13 Boron group The boron group is the series of elements in group 13 of the periodic table, comprising boron , aluminium , gallium , indium , thallium , and ununtrium . The elements in the boron group are characterized by having three electrons in their outer energy levels...	14 Carbon group The carbon group is a periodic table group consisting of carbon , silicon , germanium , tin , lead , and ununquadium ....	15	16 Chalcogen The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...	17 Halogen The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
	2 Period 2 element A period 2 element is one of the chemical elements in the second row of the periodic table. The periodic table is laid out in rows to illustrate recurring trends in the chemical behavior of the elements as their atomic number increases; a new row is started when chemical behavior begins to...	B Boron Boron Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...	C Carbon Carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...	N Nitrogen	O Oxygen	F Fluorine
	3 Period 3 element A period 3 element is one of the chemical elements in the third row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical...	Al* Aluminium Aluminium Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....	Si Silicon Silicon Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...	P Phosphorus	S Sulfur	Cl Chlorine
	4 Period 4 element A period 4 element is one of the chemical elements in the fourth row of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour...	Ga Gallium	Ge Germanium Germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....	As Arsenic Arsenic Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...	Se Selenium Selenium Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...	Br Bromine
	5 Period 5 element A period 5 element is one of the chemical elements in the fifth row of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour...	In Indium	Sn Tin	Sb Antimony Antimony Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...	Te Tellurium	I Iodine
	6 Period 6 element A period 6 element is one of the chemical elements in the sixth row of the periodic table of the elements, including the lanthanides...	Tl Thallium	Pb Lead	Bi Bismuth	Po* Polonium Polonium Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...	At* Astatine Astatine Astatine is a radioactive chemical element with the symbol At and atomic number 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the periodic table...
			Common		*The metalloid status of Al, Po and At is disputed.
			Less common
			Uncommon
			Rare
	Indicative (relative) frequency with which some elements appear in metalloid lists. Frequencies are from the list of metalloid lists and occur in a more or less geometric progression Geometric progression In mathematics, a geometric progression, also known as a geometric sequence, is a sequence of numbers where each term after the first is found by multiplying the previous one by a fixed non-zero number called the common ratio. For example, the sequence 2, 6, 18, 54, ... is a geometric progression...  of clusters. The common elements have appearance frequencies clustering around the low 90s; 'less common' elements appear half as often (clustering around ~45 per cent); and the single 'uncommon' representative (Se) and the following cluster of 'rare' elements have appearance frequencies each around half that of their immediate precursors. The series continues with the still less frequently appearing elements but this is not shown above on account of the relatively small sample size. The grey stair step is a typical example of the arbitrary dividing line between metals and nonmetals that can be found on some periodic tables. That germanium, if classified as a non-metal, then appears to fall on the wrong side of the metal-nonmetal divide, is an outcome of the publicity this form of the line received in the late 1920s and early 30s, and the view (held up to at least the late 1930s) that germanium was a poorly conducting metal.

Metalloid is a term used in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 when classifying the chemical element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s. On the basis of their general physical

Physical property

A physical property is any property that is measurable whose value describes a physical system's state. The changes in the physical properties of a system can be used to describe its transformations ....

 and chemical properties, each element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 can usually be classified as a metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 or a nonmetal

Nonmetal

Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

. However, some elements with intermediate or mixed properties can be harder to characterize. These elements are sometimes classified as metalloids, from the Latin metallum = "metal" and the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 oeides = "resembling in form or appearance". They have been described as forming a (fuzzy

Fuzzy logic

Fuzzy logic is a form of many-valued logic; it deals with reasoning that is approximate rather than fixed and exact. In contrast with traditional logic theory, where binary sets have two-valued logic: true or false, fuzzy logic variables may have a truth value that ranges in degree between 0 and 1...

) buffer zone between metals and nonmetals, the make-up and size of which depends on the applicable classification criteria.

The terms amphoteric element, half-metal, half-way element, near metal, semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 and semimetal are sometimes used synonymously. However, most of these terms have other meanings, which may not be interchangeable:

• 'amphoteric element' is sometimes used more broadly to include transition metal
  Transition metal
  The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
  
  s capable of forming oxyanion
  Oxyanion
  An oxyanion or oxoanion is a chemical compound with the generic formula AxOyz− . Oxoanions are formed by a large majority of the chemical elements. The formulae of simple oxoanions are determined by the octet rule...
  
  s, such as chromium
  Chromium
  Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
  
   and manganese
  Manganese
  Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
  
  ;
• 'half-metal' is sometimes instead used to refer to the poor metals; it also has an unrelated meaning
  Half-metal
  A half-metal is any substance that acts as a conductor to electrons of one spin orientation, but as an insulator to those of the opposite orientation....
  
  , in physics, of a compound (such as chromium dioxide) or alloy capable of acting as a conductor and an insulator;
• 'semimetal' is used to refer, more or less frequently and definitively, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure—examples include gallium, ytterbium, bismuth and neptunium.

In addition, some elements otherwise referred to as metalloids are not known to exhibit marked amphoteric

Amphoterism

In chemistry, an amphoteric species is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word amphoteroi meaning "both"...

 behaviour, or exhibit semiconductivity

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 in their most stable forms.

Metalloids are generally regarded as a third classification of chemical elements, alongside metals and nonmetals. On some occasions they have instead been grouped with the metals, regarded as nonmetals or treated as a sub-category of same.

Properties

There is no universally agreed or rigorous definition of the term metalloid and the classification of any particular element as such has been described as 'arbitrary'.

Notwithstanding, properties associated with metalloids are set out in the following two tables, alongside (and in comparison to) those of metals and non-metals. Shading to either side of the metalloids column denotes immediately apparent commonalties.

Physical

Property	Metals	Metalloids	Non-metals
Form	solid; a few liquid at or near room temperature (Ga, Hg, Cs, Fr)	solid	mostly gases
Appearance	characteristic lustre	metallic lustre	colourless; red, yellow, green, black, or intermediate shades
Allotropy	many show metallic allotropes; Bi, Sn have semiconducting allotropes	tend to exist in several (conspicuously) 'metallic' and non-metallic allotropic forms	show non-metallic allotropy (O, S), with elements close to the metal-non-metal line (C, P, Se) showing more 'metallic' allotropes
Density	generally high, with some exceptions such as the alkali metals	densities lower than neighbouring poor metals but higher than those of neighbouring nonmetals	often low
Elasticity	typically elastic, ductile, malleable (when solid)	brittle	brittle (when solid)
Electrical conductivity	good to high	intermediate to good	poor to intermediate
Thermal conductivity	medium to high	mostly intermediate; Si is high	almost negligible to very high
Packing	close-packed crystal structures; high coordination numbers	have relatively open crystal structures, with medium coordination number Coordination number In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals.... s, in contrast to the close-packed crystal structures of metals	low coordination numbers
Melting behaviour	volume generally expands	some contract, unlike (most) metals	volume generally expands
Enthalpy of fusion	may be high	often have abnormally high enthalpy of fusion values (compared to other close-packed metals)	often low
Liquid electrical conductivity	metallic	most exhibit metallic conductivity in liquid form	non-metallic
Band structure	metallic (Bi = semimetallic)	are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms	semiconductor or insulator
Electron behaviour	"free" electrons	• valence electrons not as freely delocalized as in metals; considerable covalent bonding present • have Goldhammer-Herzfeld Karl Herzfeld Karl Ferdinand Herzfeld was an Austrian-American physicist.-Education:...  criterion ratios straddling unity	no "free" electrons

Chemical

Property	Metals	Metalloids	Non-metals
General behaviour	metallic	non-metallic	non-metallic
Ionization energy	relatively low	intermediate ionization energies, usually falling between those of metals and nonmetals	high
Electronegativity	low	have electronegativity values close to 2 (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)	high
Ion formation	tend to form cations	• have a reduced tendency to form anions in water, when compared to ordinary nonmetals • solution chemistry is dominated by the formation and reactions of oxyanions	tend to form anions
Bonds	seldom form covalent	can form salts as well as covalent compounds	form many covalent
Oxidation number	nearly always positive	positive or negative	positive or negative
+Metals	give alloys	can form alloys	ionic or interstitial compounds formed
Oxides	• lower oxides are ionic and basic • higher oxides are increasingly covalent and acidic • very few glass formers	• polymeric in structure; tend to be amphoteric or weakly acidic • are glass formers (B, Si, Ge, As, Sb, Te)	• covalent, acidic • few glass formers (P, S, Se)
Halides, esp. chlorides	• ionic • water soluble (not hydrolysed)	• covalent, volatile • some partly reversibly hydrolysed	• covalent • hydrolysed by water
Hydrides	• active metals form ionic, solid hydrides with high melting points; • transition metals form metallic hydrides; • poor metals form covalent hydrides	covalent, volatile hydrides	covalent, gaseous or liquid hydrides
Organometallic compounds	many form such	can form	not formed

Distinctive

Of the above physical

Physical property

A physical property is any property that is measurable whose value describes a physical system's state. The changes in the physical properties of a system can be used to describe its transformations ....

 and chemical

Chemical property

A chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity...

 properties, brittleness or semiconductivity or both have been cited or used as singularly distinguishing indicators of metalloid status. Metallic lustre together with very marked dualistic chemical behaviour—by way of, for example, amphoteric oxides

Amphoterism

In chemistry, an amphoteric species is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word amphoteroi meaning "both"...

—has also been cited as a benchmark criterion.

Although metalloids are all reckoned to be solid as well as showing metallic lustre, their other properties vary from element to element. Noting metallic character is a combination of several properties, Hawkes suggests judging metalloid status separately for each element, based on the extent to which they exhibit the properties relevant to such status.

The concepts of metalloid and semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 should not be confused. 'Metalloid' is chemistry-based concept referring to the physical (including electronic) and chemical properties of certain elements in relation to the periodic table

Periodic table

The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

. 'Semiconductor' is a physics-based concept referring to the electronic properties of materials (including elements and compounds). Not all elements classified in the literature as metalloids necessarily exhibit semiconductivity, although most do.

Variability

There is no universally agreed or rigorous definition of the term metalloid. Accordingly, the answer to the question "Which elements are metalloids?" can vary, depending on the author and their inclusion criteria. Emsley, for example, recognised only four metalloids: germanium, arsenic, antimony and tellurium. Selwood, on the other hand, listed twelve: boron, aluminium, silicon, gallium, germanium, arsenic, tin, antimony, tellurium, bismuth, polonium, and astatine.

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

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

Common metalloids

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

• Boron
  Boron
  Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
  
  
• Silicon
  Silicon
  Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
  
  
• Germanium
  Germanium
  Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
  
  
• Arsenic
  Arsenic
  Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
  
  
• Antimony
  Antimony
  Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
  
  
• Tellurium

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

Selenium, polonium and astatine

Selenium shows borderline metalloid or non-metal behaviour.

Its most stable form, the grey trigonal allotrope, is sometimes called 'metallic' selenium since its electrical conductivity is several orders of magnitude greater than that of the red monoclinic form. The metallic character of selenium is further indicated by its lustre; its crystalline structure, which is thought to include weakly 'metallic' interchain bonding; its capacity, when molten, to be drawn into thin threads; its reluctance to acquire 'the high positive oxidation numbers characteristic of nonmetals'; and the existence of a hydrolysed cationic salt in the form of trihydroxoselenium (IV) perchlorate Se(OH)₃⁺ClO₄^–.

The non-metallic character of selenium is indicated by its brittleness; its band structure, which is that of a semiconductor; its low electrical conductivity which, at ~10⁻⁹ to 10⁻¹² S·cm⁻¹ when highly purified, is comparable to or less than that of bromine (7.95 S·cm⁻¹), a nonmetal; its relatively high electronegativity (2.55 revised Pauling); the retention of its semiconducting properties in liquid form; and its reaction chemistry, which is mainly that of its nonmetallic anionic forms Se^2–, SeO and SeO, although it shares with sulfur and tellurium the capacity to form cyclic polycations (such as Se) when dissolved in oleum

Oleum

Oleum , or fuming sulfuric acid refers to a solution of various compositions of sulfur trioxide in sulfuric acid or sometimes more specifically to disulfuric acid ....

s.

Polonium is 'distinctly metallic' in some ways, as indicated by the many salts it forms, the presence of the rose-coloured Po²⁺ cation in aqueous solution, and the metallic conductivity of both of its allotropic forms. However, it also shows nonmetallic character by forming numerous metal polonides containing the Po^2– anion.

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

In terms of non-metallic indicators:

• Batsanov gives a calculated band gap energy of 0.7 eV, this being consistent with nonmetals (in physics) having separated valence and conduction bands and thereby being either semiconductors or insulators;
• it has the narrow liquid range ordinarily associated with non-metals, given its estimated melting point of 575 K and estimated boiling point of 610 K;
• its chemistry in aqueous solution is predominately characterised by the formation of various anionic species; and
• most of its known compounds, which include astatides (XAt), astatates (XAtO₃), and monovalent interhalogen compounds, are analogous to those of iodine, which is a halogen and a nonmetal.

In terms of metallic indicators:

• Samsonov observes that, '[L]ike typical metals, it is precipitated by hydrogen sulfide even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the cathode on electrolysis.'
• Rossler cites further indications of a tendency for astatine to behave like a (heavy) metal as: '…the formation of pseudohalide compounds…complexes of astatine cations…complex anions of trivalent astatine…as well as complexes with a variety of organic solvents.'
• Rao and Ganguly note that elements with an enthalpy of vaporization (EoV) greater than ~42 kJ/mol are metallic in the liquid state. Such elements include boron, silicon, germanium, antimony, selenium and tellurium. Vásaros & Berei give estimated values for the EoV of diatomic astatine, the lowest of these being 50 kJ/mol. On this basis astatine may also be metallic in the liquid state. Diatomic iodine, with an EoV of 41.71 falls just short of the threshold figure.
• Champion et al. argue that astatine demonstrates cationic behaviour, in strongly acidic aqueous solutions, by way of the existence of stable At⁺ and AtO⁺ forms.

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

Semi-quantitative characterization

	Element	IE	EN	Band structure
	Boron	191	2.04	semiconductor
	Silicon	187	1.90	same
	Germanium	182	2.01	same
	Arsenic	225	2.18	semimetal
	Antimony	198	2.05	same
	Tellurium	207	2.10	semiconductor
	average	198	2.05
	The common metalloids, and their ionization energies (kcal/mol); electronegativities (revised Pauling); and electronic band structures (most thermodynamically stable forms under ambient conditions).

Metalloids tend to be collectively characterized in terms of generalities or a few broadly indicative physical or chemical properties. A single quantitative criterion is also occasionally mentioned.

In a somewhat more specific treatment, Masterton and Slowinski wrote that metalloids have ionization energies clustering around 200 kcal/mol, and electronegativity values close to 2.0, and that they are typically semiconductors, 'although antimony and arsenic [being semimetals in the physics-based sense] have electrical conductivities which approach those of metals.'

Their description, in terms of these three more or less clearly defined properties, encompasses the six common metalloids (see table, right).

Selenium and polonium are probably excluded from this scheme; astatine may or may not be included.

In other quantitative terms, the common metalloids show packing efficiencies

Atomic packing factor

In crystallography, atomic packing factor or packing fraction is the fraction of volume in a crystal structure that is occupied by atoms. It is dimensionless and always less than unity. For practical purposes, the APF of a crystal structure is determined by assuming that atoms are rigid spheres...

 of between 34 to 41 per cent (boron 38; silicon and germanium 34; arsenic 38.5; antimony 41; tellurium 36.4). These values are lower than those of most metals, more than 80 per cent of which have a packing efficiency of at least 68 per cent, but higher than those of elements ostensibly classified as non-metals, such as graphite (17 per cent), sulphur (19.2), iodine (23.9), selenium (24.2), and black phosphorus (28.5).

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

Other metalloids

The lack of an agreed definition of a metalloid has meant that hydrogen, beryllium, carbon, nitrogen, aluminium, phosphorus, sulfur, zinc, gallium, tin, iodine, lead, bismuth and radon are occasionally classified as metalloids.

The term metalloid has also been used to refer to:

• elements that exhibit metallic lustre and electrical conductivity and that are also amphoteric
  Amphoterism
  In chemistry, an amphoteric species is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word amphoteroi meaning "both"...
  
  , such as arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead and aluminium;
• elements that are otherwise sometimes referred to as poor metals; and
• non-metallic elements (for example, nitrogen; carbon) that can form alloy
  Alloy
  An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
  
  s with, or modify the properties of, metals.

Aluminium

Aluminium is ordinarily classified as a metal, given its lustre, malleability and ductility, high electrical and thermal conductivity and close-packed crystalline structure.

It does however have some properties that are unusual for a metal and, taken together, these are sometimes used as a basis to classify aluminium as a metalloid:

• its crystalline structure shows some evidence of directional bonding
• although it forms an Al³⁺ cation in some compounds, it bonds covalently in most others
• its oxide is amphoteric, and a conditional glass-former
• it forms anionic aluminate
  Aluminate
  An aluminate is a compound containing aluminium and oxygen. When precipitated from solution, the anion condenses with more electropositive elements. The generally accepted form of the aluminate is a mononuclear tetrahedral complex that is negatively charged, Al4− or AlO2−...
  
  s, such behaviour being considered non-metallic in character.

Stott labels aluminium as weak metal, having the physical properties of a good metal but some of the chemical properties of a non-metal. Steele notes the somewhat paradoxical chemical behaviour of aluminium: it resembles a weak metal with its amphoteric oxide and the covalent character of many of its compounds yet it is also a strongly electropositive metal, with a high negative electrode potential.

The notion of aluminium as a metalloid is sometimes disputed on account of its many metallic properties and to emphasize that it represents an exception to the mnemonic that elements adjacent to the metal-nonmetal dividing line are metalloids.

Near metalloids

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

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

In 1954, Szabó & Lakatos included beryllium and aluminium in their list of metalloids, together with boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine.

In 1957, Sanderson included carbon, phosphorus, selenium, and iodine as part of an intermediary class of elements with 'certain metallic properties', alongside boron, silicon, arsenic, tellurium, and astatine. Germanium, antimony and polonium were counted as metals.

More recently, in 2007, Petty included carbon, phosphorus, selenium, tin and bismuth in his list of metalloids, as well as boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine.

Elements such as these, which are in the proximity of the common metalloids, and otherwise ordinarily classified as either metals or non-metals, are occasionally called, or described as, near-metalloids, or the like.

Metals falling into this loose category—aluminium, tin and bismuth, for example—tend to show 'odd' packing structures, marked covalent chemistry (molecular or polymeric), and amphoteric behaviour. They are also referred to as (chemically) weak metals, poor metals, post-transition metals,

Post-transition metal

In chemistry, the term post-transition metal is used to describe the category of metallic elements to the right of the transition elements on the periodic table...

or semimetals (in the aforementioned sense of metals with incomplete metallic character), classification groupings that generally cohabit the same periodic table territory but which are not necessarily mutually inclusive.

Nonmetals in this category, including carbon, phosphorus, selenium and iodine, exhibit metallic lustre, semiconducting properties (for example, intermediate electrical conductivity; a relatively narrow band gap; and light sensitivity) and bonding or valence bands with delocalized character, in their most thermodynamically stable forms under ambient conditions (carbon as graphite; phosphorus as black phosphorus; selenium as grey selenium). These elements are alternatively described as being 'near metalloidal', showing metalloidal character, or having metalloid-like or some metalloid(al) or metallic properties.

Allotropes

Some allotropes of the elements exhibit more pronounced metallic, metalloidal or non-metallic behavior than others. For example, the diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

 allotrope of carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 is clearly non-metallic, but the graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 allotrope displays limited electrical conductivity more characteristic of a metalloid. Phosphorus

Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

, selenium

Selenium

Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

, tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

, and bismuth

Bismuth

Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

 also have allotropes that display borderline or either metallic or non-metallic behavior.

Location and identification

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

This line has been called the metal-nonmetal line, the metalloid line, the semimetal line, the Zintl border or the Zintl line. The latter two terms also refer to a vertical line

Zintl phase

In chemistry a Zintl phase is the product of a reaction between*group 1 or group 2 and*post transition metals or metalloids from group 13, 14, 15 or 16....

 sometimes drawn between groups 13 and 14, which was christened by Laves

Fritz Laves

-References:...

 in 1941, and used to differentiate intermetallic compounds

Intermetallics

Intermetallics or intermetallic compounds is a term that is used in a number of different ways. Most commonly it refers to solid-state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term...

 generally formed by group 13 elements with electropositive metals, from the salt-like compounds usually formed by elements in and to the right of group 14.

References to the concept of such a dividing line between metals and non-metals appear in the literature as far back as at least 1869.

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

In 1906, Alexander Smith

Alexander Smith (chemist)

Alexander Smith was an American chemist, born in Edinburgh, Scotland. He graduated from the University of Edinburgh in 1886 and received the degree of Ph.D. at Munich in 1889. After coming to the United States, Smith was professor of chemistry and mineralogy at Wabash College and later a faculty...

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

In 1923, Horace Groves Deming, an American chemist, published short (Mendeleev style) and medium (18-column) form periodic tables each of which each included a regular stepped line separating metals from non-metals, in his textbook General Chemistry: An elementary survey. Merck and Company prepared a handout form of Deming's 18-column table, in 1928, which was widely circulated in American schools and by the 1930s his table was appearing in handbooks and encyclopaedias of chemistry. It was also distributed for many years by the Sargent-Welch Scientific Company.

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

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

A dividing line between metals and nonmetals is sometimes replaced by two dividing lines: one between metals and metalloids; the second between metalloids and nonmetals.

Some periodic tables distinguish elements that are metalloids in the absence of any formal dividing line between metals and non-metals. Metalloids are instead shown as occurring in a diagonal fixed band or diffuse region, running from upper left to lower right and centred around arsenic.

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

Several other sources note confusion or ambiguity as to the location of the dividing line; suggest its apparent arbitrariness provides grounds for refuting its validity; and comment as to its misleading, contentious or approximate nature. Deming himself noted that the line could not be drawn very accurately.

Typical applications

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

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

Typical applications of the common metalloids have instead encompassed: use of their oxides as glass-formers; their inclusion as alloying components or additives; and their employment as semiconductors, dopants or semiconductor constituents.

Glass formation

The oxides B₂O₃, SiO₂, GeO₂, As₂O₃ and Sb₂O₃ readily form glasses; TeO₂ will also form a glass but to do so requires either a 'heroic quench rate' (to avoid the formation of the crystalline form) or the addition of an impurity. These compounds have found or continue to find practical uses in chemical, domestic and industrial glassware and optics (especially Ge and Te).

Alloys

In 1914 Desch wrote that 'certain non-metallic elements are capable of forming compounds of distinctly metallic character with metals, and these elements may therefore enter into the composition of alloys'. He associated silicon, arsenic and tellurium, in particular, with the alloy-forming elements. Phillips and Williams later noted that compounds of silicon, germanium, arsenic and antimony with the poor metals, 'are probably best classed as alloys'.

Boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

 can form intermetallic compounds

Intermetallics

Intermetallics or intermetallic compounds is a term that is used in a number of different ways. Most commonly it refers to solid-state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term...

 and alloys with transition metal

Transition metal

The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...

s, of the composition M_nB, if n > 2.

Sanderson commented that silicon

Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 'is metalloid in nature, appearing quite metallic in its ability to alloy with metals.'

Germanium

Germanium

Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

 forms a considerable number of alloys, most importantly with the coinage metals

Group 11 element

A Group 11 element is one in the series of elements in group 11 in the periodic table, consisting of transition metals which are the traditional coinage metals of copper , silver , and gold...

.

Arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 can form alloys with metals, including platinum and copper.

Antimony

Antimony

Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

 is well known as an alloy former, as exemplified by type metal

Type metal

In printing, type metal refers to the metal alloys used in traditional typefounding and hot metal typesetting. Lead is the main constituent of these alloys...

 (a lead alloy with up to 25 per cent, by weight, antimony) and pewter

Pewter

Pewter is a malleable metal alloy, traditionally 85–99% tin, with the remainder consisting of copper, antimony, bismuth and lead. Copper and antimony act as hardeners while lead is common in the lower grades of pewter, which have a bluish tint. It has a low melting point, around 170–230 °C ,...

 (a tin alloy with up to 20 per cent antimony).

In 1973 the US Geological Survey reported that about 18 percent of tellurium production was sold as copper tellurium alloys (40‒50 percent tellurium) and ferrotellurium (50‒58 percent tellurium).

Semiconductors and electronics

All the common metalloids or their compounds have found application in the semiconductor or solid-state electronic industries. The relative difficulty of obtaining single crystals of boron, combined with its high melting point, and the difficulty of introducing and retaining controlled impurities, have retarded its use as a semiconductor.

Pre-1800

Ancient conceptions of metals as solid, fusible and malleable substances can be found in Plato's

Plato

Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 Timaeus

Timaeus (dialogue)

Timaeus is one of Plato's dialogues, mostly in the form of a long monologue given by the title character, written circa 360 BC. The work puts forward speculation on the nature of the physical world and human beings. It is followed by the dialogue Critias.Speakers of the dialogue are Socrates,...

 (c. 360 BCE) and Aristotle’s

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 Meteorology

Meteorology (Aristotle)

Meteorology is a treatise by Aristotle which contains his theories about the earth sciences. These include early accounts of water evaporation, weather phenomena, and earthquakes....

.

At an early date, attempts were made by Pseudo-Geber

Pseudo-Geber

Pseudo-Geber is the name assigned by modern scholars to an anonymous European alchemist born in the 13th century, sometimes identified with Paul of Taranto, who wrote books on alchemy and metallurgy, in Latin, under the pen name of "Geber". "Geber" is the shortened and Latinised form of the name...

 (c. 1310), Basil Valentine (Conclusiones), Paracelsus

Paracelsus

Paracelsus was a German-Swiss Renaissance physician, botanist, alchemist, astrologer, and general occultist....

 (1539?), and Boerhaave (Elementa Chemiæ, 1733) to adopt a system of classification which would separate the more characteristic metals from substances possessing those characteristics to a lesser degree, such as zinc, antimony, bismuth, stibnite, pyrite and galena, all of the latter then being called semi-metals or bastard metals.

In 1735 Brandt proposed to make the presence or absence of malleability the principle of this classification and on that basis he separated mercury from the metals. The same view was adopted by Vogel (1755, Institutiones Chemiæ) and Buffon

Georges-Louis Leclerc, Comte de Buffon

Georges-Louis Leclerc, Comte de Buffon was a French naturalist, mathematician, cosmologist, and encyclopedic author.His works influenced the next two generations of naturalists, including Jean-Baptiste Lamarck and Georges Cuvier...

 (1785, Histoire naturelle des Minéraux). Subsequently, when Braun had observed the solification of mercury by cold in 1759–60, and this had been confirmed by Hutchins and Cavendish

Henry Cavendish

Henry Cavendish FRS was a British scientist noted for his discovery of hydrogen or what he called "inflammable air". He described the density of inflammable air, which formed water on combustion, in a 1766 paper "On Factitious Airs". Antoine Lavoisier later reproduced Cavendish's experiment and...

 in 1783, the malleability of mercury became known, and it was included amongst the metals.

The insufficiency of the distinction which had been drawn between metals and semi-metals was pointed out by Fourcroy (1789, Eleméns d’Histoire Naturalle et de Chemie, ii. 380) as being evident from the fact that

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

This concept of a semi-metal, as a brittle (and thereby imperfect) metal, was gradually discarded following the publication, in 1789, of Lavoisier's 'revolutionary' Elementary Treatise on Chemistry.

1800–1950s

In 1807, possibly '[in] an attempt to revive this old distinction between metals and substances resembling metals', Erman and Simon suggested using the term metalloid to refer to the newly discovered elements sodium and potassium, since these were lighter than water and for that reason many chemists did not regard them as proper metals. Their suggestion was ignored by the chemical community.

In 1811 or 1812, Berzelius

Berzelius

Berzelius is a secret society at Yale University named for the Swedish scientist Jöns Jakob Berzelius, considered one of the founding fathers of modern chemistry...

 referred to non-metallic elements as metalloids, in reference to their ability to form oxyanions (such as sulfur, in the form of the sulfate ion,
SO, a property likewise exhibited by many of the metals, such as chromium, by way of the chromate ion, CrO). The terminology of Berzelius was widely adopted although it was subsequently regarded by some commentators as counterintuitive, misapplied, incorrect or invalid.

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

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

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

In 1864, use of the term metalloid for non-metals was still sanctioned 'by the best authorities' although its usage as such did not always seem appropriate and the greater propriety of its application to other elements, such as arsenic, had been considered.

By as early as 1866 some authors were instead using the term non-metal, rather than metalloid, to refer to non-metallic elements.

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

As late as 1888 the division of the elements into metals, metalloids, and non-metals, rather than metals and metalloids, was still considered to be peculiar and a potential source of confusion.

Beach, writing in 1911, explained it this way:

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

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

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

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

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

In 1947, Pauling

Linus Pauling

Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...

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

In 1959 the International Union of Pure and Applied Chemistry

International Union of Pure and Applied Chemistry

The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

 (IUPAC) recommended that '[t]he word metalloid should not be used to denote non-metals' even though it was still being used in this sense (around that time) by, for example, the French.

1960–

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

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

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

Monographs

• Chedd G 1969, Half-way elements: The technology of metalloids, Doubleday, New York
• Dunstan S 1968, 'The metalloids', in Principles of chemistry, D. Van Nostrand Company, London, pp. 407–439
• Goldsmith RH 1982, 'Metalloids', Journal of Chemical Education, vol. 59, no. 6, pp. 526–527, Bibcode
  Bibcode
  The bibcode is an identifier used by a number of astronomical data systems to specify literature references. The bibcode was developed to be used in SIMBAD and the NASA/IPAC Extragalactic Database , but is now used more widely, for example, in the NASA Astrophysics Data System...
  
   1982JChEd..59..526G, doi
  Digital object identifier
  A digital object identifier is a character string used to uniquely identify an object such as an electronic document. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found...
  
   10.1021/ed059p526
• Hawkes SJ 2001, 'Semimetallicity', Journal of Chemical Education, vol. 78, no. 12, pp. 1686–87, Bibcode
  Bibcode
  The bibcode is an identifier used by a number of astronomical data systems to specify literature references. The bibcode was developed to be used in SIMBAD and the NASA/IPAC Extragalactic Database , but is now used more widely, for example, in the NASA Astrophysics Data System...
  
   2001JChEd..78.1686H, doi
  Digital object identifier
  A digital object identifier is a character string used to uniquely identify an object such as an electronic document. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found...
  
  10.1021/ed078p1686
• Rochow EG 1966, The metalloids, DC Heath and Company, Boston