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A metalloid is a type of
chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
which has a preponderance of properties in between, or that are a mixture of, those of
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
s and
nonmetal In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differentl ...
s. There is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature of
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
. The six commonly recognised metalloids are
boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the '' boron group'' it has t ...
,
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
,
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors ...
,
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...
,
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
, and
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
. Five elements are less frequently so classified:
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
, selenium, polonium, and
astatine Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-live ...
. On a standard periodic table, all eleven elements are in a diagonal region of the
p-block A block of the periodic table is a set of elements unified by the atomic orbitals their valence electrons or vacancies lie in. The term appears to have been first used by Charles Janet. Each block is named after its characteristic orbital: s-blo ...
extending from boron at the upper left to astatine at lower right. Some periodic tables include a dividing line between metals and nonmetals, and the metalloids may be found close to this line. Typical metalloids have a metallic appearance, but they are brittle and only fair conductors of electricity. Chemically, they behave mostly as nonmetals. They can form
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
s with metals. Most of their other physical properties and chemical properties are intermediate in nature. Metalloids are usually too brittle to have any structural uses. They and their compounds are used in alloys, biological agents,
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s, flame retardants,
glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
es, optical storage and optoelectronics,
pyrotechnics Pyrotechnics is the science and craft of creating such things as fireworks, safety matches, oxygen candles, explosive bolts and other fasteners, parts of automotive airbags, as well as gas-pressure blasting in mining, quarrying, and demolition ...
,
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
s, and electronics. The electrical properties of silicon and germanium enabled the establishment of the
semiconductor industry The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. It formed around 1960, once the fabrication of semiconduc ...
in the 1950s and the development of solid-state electronics from the early 1960s. The term ''metalloid'' originally referred to nonmetals. Its more recent meaning, as a category of elements with intermediate or hybrid properties, became widespread in 1940–1960. Metalloids are sometimes called semimetals, a practice that has been discouraged, as the term '' semimetal'' has a different meaning in
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
than in chemistry. In physics, it refers to a specific kind of
electronic band structure In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called '' band gaps'' or ...
of a substance. In this context, only arsenic and antimony are semimetals, and commonly recognised as metalloids.


Definitions


Judgment-based

A metalloid is an element that possesses a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals, and which is therefore hard to classify as either a metal or a nonmetal. This is a generic definition that draws on metalloid attributes consistently cited in the literature. Difficulty of categorisation is a key attribute. Most elements have a mixture of metallic and nonmetallic properties, Hopkins & Bailar 1956, p. 458 and can be classified according to which set of properties is more pronounced. Only the elements at or near the margins, lacking a sufficiently clear preponderance of either 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 Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-live ...
are sometimes added to the list. Boron sometimes is excluded, by itself, or with silicon. Sometimes
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
is not regarded as a metalloid. Swift & Schaefer 1962, p. 100 The inclusion of
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
, polonium, and astatine as metalloids has been questioned. Other elements are occasionally classified as metalloids. These elements include hydrogen,
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...
,
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, Rausch 1960
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
,
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
gallium Gallium is a chemical element with the Symbol (chemistry), symbol Ga and atomic number 31. Discovered by France, French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in boron group, group 13 of the periodic table and is similar to ...
, tin,
iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
, bismuth, and radon. The term metalloid has also been used for elements that exhibit metallic lustre and electrical conductivity, and that are amphoteric, such as arsenic, antimony, vanadium,
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...
,
molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...
,
tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
, tin, lead, and aluminium. The p-block metals, and nonmetals (such as carbon or nitrogen) that can form
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
s with metals or modify their properties have also occasionally been considered as metalloids.


Criteria-based

No widely accepted definition of a metalloid exists, nor any division of the periodic table into metals, metalloids, and nonmetals; Hawkes Hawkes 2001, p. 1687 questioned the feasibility of establishing a specific definition, noting that anomalies can be found in several attempted constructs. Classifying an element as a metalloid has been described by Sharp Sharp 1981, p. 299 as "arbitrary". The number and identities of metalloids depend on what classification criteria are used. Emsley recognised four metalloids (germanium, arsenic, antimony, and tellurium); James et al. listed twelve (Emsley's plus boron, carbon, silicon, selenium, bismuth, polonium, moscovium, and livermorium). On average, seven elements are included in such lists; individual classification arrangements tend to share common ground and vary in the ill-defined margins. A single quantitative criterion such as electronegativity is commonly used, metalloids having electronegativity values from 1.8 or 1.9 to 2.2. Further examples include packing efficiency (the fraction of volume in a
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
occupied by atoms) and the Goldhammer-Herzfeld criterion ratio. The commonly recognised metalloids have packing efficiencies of between 34% and 41%. The Goldhammer-Herzfeld ratio, roughly equal to the cube of the atomic radius divided by the molar volume, is a simple measure of how metallic an element is, the recognised metalloids having ratios from around 0.85 to 1.1 and averaging 1.0. Other authors have relied on, for example, atomic conductance or bulk coordination number. Jones, writing on the role of classification in science, observed that " lassesare usually defined by more than two attributes". Masterton and Slowinski used three criteria to describe the six elements commonly recognised as metalloids: metalloids have
ionization energies Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
around 200 kcal/mol (837 kJ/mol) and electronegativity values close to 2.0. They also said that metalloids are typically semiconductors, though antimony and arsenic (semimetals from a physics perspective) have electrical conductivities approaching those of metals. Selenium and polonium are suspected as not in this scheme, while astatine's status is uncertain. In this context, Vernon proposed that a metalloid is a chemical element that, in its standard state, has (a) the electronic band structure of a semiconductor or a semimetal; and (b) an intermediate first ionization potential "(say 750−1,000 kJ/mol)"; and (c) an intermediate electronegativity (1.9–2.2).


Periodic table territory


Location

Metalloids lie on either side of the dividing line between metals and nonmetals. This can be found, in varying configurations, on some
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
s. 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 Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. * Critical Software, a company specializing ...
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
groups A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...
. Gray 2009, p. 9 A related effect can be seen in other diagonal similarities between some elements and their lower right neighbours, specifically lithium-magnesium, beryllium-aluminium, and boron-silicon. Rayner-Canham Rayner-Canham 2011 has argued that these similarities extend to carbon-phosphorus, nitrogen-sulfur, and into three d-block series. This exception arises due to competing horizontal and vertical trends in the nuclear charge. Going along a period, the nuclear charge increases with
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...
as do the number of electrons. The additional pull on outer electrons as nuclear charge increases generally outweighs the screening effect 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 In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as ...
, the effect of increasing nuclear charge is generally outweighed by the effect of additional electrons being further away from the nucleus. Atoms generally become larger, ionization energy falls, and metallic character increases. The net effect is that the location of the metal–nonmetal transition zone shifts to the right in going down a group, and analogous diagonal similarities are seen elsewhere in the periodic table, as noted.


Alternative treatments

Elements bordering the metal–nonmetal dividing line are not always classified as metalloids, noting a binary classification can facilitate the establishment of rules for determining bond types between metals and nonmetals. Roher 2001, pp. 4–6 In such cases, the authors concerned focus on one or more attributes of interest to make their classification decisions, rather than being concerned about the marginal nature of the elements in question. Their considerations may or not be made explicit and may, at times, seem arbitrary. Metalloids may be grouped with metals; or regarded as nonmetals; or treated as a sub-category of nonmetals. Other authors have suggested classifying some elements as metalloids "emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table". Brown & Holme 2006, p. 57 Some periodic tables distinguish elements that are metalloids and display no formal dividing line between metals and nonmetals. Metalloids are instead shown as occurring in a diagonal band or diffuse region. The key consideration is to explain the context for the taxonomy in use.


Properties

Metalloids usually look like metals but behave largely like nonmetals. Physically, they are shiny, 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 amphoteric or weakly acidic
oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s. They can form alloys with metals. Most of their other physical and chemical properties are intermediate in nature.


Compared to 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. The above table reflects the hybrid nature of metalloids. 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.


Common applications

:''The focus of this section is on the recognised metalloids. Elements less often recognised as metalloids are ordinarily classified as either metals or nonmetals; some of these are included here for comparative purposes.'' Metalloids are too brittle to have any structural uses in their pure forms. They and their compounds are used as (or in) alloying components, biological agents (toxicological, nutritional, and medicinal), catalysts, flame retardants, glasses (oxide and metallic), optical storage media and optoelectronics, pyrotechnics, semiconductors, and electronics.


Alloys

Writing early in the history of intermetallic compounds, the British metallurgist Cecil Desch observed that "certain non-metallic elements are capable of forming compounds of distinctly metallic character with metals, and these elements may therefore enter into the composition of alloys". He associated silicon, arsenic, and tellurium, in particular, with the alloy-forming elements. Phillips and Williams suggested that compounds of silicon, germanium, arsenic, and antimony with B metals, "are probably best classed as alloys". Among the lighter metalloids, alloys with
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s are well-represented. Boron can form intermetallic compounds and alloys with such metals of the composition M''n''B, if ''n'' > 2. Ferroboron (15% boron) is used to introduce boron into
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...
; nickel-boron alloys are ingredients in welding alloys and case hardening compositions for the engineering industry. Alloys of silicon with
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and with aluminium are widely used by the steel and automotive industries, respectively. Germanium forms many alloys, most importantly with the coinage metals. The heavier metalloids continue the theme. Arsenic can form alloys with metals, including
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
and
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
; 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 include 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 less often recognised as metalloids, selenium – in the form of ferroselenium (50–58% selenium) – is used to improve the machinability of stainless steels.


Biological agents

All six of the elements commonly recognised as metalloids have toxic, dietary or medicinal properties. Arsenic and antimony compounds are especially toxic; boron, silicon, and possibly arsenic, are essential trace elements. Boron, silicon, arsenic, and antimony have medical applications, and germanium and tellurium are thought to have potential. Boron is used in insecticides and herbicides. It is an essential trace element. Science Learning Hub 2009 As boric acid, it has antiseptic, antifungal, and antiviral properties. Silicon is present in silatrane, a highly toxic rodenticide. Long-term inhalation of silica dust causes
silicosis Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Silic ...
, a fatal disease of the lungs. Silicon is an essential trace element.
Silicone A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cookin ...
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. There is interest in the pharmacological actions of germanium compounds but no licensed medicine as yet. Arsenic is notoriously poisonous and may also be an
essential element In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight ( oxygen, hydrogen, ca ...
in ultratrace amounts. Nielsen 1998 During
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...
, both sides used "arsenic-based sneezing and vomiting agents…to force enemy soldiers to remove their
gas mask A gas mask is a mask used to protect the wearer from inhaling airborne pollutants and toxic gases. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Most gas mas ...
s before firing mustard or
phosgene Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, esp ...
at them in a second
salvo A salvo is the simultaneous discharge of artillery or firearms including the firing of guns either to hit a target or to perform a salute. As a tactic in warfare, the intent is to cripple an enemy in one blow and prevent them from fightin ...
." It has been used as a pharmaceutical agent since antiquity, including for the treatment of
syphilis Syphilis () is a sexually transmitted infection caused by the bacterium '' Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, a ...
before the development of
antibiotics An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and preventio ...
. Jaouen & Gibaud 2010 Arsenic is also a component of melarsoprol, a medicinal drug used in the treatment of human
African trypanosomiasis African trypanosomiasis, also known as African sleeping sickness or simply sleeping sickness, is an insect-borne parasitic infection of humans and other animals. It is caused by the species '' Trypanosoma brucei''. Humans are infected by two t ...
or sleeping sickness. In 2003, arsenic trioxide (under the trade name
Trisenox Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...
) was re-introduced for the treatment of
acute promyelocytic leukaemia Acute promyelocytic leukemia (APML, APL) is a subtype of acute myeloid leukemia (AML), a cancer of the white blood cells. In APL, there is an abnormal accumulation of immature granulocytes called promyelocytes. The disease is characterized by ...
, a cancer of the blood and bone marrow. Arsenic in drinking water, which causes lung and bladder cancer, has been associated with a reduction in breast cancer mortality rates. Metallic antimony is relatively non-toxic, but most antimony compounds are poisonous. Two antimony compounds,
sodium stibogluconate Sodium stibogluconate, sold under the brand name Pentostam among others, is a medication used to treat leishmaniasis. This includes leishmaniasis of the cutaneous, visceral, and mucosal types. Some combination of miltefosine, paramycin and lipos ...
and stibophen, are used as antiparasitical drugs. Elemental tellurium is not considered particularly toxic; 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. Tellurium dioxide has been used to treat seborrhoeic dermatitis; other tellurium compounds were used as antimicrobial agents before the development of antibiotics. In the future, such compounds may need to be substituted for antibiotics that have become ineffective due to bacterial resistance. Of the elements less often recognised as metalloids, beryllium and lead are noted for their toxicity; lead arsenate has been extensively used as an insecticide. Sulfur is one of the oldest of the fungicides and pesticides. Phosphorus, sulfur, zinc, selenium, and iodine are essential nutrients, and aluminium, tin, and lead may be. Sulfur, gallium, selenium, 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; gallium citrate, a radiopharmaceutical, facilitates imaging of inflamed body areas. Selenium sulfide is used in medicinal shampoos and to treat skin infections such as tinea versicolor. Iodine is used as a disinfectant in various forms. Bismuth is an ingredient in some
antibacterial An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
s.


Catalysts

Boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bond ...
and trichloride are used as
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s in organic synthesis and electronics; the
tribromide Tribromide is the anion with the chemical formula Br3−, or salts containing it: * Tetrabutylammonium tribromide * Tetrabromophosphonium tribromide * Pyridinium perbromide Sodium and potassium tribromides can be prepared by reacting NaBr or K ...
is used in the manufacture of diborane. Non-toxic boron
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s could replace toxic phosphorus ligands in some transition metal catalysts. Silica sulfuric acid (SiO2OSO3H) is used in organic reactions. Germanium dioxide is sometimes used as a catalyst in the production of PET plastic for containers; cheaper antimony compounds, such as the trioxide or triacetate, are more commonly employed for the same purpose despite concerns about antimony contamination of food and drinks. Arsenic trioxide has been used in the production of
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
, to boost the removal of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
, as have selenous acid and
tellurous acid Tellurous acid is an inorganic compound with the formula H2TeO3. It is the oxoacid of tellurium(IV). This compound is not well characterized. An alternative way of writing its formula is (HO)2TeO. In principle, tellurous acid would form by trea ...
. Selenium acts as a catalyst in some microorganisms. Tellurium, its dioxide, and its tetrachloride are strong catalysts for air oxidation of carbon above 500 °C. Graphite oxide can be used as a catalyst in the synthesis of imines and their derivatives.
Activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that increase the surface area availa ...
and alumina have been used as catalysts for the removal of sulfur contaminants from natural gas.
Titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...
doped aluminium has been identified as a substitute for expensive noble metal catalysts used in the production of industrial chemicals.


Flame retardants

Compounds of boron, silicon, arsenic, and antimony have been used as flame retardants. Boron, in the form of
borax Borax is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form ...
, has been used as a textile flame retardant since at least the 18th century. Silicon compounds such as silicones, silanes,
silsesquioxane 220x220px, thumbnail, A cubic silsesquioxane. A silsesquioxane is an organosilicon compound with the chemical formula SiO3/2sub>n (R = H, alkyl, aryl, alkenyl or alkoxyl.). Silsesquioxanes are colorless solids that adopt cage-like or polymeric ...
,
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
, and
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...
s, some of which were developed as alternatives to more toxic halogenated products, can considerably improve the flame retardancy of plastic materials. Arsenic compounds such as sodium arsenite or sodium arsenate are effective flame retardants for wood but have been less frequently used due to their toxicity. Antimony trioxide is a flame retardant.
Aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic ...
has been used as a wood-fibre, rubber, plastic, and textile flame retardant since the 1890s. Apart from aluminium hydroxide, use of phosphorus based flame-retardants – in the form of, for example,
organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered ...
s – now exceeds that of any of the other main retardant types. These employ boron, antimony, or halogenated hydrocarbon compounds.


Glass formation

The oxides B2O3, SiO2, GeO2, As2O3, and Sb2O3 readily form
glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
es. TeO2 forms a glass but this requires a "heroic quench rate" or the addition of an impurity; otherwise the crystalline form results. Kaminow & Li 2002, p. 118 These compounds are used in chemical, domestic, and industrial glassware and optics. Boron trioxide is used as a
glass fibre Glass fiber ( or glass fibre) is a material consisting of numerous extremely fine fibers of glass. Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the inventio ...
additive, and is also a component of borosilicate glass, widely used for laboratory glassware and domestic ovenware for its low thermal expansion. Most ordinary glassware is made from silicon dioxide. 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 (for the removal of bubbles), as is antimony trioxide. Tellurium dioxide finds application in laser and nonlinear optics.
Amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
metallic glass An amorphous metal (also known as metallic glass, glassy metal, or shiny metal) is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a hig ...
es 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 an alloy of composition Au75Si25 reported in 1960. A metallic glass having a strength and toughness not previously seen, of composition Pd82.5P6Si9.5Ge2, was reported in 2011. Phosphorus, selenium, and lead, which are less often recognised as metalloids, are also used in glasses.
Phosphate glass Phosphate glass is a class of optical glasses composed of metaphosphates of various metals. Instead of SiO2 in silicate glasses, the glass forming substrate is P2O5. Discovery Dr. Alexis G. Pincus of the American Optical Company supplied alum ...
has a substrate of phosphorus pentoxide (P2O5), rather than the silica (SiO2) of conventional silicate glasses. It is used, for example, to make
sodium lamp A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589  nm. Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
s. Selenium compounds can be used both as decolourising agents and to add a red colour to glass. Decorative glassware made of traditional
lead glass Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically a ...
contains at least 30%
lead(II) oxide Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula Pb O. PbO occurs in two polymorphs: litharge having a tetragonal crystal structure, and massicot having an orthorhombic crystal structure. Modern ap ...
(PbO); lead glass used for radiation shielding may have up to 65% PbO. Lead-based glasses have also been extensively used in electronic 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 and optoelectronics

Varying compositions of GeSbTe ("GST alloys") and Ag- and In- doped Sb2Te ("AIST alloys"), being examples of
phase-change material A phase change material (PCM) is a substance which releases/absorbs sufficient energy at phase transition to provide useful heat or cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and li ...
s, are widely used in rewritable
optical disc In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data ( bits) in the form of pits and lands on a special material, often aluminum, on one of its flat surface ...
s and phase-change memory devices. By applying heat, they can be switched between amorphous (glassy) and crystalline states. The change in optical and electrical properties can be used for information storage purposes. Future applications for GeSbTe may include, "ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses, and artificial retina devices."


Pyrotechnics

The recognised metalloids have either pyrotechnic applications or associated properties. Boron and silicon are commonly encountered; Kosanke 2002, p. 110 they act somewhat like metal fuels. Boron is used in
pyrotechnic initiator In pyrotechnics, a pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic composition used primarily to ignite other, more difficult-to-ignite materials, such as thermites, gas generators, and solid-fuel rockets. The ...
compositions (for igniting other hard-to-start compositions), and in delay compositions that burn at a constant rate. Conkling & Mocella 2010, p. 82
Boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron– carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers ...
has been identified as a possible replacement for more toxic
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
or hexachloroethane mixtures in smoke munitions, signal flares, and fireworks. Silicon, like boron, is a component of initiator and delay mixtures. Doped germanium can act as a variable speed thermite fuel. Arsenic trisulfide As2S3 was used in old naval signal lights; in fireworks to make white stars; in yellow smoke screen mixtures; and in initiator compositions. Antimony trisulfide Sb2S3 is found in white-light fireworks and in flash and sound mixtures. Tellurium has been used in delay mixtures and in blasting cap initiator compositions. Ellern 1968, pp. 209–10, 322 Carbon, aluminium, phosphorus, and selenium continue the theme. Carbon, in
black powder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). T ...
, is a constituent of fireworks rocket propellants, bursting charges, and effects mixtures, and military delay fuses and igniters. Aluminium is a common pyrotechnic ingredient, and is widely employed for its capacity to generate light and heat, including in thermite mixtures. Phosphorus can be found in smoke and incendiary munitions, paper caps used in toy guns, and
party popper A party popper is a handheld pyrotechnic device commonly used at parties. It emits a loud popping noise by means of a small friction-actuated explosive charge that is activated by pulling a string. The explosive charge comes from a very small amo ...
s. Selenium has been used in the same way as tellurium.


Semiconductors and electronics

All the elements commonly recognised as metalloids (or their compounds) have been used in the semiconductor or solid-state electronic industries. Some properties of boron have limited its use as a semiconductor. It has a high melting point, single
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
s 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) Bomgardner 2013, p. 20 and information and communication technologies. This was despite the study of semiconductors, early in the 20th century, having been regarded as the "physics of dirt" and not deserving of close attention. Germanium has largely been replaced by silicon in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process. Russell & Lee 2005, p. 401; Büchel, Moretto & Woditsch 2003, p. 278 Germanium is still a constituent of semiconducting silicon-germanium "alloys" and these have been growing in use, particularly for wireless communication devices; such alloys exploit the higher carrier mobility of germanium. The synthesis of gram-scale quantities of semiconducting germanane was reported in 2013. This consists of one-atom thick sheets of hydrogen-terminated germanium atoms, analogous to
graphane Graphane is a two-dimensional polymer of carbon and hydrogen with the formula unit (CH)n where ''n'' is large. Partial hydrogenation results in hydrogenated graphene, which was reported by Elias et al in 2009 by a TEM study to be "direct evidenc ...
. It conducts electrons more than ten times faster than silicon and five times faster than germanium, and is thought to have potential for optoelectronic and sensing applications. The development of a germanium-wire based anode that more than doubles the capacity of
lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
was reported in 2014. In the same year, Lee et al. reported that defect-free crystals of
graphene Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure.
large enough to have electronic uses could be grown on, and removed from, a germanium substrate. Arsenic and antimony are not semiconductors in their standard state#Liquids and solids, 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. Antimony nanocrystals may enable
lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
to be replaced by more powerful sodium ion batteries. Tellurium, which is a semiconductor in its standard state, is used mainly as a component in type II/VI semiconducting- chalcogenides; these have applications in electro-optics and electronics. Cadmium telluride (CdTe) is used in solar modules for its high conversion efficiency, low manufacturing costs, and large band gap of 1.44 eV, letting it absorb a wide range of 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 expected to find such use. Of the less often recognised metalloids, phosphorus, gallium (in particular) and selenium have semiconductor applications. Phosphorus is used in trace amounts as a
dopant A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. Whe ...
for
n-type semiconductor An extrinsic semiconductor is one that has been '' doped''; during manufacture of the semiconductor crystal a trace element or chemical called a doping agent has been incorporated chemically into the crystal, for the purpose of giving it differen ...
s. The commercial use of gallium compounds is dominated by semiconductor applications – in integrated circuits, cell phones,
laser diode The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with ...
s,
light-emitting diode A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s, photodetectors, and
solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.
s. Selenium is used in the production of solar cells and in high-energy surge protectors. Boron, silicon, germanium, antimony, and tellurium, as well as heavier metals and metalloids such as Sm, Hg, Tl, Pb, Bi, and Se, can be found in topological insulators. These are alloys or compounds which, at ultracold temperatures or room temperature (depending on their composition), are metallic conductors on their surfaces but insulators through their interiors. Cadmium arsenide Cd3As2, at about 1 K, is a Dirac-semimetal – a bulk electronic analogue of graphene – in which electrons travel effectively as massless particles. These two classes of material are thought to have potential
quantum computing Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...
applications.


Nomenclature and history


Derivation and other names

The word metalloid comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''metallum'' ("metal") and the Greek ''oeides'' ("resembling in form or appearance"). Several names are sometimes used synonymously although some of these have other meanings that are not necessarily interchangeable: ''amphoteric element,'' ''boundary element,'' ''half-metal,'' Klemm 1950, pp. 133–42; Reilly 2004, p. 4 ''half-way element,'' ''near metal,'' Tyler 1948, p. 105 ''meta-metal,'' ''semiconductor,'' ''semimetal'' and ''submetal''. "Amphoteric element" is sometimes used more broadly to include transition metals capable of forming oxyanions, such as chromium and
manganese Manganese is a chemical element with the Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of ...
. " Half-metal" is used in physics to refer to a compound (such as
chromium dioxide Chromium dioxide or chromium(IV) oxide is an inorganic compound with the formula CrO2. It is a black synthetic magnetic solid. It once was widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chrom ...
) 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. The names ''amphoteric element'' and ''semiconductor'' are problematic as some elements referred to as metalloids do not show marked amphoteric behaviour (bismuth, for example) or semiconductivity (polonium) in their most stable forms.


Origin and usage

The origin and usage of the term ''metalloid'' is convoluted. Its origin lies in attempts, dating from antiquity, to describe metals and to distinguish between typical and less typical forms. It was first applied in the early 19th century to metals that floated on water (sodium and potassium), and then more popularly to nonmetals. Earlier usage in
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...
, to describe a mineral having a metallic appearance, can be sourced to as early as 1800. Since the mid-20th century it has been used to refer to intermediate or borderline chemical elements. Goldsmith 1982, p. 526 The
International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
(IUPAC) previously recommended abandoning the term metalloid, and suggested using the term ''semimetal'' instead. Use of this latter term has more recently been discouraged by Atkins et al. Atkins et al. 2010, p. 20 as it has a different meaning in physics – one that more specifically refers to the
electronic band structure In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called '' band gaps'' or ...
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 of the terms metalloid or semimetal.


Elements commonly recognised as metalloids

:''Properties noted in this section refer to the elements in their most thermodynamically stable forms under ambient conditions.''


Boron

Pure boron is a shiny, silver-grey crystalline solid. It is less dense than aluminium (2.34 vs. 2.70 g/cm3), and is hard and brittle. It is barely reactive under normal conditions, except for attack by
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
, and has a melting point of 2076 °C (cf. steel ~1370 °C). Boron is a semiconductor; its room temperature electrical conductivity is 1.5 × 10−6 S•cm−1 (about 200 times less than that of tap water) and it has a band gap of about 1.56 eV. Mendeleev commented that, "Boron appears in a free state in several forms which are intermediate between the metals and the nonmmetals." The structural chemistry of boron is dominated by its small atomic size, and relatively high ionization energy. With only three valence electrons per boron atom, simple covalent bonding cannot fulfil the octet rule. Rayner-Canham & Overton 2006, p. 291 Metallic bonding is the usual result among the heavier congenors of boron but this generally requires low ionization energies. Instead, because of its small size and high ionization energies, the basic structural unit of boron (and nearly all of its allotropes) is the icosahedral B12 cluster. Of the 36 electrons associated with 12 boron atoms, 26 reside in 13 delocalized molecular orbitals; the other 10 electrons are used to form two- and three-centre covalent bonds between icosahedra. The same motif can be seen, as are deltahedral variants or fragments, in metal borides and hydride derivatives, and in some halides. The bonding in boron has been described as being characteristic of behaviour intermediate between metals and nonmetallic
covalent network A network solid or covalent network solid (also called atomic crystalline solids or giant covalent structures) is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the mat ...
solids (such as
diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...
). 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 the proximity of boron to the metal-nonmetal borderline. Fehlner 1990, p. 202 Most of the chemistry of boron is nonmetallic in nature. Unlike its heavier congeners, it is not known to form a simple B3+ or hydrated (H2O)4sup>3+ cation. The small size of the boron atom enables the preparation of many interstitial alloy-type borides. Greenwood & Earnshaw 2002, p. 145 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 6H6sup>2− and u6(CO)18sup>2−. The aqueous chemistry of boron is characterised by the formation of many different polyborate anions. Given its high charge-to-size ratio, boron bonds covalently in nearly all of its compounds; the exceptions are the borides as these include, depending on their composition, covalent, ionic, and metallic bonding components. Simple binary compounds, such as boron trichloride are Lewis acids as the formation of three covalent bonds leaves a hole in the
octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 com ...
which can be filled by an electron-pair donated by a
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. Boron has a strong affinity for
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and a duly extensive borate chemistry. The oxide B2O3 is polymeric in structure, Puddephatt & Monaghan 1989, p. 59 weakly acidic, and a glass former. Rao 2002, p. 22 Organometallic compounds of boron have been known since the 19th century (see
organoboron chemistry Organoborane or organoboron compounds are chemical compounds of boron and carbon that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds. Organoboron ...
).


Silicon

Silicon is a crystalline solid with a blue-grey metallic lustre. Greenwood & Earnshaw 2002, p. 331 Like boron, it is less dense (at 2.33 g/cm3) than aluminium, and is hard and brittle. It is a relatively unreactive element. According to Rochow, the massive crystalline form (especially if pure) is "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 dissolves in hot aqueous
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
s with the evolution of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, as do metals such as beryllium, aluminium, zinc, gallium or indium. It melts at 1414 °C. Silicon is a semiconductor with an electrical conductivity of 10−4 S•cm−1 and a band gap of about 1.11 eV. Russell & Lee 2005, p. 393 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 is not known to form a cation. Silicon can form alloys with metals such as iron and copper. It shows fewer tendencies to anionic behaviour than ordinary nonmetals. Cox 2004, p. 27 Its solution chemistry is characterised by the formation of oxyanions. Hiller & Herber 1960, inside front cover; p. 225 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 Organosilicon compounds are organometallic compounds containing carbon–silicon bonds. Organosilicon chemistry is the corresponding science of their preparation and properties. Most organosilicon compounds are similar to the ordinary organic c ...
).


Germanium

Germanium is a shiny grey-white solid. It has a density of 5.323 g/cm3 and is hard and brittle. It is mostly unreactive at room temperature but is slowly attacked by hot concentrated
sulfuric Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
or
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
. Greenwood & Earnshaw 2002, p. 373 Germanium also reacts with molten caustic soda to yield sodium germanate Na2GeO3 and hydrogen gas. It melts at 938 °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 similar to that of liquid mercury. Most of the chemistry of germanium is characteristic of a nonmetal. Whether or not germanium forms a cation is unclear, aside from the reported existence of the Ge2+ ion in a few esoteric compounds. It can form alloys with metals such as aluminium and
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
. It 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. Eggins 1972, p. 66; Wiberg 2001, p. 895 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·H2Oand 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 (the monoxide GeO, is even more so), and this is sometimes used to classify germanium as a metal. Up to the 1930s germanium was considered to be a poorly conducting metal; it has occasionally been classified as a metal by later writers. As with all the elements commonly recognised as metalloids, germanium has an established organometallic chemistry (see Organogermanium chemistry).


Arsenic

Arsenic is a grey, metallic looking solid. It has a density of 5.727 g/cm3 and is brittle, and moderately hard (more than aluminium; less than
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
). Gray, Whitby & Mann 2011 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 and hydrogen gas. Greenwood & Earnshaw 2002, p. 552 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 Carapella 1968, p. 30 and a band overlap of 0.5 eV. Barfuß et al. 1981, p. 967 Liquid arsenic is a semiconductor with a band gap of 0.15 eV. The chemistry of arsenic is predominately nonmetallic. Whether or not arsenic forms a cation is unclear. Its many metal alloys are mostly brittle. It 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. Massey 2000, p. 267 The halides, and the oxides and their derivatives are illustrative examples. Bailar, Moeller & Kleinberg 1965, p. 513 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, as noted below, (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

Antimony is a silver-white solid with a blue tint and a brilliant lustre. It has a density of 6.697 g/cm3 and is 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. Greenwood & Earnshaw 2002, p. 553 It is not affected by molten alkali. Antimony is capable of displacing hydrogen from water, when heated: 2 Sb + 3 H2O → Sb2O3 + 3 H2. It melts at 631 °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. Antimony has some definite cationic chemistry, SbO+ and Sb(OH)2+ being present in acidic aqueous solution; the compound Sb8(GaCl4)2, which contains the homopolycation, Sb82+, was prepared in 2004. It can form alloys with one or more metals such as aluminium, iron,
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
, 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 (being a heavier atom) to
oxidize Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
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 Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...
Sb(NO3)3,
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...
SbPO4, sulfate Sb2(SO4)3 and
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. ...
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 In chemistry, an oxycation is a polyatomic ion with a positive charge that contains oxygen. Examples * Dioxygenyl ion, * Nitrosonium ion, * Nitronium ion, * Vanadyl ion, VO2+, a very stable oxycation * Uranyl ion, , all natural U6+ occurs ...
SbO. The oxide Sb2O3 is polymeric, amphoteric, Wiberg 2001, p. 764 and a glass former. Antimony has an extensive organometallic chemistry (see
Organoantimony chemistry Organoantimony chemistry is the chemistry of compounds containing a carbon to antimony (Sb) chemical bond. Relevant oxidation states are Sb(V) and Sb(III). The toxicity of antimony limits practical application in organic chemistry. Organoantimo ...
).


Tellurium

Tellurium is a silvery-white shiny solid. It has a density of 6.24 g/cm3, is brittle, and is 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. Kudryavtsev 1974, p. 78 It reacts (to varying degrees) with nitric, sulfuric, and hydrochloric acids to give compounds such as the
sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
TeSO3 or
tellurous acid Tellurous acid is an inorganic compound with the formula H2TeO3. It is the oxoacid of tellurium(IV). This compound is not well characterized. An alternative way of writing its formula is (HO)2TeO. In principle, tellurous acid would form by trea ...
H2TeO3, the basic nitrate (Te2O4H)+(NO3), or the oxide sulfate Te2O3(SO4). It dissolves in boiling alkalis, to give the
tellurite The tellurite ion is . A tellurite (compound), for example sodium tellurite, is a compound that contains this ion. They are typically colorless or white salts, which in some ways are comparable to sulfite. A mineral with the formula TeO2 is c ...
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 449.51 °C. Crystalline tellurium has a structure consisting of parallel infinite spiral chains. The bonding between adjacent atoms in a chain is covalent, but there is evidence of a weak metallic interaction between the neighbouring atoms of different chains. Stuke 1974, p. 178; Donohue 1982, pp. 386–87; Cotton et al. 1999, p. 501 Tellurium is a semiconductor with an electrical conductivity of around 1.0 S•cm−1 and a band gap of 0.32 to 0.38 eV. Berger 1997, p. 90 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 shows some cationic behaviour. The dioxide dissolves in acid to yield the trihydroxotellurium(IV) Te(OH)3+ ion; the red Te42+ and yellow-orange Te62+ ions form when tellurium is oxidized in
fluorosulfuric acid Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, H2SO ...
(HSO3F), or liquid
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...
(SO2), respectively. It can form alloys with aluminium,
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
, 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. The +4 state is the most stable. Tellurides of composition X''x''Te''y'' are easily formed with most other elements and represent the most common tellurium minerals.
Nonstoichiometry In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mo ...
is pervasive, especially with transition metals. Many tellurides can be regarded 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 BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
selenate 2TeO2·SeO3 and an analogous perchlorate and periodate 2TeO2·HXO4. Tellurium forms a polymeric, amphoteric, glass-forming oxide TeO2. It 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

Carbon is ordinarily classified as a nonmetal but has some metallic properties and is occasionally classified as a metalloid. Hexagonal graphitic carbon (graphite) is the most thermodynamically stable
allotrope Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical State of matter, state, known as allotropes of the elements. Allotropes are different structural modifications o ...
of carbon under ambient conditions. It has a lustrous appearance and is a fairly good electrical conductor. Graphite has a layered structure. Each layer consists of carbon atoms bonded to three other carbon atoms in a
hexagonal lattice The hexagonal lattice or triangular lattice is one of the five two-dimensional Bravais lattice types. The symmetry category of the lattice is wallpaper group p6m. The primitive translation vectors of the hexagonal lattice form an angle of 120� ...
arrangement. The layers are stacked together and held loosely by
van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and ...
s and delocalized valence electrons. Like a metal, the conductivity of graphite in the direction of its planes decreases as the temperature is raised; Atkins et al. 2006, pp. 320–21 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. The resulting materials are referred to as "carbon alloys". Carbon can form ionic salts, including a hydrogen sulfate, perchlorate, and nitrate (CX.2HX, where X = HSO4, ClO4; and CNO.3HNO3). In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
, carbon can form complex cationstermed ''carbocations''in which the positive charge is on the carbon atom; examples are and , and their derivatives. Carbon is brittle, and behaves as a semiconductor in a direction perpendicular to 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 C4− (
methanide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...
), C ( acetylide), and C ( sesquicarbide or allylenide), in compounds with metals of main groups 1–3, and with the
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...
s and
actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...
s. Its oxide CO2 forms carbonic acid H2CO3.


Aluminium

Aluminium is ordinarily classified as a metal. It is lustrous, malleable and ductile, and has high electrical and thermal conductivity. Like most metals it has a close-packed crystalline structure, and forms a cation in aqueous solution. It has some properties that are unusual for a metal; taken together, Metcalfe, Williams & Castka 1974, p. 539 these are sometimes used as a basis to classify aluminium as a metalloid. Its crystalline structure shows some evidence of directional bonding. Aluminium bonds covalently in most compounds. The oxide Al2O3 is amphoteric and a conditional glass-former. Aluminium can form anionic
aluminate In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminum atom. Aluminate oxyanions Alu ...
s, such behaviour being considered nonmetallic in character. Hamm 1969, p. 653 Classifying aluminium as a metalloid has been disputed given its many metallic properties. It is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal–nonmetal dividing line are metalloids. 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 in its amphoteric oxide and in the covalent character of many of its compounds ... Yet it is a highly electropositive metal ... itha high negative electrode potential". Moody says that, "aluminium is on the 'diagonal borderland' between metals and non-metals in the chemical sense."


Selenium

Selenium shows borderline metalloid or nonmetal behaviour. Its most stable form, the grey trigonal allotrope, is sometimes called "metallic" selenium because its electrical conductivity is several orders of magnitude greater than that of the red
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
form. The metallic character of selenium is further shown by its lustre, Glinka 1965, p. 356 and its crystalline structure, which is thought to include weakly "metallic" interchain bonding. Selenium can be drawn into thin threads when molten and viscous. It shows reluctance to acquire "the high positive oxidation numbers characteristic of nonmetals". It can form cyclic polycations (such as Se) when dissolved in
oleum Oleum (Latin ''oleum'', meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Ol ...
s (an attribute it shares with sulfur and tellurium), and a hydrolysed cationic salt in the form of trihydroxoselenium(IV) perchlorate e(OH)3sup>+·ClO. 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. Kozyrev 1959, p. 104; Chizhikov & Shchastlivyi 1968, p. 25; Glazov, Chizhevskaya & Glagoleva 1969, p. 86 This is comparable to or less than that of
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
(7.95 S•cm−1), a nonmetal. Selenium has the electronic band structure of a
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
Berger 1997, pp. 86–87 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 Se2−, SeO and SeO. Selenium is commonly described as a metalloid in the
environmental chemistry Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source. It can be defined as ...
literature. It moves through the aquatic environment similarly to arsenic and antimony; its water-soluble salts, in higher concentrations, have a similar toxicological profile to that of arsenic.


Polonium

Polonium is "distinctly metallic" in some ways. Cotton et al. 1999, p. 502 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. Bagnall 1966, p. 41; Nickless 1968, p. 79 Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly reducing conditions are required for the formation of the Po2− anion in aqueous solution. Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants. It has a simple cubic crystalline structure. Such a structure has few slip systems and "leads to very low ductility and hence low fracture resistance". Polonium shows nonmetallic character in its halides, and by the existence of
polonide A polonide is a chemical compound of the radioactive element polonium with any element less electronegative than polonium. Polonides are usually prepared by a direct reaction between the elements at temperatures of around 300–400 °C... They a ...
s. 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 Po2− anion, are also known.


Astatine

As a
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this grou ...
, astatine tends to be classified as a nonmetal. It has some marked metallic properties and is sometimes instead classified as either a metalloid or (less often) as 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, with 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 (∆Hvap) greater than ~42 kJ/mol are metallic when liquid. Rao & Ganguly 1986 Such elements include boron, silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆Hvap of
diatomic Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear. O ...
astatine are 50 kJ/mol or higher; diatomic iodine, with a ∆Hvap of 41.71, falls just short of the threshold figure. "Like typical metals, it statineis precipitated by
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...
even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
on
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
." 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 astatine 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 been extrapolated to have the narrow liquid range ordinarily associated with nonmetals (mp 302 °C; bp 337 °C), although experimental indications suggest a lower boiling point of about 230±3 °C. 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 band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
s and thereby being either semiconductors or insulators. The chemistry of astatine in aqueous solution is mainly 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 polonium-like than halogen-like. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.


Related concepts


Near metalloids

In the periodic table, some of the elements adjacent 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 unusual 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

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. White tin is a very shiny, ductile and malleable metal. It is the stable form 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, and can also be prepared in brittle semi-lustrous crystalline or polycrystalline forms. It is the stable form below 13.2 °C 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 diamond. It behaves as a semiconductor (as if it had 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. Vernon 2013, p. 1705 The diamond allotrope of carbon is clearly nonmetallic, being translucent and having a low 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 different behaviours.


Abundance, extraction, and cost


Abundance

The table gives crustal abundances of the elements commonly to rarely recognised as metalloids. Some other elements are included for comparison: oxygen and xenon (the most and least abundant elements with stable isotopes); iron and the coinage metals copper, silver, and gold; and rhenium, the least abundant stable metal (aluminium is normally the most abundant metal). Various abundance estimates have been published; these often disagree to some extent.


Extraction

The recognised metalloids can be obtained by chemical reduction of either their oxides or their
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds la ...
s. Simpler or more complex extraction methods may be employed depending on the starting form and economic factors. Boron is routinely obtained by reducing the trioxide with magnesium: B2O3 + 3 Mg → 2 B + 3MgO; after secondary processing the resulting brown powder has a purity of up to 97%. Boron of higher purity (> 99%) is prepared by heating volatile boron compounds, such as BCl3 or BBr3, either in a hydrogen atmosphere (2 BX3 + 3 H2 → 2 B + 6 HX) or to the point of thermal decomposition. Silicon and germanium are obtained from their oxides by heating the oxide with carbon or hydrogen: SiO2 + C → Si + CO2; GeO2 + 2 H2 → Ge + 2 H2O. Arsenic is isolated from its pyrite (FeAsS) or arsenical pyrite (FeAs2) by heating; alternatively, it can be obtained from its oxide by reduction with carbon: 2 As2O3 + 3 C → 2 As + 3 CO2. Antimony is derived from its sulfide by reduction with iron: Sb2S3 → 2 Sb + 3 FeS. Tellurium is prepared from its oxide by dissolving it in aqueous NaOH, yielding tellurite, then by
electrolytic reduction An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two electrode ...
: TeO2 + 2 NaOH → Na2TeO3 + H2O; Na2TeO3 + H2O → Te + 2 NaOH + O2. Another option is reduction of the oxide by roasting with carbon: TeO2 + C → Te + CO2. Production methods for the elements less frequently recognised as metalloids involve natural processing, electrolytic or chemical reduction, or irradiation. Carbon (as graphite) occurs naturally and is extracted by crushing the parent rock and floating the lighter graphite to the surface. Aluminium is extracted by dissolving its oxide Al2O3 in molten cryolite Na3AlF6 and then by high temperature electrolytic reduction. Selenium is produced by roasting the coinage metal selenides X2Se (X = Cu, Ag, Au) with soda ash to give the selenite: X2Se + O2 + Na2CO3 → Na2SeO3 + 2 X + CO2; the selenide is neutralized by sulfuric acid H2SO4 to give selenous acid H2SeO3; this is reduced by bubbling with SO2 to yield elemental selenium. Polonium and astatine are produced in minute quantities by irradiating bismuth.


Cost

The recognised metalloids and their closer neighbours mostly cost less than silver; only polonium and astatine are more expensive than gold, on account of their significant radioactivity. As of 5 April 2014, prices for small samples (up to 100 g) of silicon, antimony and tellurium, and graphite, aluminium and selenium, average around one third the cost of silver (US$1.5 per gram or about $45 an ounce). Boron, germanium, and arsenic samples average about three-and-a-half times the cost of silver. Polonium is available for about $100 per microgram. Zalutsky and Pruszynski Zalutsky & Pruszynski 2011, p. 181 estimate a similar cost for producing astatine. Prices for the applicable elements traded as commodities tend to range from two to three times cheaper than the sample price (Ge), to nearly three thousand times cheaper (As).


Notes


References


Sources

*Addison WE 1964, ''The Allotropy of the Elements,'' Oldbourne Press, London *Addison CC & Sowerby DB 1972, ''Main Group Elements: Groups V and VI,'' Butterworths, London, *Adler D 1969, 'Half-way Elements: The Technology of Metalloids', book review, ''Technology Review,'' vol. 72, no. 1, Oct/Nov, pp. 18–19, *Ahmed MAK, Fjellvåg H & Kjekshus A 2000, 'Synthesis, Structure and Thermal Stability of Tellurium Oxides and Oxide Sulfate Formed from Reactions in Refluxing Sulfuric Acid', ''Journal of the Chemical Society, Dalton Transactions,'' no. 24, pp. 4542–49, *Ahmeda E & Rucka M 2011, 'Homo- and heteroatomic polycations of groups 15 and 16. Recent advances in synthesis and isolation using room temperature ionic liquids', ''Coordination Chemistry Reviews,'' vol. 255, nos 23–24, pp. 2892–903, *Allen DS & Ordway RJ 1968, ''Physical Science,'' 2nd ed., Van Nostrand, Princeton, New Jersey, *Allen PB & Broughton JQ 1987, 'Electrical Conductivity and Electronic Properties of Liquid Silicon', ''Journal of Physical Chemistry,'' vol. 91, no. 19, pp. 4964–70, *Alloul H 2010
''Introduction to the Physics of Electrons in Solids,''
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American Journal of Therapeutics The ''American Journal of Therapeutics'' is a bimonthly medical journal covering advances in drug therapy, comparative effectiveness research, and post-marketing surveillance. The journal was established in 1994 by John Somberg MD and is published ...
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Further reading

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