey, metallic looking solid. It is about one-third less dense 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 sulfuric acid. It reacts with fused caustic soda to give the arsenate Na₃AsO₃, together with the evolution of hydrogen. Arsenic sublimes at 615 °C. The vapour is lemon-yellow and smells like garlic. Arsenic only melts under a pressure of 38.6 atm, at 817 °C. It 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 nonmetallic, and with many metals it forms alloys; most of these are 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. The halides are hydrolysed by water but these reactions, particularly those of the chloride, are reversible with the addition of a hydrohalic acid. The oxide is acidic but weakly amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties. 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, showing its brilliant lustre

Antimony is a silver-white solid with a blue tint and a brilliant lustre. It is about 15% less dense 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 gives the pentachloride SbCl₅ and (hot) concentrated sulfuric acid results in the sulfate Sb₂(SO₄)₃. It is not affected by molten alkali. Antimony is capable of displacing hydrogen from water, when heated: 2Sb + 3 H₂O → Sb₂O₃ + 3 H₂. It melts at 630.63 °C. 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 nonmetal. It forms alloys with one or more metals such as aluminium, iron, nickel, copper, zinc, tin, lead and bismuth. Antimony has 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 with arsenic. This is explained by the poor shielding afforded the arsenic nucleus by its 3d¹⁰ electrons. In comparison, the tendency of antimony to oxidize more easily partially offsets the effect of its 4d¹⁰ shell. Tripositive antimony is amphoteric; pentapositive 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 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, described by Dmitri Mendeleev as forming a transition between metals and nonmetals

Tellurium is a silvery-white shiny solid. It is about 15% less dense than iron, brittle, and the softest of the commonly recognised metalloids, being marginally harder than sulfur. Large pieces of tellurium are stable in air. The finely powdered form is oxidized by air in the presence of moisture. Tellurium reacts with boiling water, or when freshly precipitated even at 50 °C, to give the dioxide and hydrogen: Te + 2 H₂O → TeO₂ + 2 H₂. It reacts (to varying degrees) with nitric, sulfuric and hydrochloric acids to give compounds such as the sulfoxide TeSO₃ or tellurous acid H₂TeO₃, the basic nitrate (Te₂O₄H)⁺(NO₃)^–, or the oxide sulfate Te₂O₃(SO₄). It dissolves in boiling alkalis, with the formation of the tellurite and telluride: 3 Te + 6 KOH = K₂TeO₃ + 2 K₂Te +3 H₂O, a reaction that proceeds or is reversible with increasing or decreasing temperature. At higher temperatures tellurium is sufficiently plastic to extrude. It melts at ca. 450 °C. 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.

Most of the chemistry of tellurium is characteristic of a nonmetal. It forms alloys with 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 valence 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 a 'conditional' glass-forming oxide—it forms a glass with a very small amount of additive. Tellurium has an extensive organometallic chemistry (see organotellurium chemistry).

Elements less commonly recognised as metalloids

Carbon

Main article: Carbon

Carbon (as graphite). Delocalized valence electrons within the layers of graphite give it a metallic appearance.

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

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, like a metal; it has the electronic band structure of a semimetal. The allotropes of carbon, including graphite, can accept 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 sulfate, perchlorate, nitrate, hydrogen selenate, and hydrogen phosphate; in organic chemistry, carbon can form complex cations—termed carbocations—in which the positive charge is on the carbon atom; examples are CH⁺
₃ and CH⁺
₅, and their derivatives.

Carbon is brittle like nonmetals 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. Carbon can form anions such as C^4– (methanide), C^2–
₂ (acetylide) and C^3–
₄ (sesquicarbide or allylenide), in compounds with metals of main groups 1–3, and with the lanthanides and actinides. Its oxide CO₂ forms a medium-strength carbonic acid H₂CO₃.

Aluminium

Main article: Aluminium

People handling high purity aluminium for the first time often question if it really is aluminium since it is very much softer than its familiar alloys.

Aluminium is classified as a metal. It is lustrous, malleable and ductile, has high electrical and thermal conductivity and a close-packed crystalline structure.

It has 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 can form anionic aluminates, such behaviour being considered nonmetallic in character.

Stott labels aluminium as a weak metal. It has the physical properties of a metal but some of the chemical properties of a nonmetal. Steele notes the 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 as it has many metallic properties. Aluminium is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal-nonmetal dividing line are metalloids.

Selenium

Main article: Selenium

Selenium. Being a photoconductor, grey selenium conducts electricity around 1,000 times better when light falls on it, a property used since the mid-1870s in various light-sensing applications.

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 shows 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. Trace amounts of selenium are essential to human health, and 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. Both of its allotropic forms are metallic conductors. It is soluble in acids, forming the rose-coloured Po²⁺ cation and displacing hydrogen: Po + 2 H⁺ → Po²⁺ + H₂. Many polonium salts are known. The oxide PoO₂ is predominantly basic in nature. Polonium is a reluctant oxidizing agent, unlike its lighter congener oxygen: highly reducing conditions are required for the formation of the Po^2– anion in aqueous solution.

Polonium also shows nonmetallic character in its halides, and by way of the existence of polonides. The 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, which is ordinarily classified as a nonmetal has some marked metallic properties, and may be either a metalloid, or a metal. Immediately following its production in 1940, early investigators considered it 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 2013, on the basis of relativistic modelling, astatine was predicted to be a monatomic metal, having a face-centred cubic crystalline structure.

Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be a metal if it could form a condensed phase. Astatine may be metallic in the liquid state on the basis 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. Estimated values for the EoV of diatomic astatine are 50 kJ/mol or higher; diatomic iodine, with an EoV of 41.71, falls just short of the threshold figure. '[L]ike typical metals, it [astatine] 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'. Further indications of a tendency for astatine to behave like a (heavy) metal are: '...the formation of pseudohalide compounds...complexes of astatine cations...complex anions of trivalent astatine...as well as complexes with a variety of organic solvents'. It has also been argued that it demonstrates cationic behaviour, by way of stable At⁺ and AtO⁺ forms, in strongly acidic aqueous solutions.

Some of astatine's reported properties are nonmetallic. It has the narrow liquid range ordinarily associated with nonmetals (mp 575 K, bp 610). 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. The chemistry of astatine in aqueous solution is predominately characterised by the formation of various anionic species. Most of its known compounds resemble those of iodine, which is a halogen and a nonmetal. Such compounds include astatides (XAt), astatates (XAtO₃), and monovalent interhalogen compounds.

Restrepo et al. reported that astatine appeared to be more like polonium than the established halogens. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.

Honourable mentions

Elements near the 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. It has an electrical conductivity of 1.7 × 10⁻⁸ S•cm⁻¹ at room temperature, higher than selenium but lower than boron, the least electrically conducting of the recognised metalloids.

Some of the elements adjacent in the periodic table to the commonly recognised metalloids, although usually classified as either metals or nonmetals, are occasionally referred to as near-metalloids or noted for their metalloidal character. To the left of the metal-nonmetal dividing line, such elements include gallium, tin and bismuth. They show 'odd' packing structures, marked covalent chemistry (molecular or polymeric), and amphoterism. To the right of the dividing line are 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.

Allotropes

White tin (left) and grey tin (right). Both forms have a metallic appearance.

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

Tin, for example, has two allotropes: tetragonal 'white' β-tin and cubic 'grey' α-tin, as shown in the picture. 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 S·cm⁻¹ (~1/6th that of copper). Grey tin usually has the appearance of a grey micro-crystalline powder although it can also be prepared in 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 S·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 a semiconductor (with a band gap of 0.08 eV), but has the electronic band structure of a semimetal. It has been referred to as either a very poor metal, a metalloid, a nonmetal or a near metalloid.

The diamond allotrope of carbon is clearly nonmetallic, being translucent and having a relatively poor electrical conductivity of 10⁻¹⁴ to 10⁻¹⁶ S·cm⁻¹. Graphite has an electrical conductivity of 3×10⁴ S·cm⁻¹, a figure more characteristic of a metal. Phosphorus, sulfur, arsenic, selenium, antimony and bismuth also have less stable allotropes that display borderline or either more or less metallic or nonmetallic behaviour.

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Monographs

• Brady JE, Humiston GE & Heikkinen H 1980, 'Chemistry of the Representative Elements: Part II, The Metalloids and Nonmetals', in General Chemistry: Principles and Structure, 2nd ed., SI version, John Wiley & Sons, New York, pp. 537–591, ISBN 0471063150
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• Rochow EG 1966, The Metalloids, DC Heath and Company, Boston
• Vernon RE 2013, 'Which Elements are Metalloids?', Journal of Chemical Education, vol. 90, no. 12, pp. 1703–7, doi:10.1021/ed3008457