Metalloid


Metalloid
  13 14 15 16 17  
2  B
Boron
C
Carbon
N
Nitrogen
O
Oxygen
F
Fluorine
3  Al*
Aluminium
Si
Silicon
P
Phosphorus
S
Sulfur
Cl
Chlorine
4  Ga
Gallium
Ge
Germanium
As
Arsenic
Se
Selenium
Br
Bromine
5  In
Indium
Sn
Tin
Sb
Antimony
Te
Tellurium
I
Iodine
6  Tl
Thallium
Pb
Lead
Bi
Bismuth
Po*
Polonium
At*
Astatine
 Common *The metalloid status of Al, Po and At is disputed.
 Less common
 Uncommon
 Rare
Indicative (relative) frequency with which some elements appear in metalloid lists. Frequencies are from the list of metalloid lists and occur in a more or less geometric progression of clusters. The common elements 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.[1]

Metalloid is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, each element can usually be classified as a metal or a nonmetal. However, some elements with intermediate[2] or mixed[3] properties can be harder to characterize.[4][5][n 1] These elements are sometimes classified as metalloids, from the Latin metallum = "metal" and the Greek oeides = "resembling in form or appearance".[9][10] They have been described as forming a (fuzzy) buffer zone between metals and nonmetals, the make-up and size of which depends on the applicable classification criteria.[n 2]

The terms amphoteric element,[20][21] half-metal,[22][23] half-way element,[24] near metal,[25] semiconductor[26] and semimetal[27] 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 metals capable of forming oxyanions, such as chromium and manganese;[28]
  • 'half-metal' is sometimes instead used to refer to the poor metals;[29] it also has an unrelated meaning, 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,[30][31][32] ytterbium,[33][34][35] bismuth[36] and neptunium.[37][38]

In addition, some elements otherwise referred to as metalloids are not known to exhibit marked amphoteric behaviour, or exhibit semiconductivity in their most stable forms.

Metalloids are generally regarded as a third classification of chemical elements, alongside metals and nonmetals.[39] On some occasions they have instead been grouped with the metals,[25][40] regarded as nonmetals[41] or treated as a sub-category of same.[42][43][44][45][46][n 3]

Contents

Properties

There is no universally agreed or rigorous definition of the term[48][49] metalloid and the classification of any particular element as such has been described as 'arbitrary'.[50]

Notwithstanding, properties associated with metalloids are set out in the following two tables, alongside (and in comparison to) those of metals and non-metals.[51] 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)[52][53] solid[54] mostly gases[55]
Appearance characteristic lustre metallic lustre[54] colourless; red, yellow, green, black, or intermediate shades[56]
Allotropy many show metallic allotropes; Bi, Sn have semiconducting allotropes tend to exist in several (conspicuously)[57] 'metallic' and non-metallic allotropic forms[58] 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[59] densities lower than neighbouring poor metals but higher than those of neighbouring nonmetals[46] often low
Elasticity typically elastic, ductile, malleable (when solid) brittle[60] brittle (when solid)
Electrical conductivity good to high[n 4] intermediate[63] to good[n 5] poor to intermediate[n 6]
Thermal conductivity medium to high[71] mostly intermediate;[60][72] Si is high almost negligible[73] to very high[74]
Packing close-packed crystal structures; high coordination numbers have relatively open crystal structures, with medium coordination numbers,[75] in contrast to the close-packed crystal structures of metals[76] low coordination numbers
Melting behaviour volume generally expands[77] some contract, unlike (most)[78] metals[79] volume generally expands[77]
Enthalpy of fusion may be high often have abnormally high enthalpy of fusion values[80] (compared to other close-packed metals)[81] often low
Liquid electrical conductivity[82] metallic most exhibit metallic conductivity in liquid form[83][84] non-metallic
Band structure metallic (Bi = semimetallic) are semiconductors or, if not (As, Sb = semimetallic), exist in semiconducting forms[44][85] semiconductor or insulator[86]
Electron behaviour "free" electrons valence electrons not as freely delocalized as in metals; considerable covalent bonding present[87]
have Goldhammer-Herzfeld criterion[n 7] ratios straddling unity[83][92]
no "free" electrons

Chemical

Property Metals Metalloids Non-metals
General behaviour metallic non-metallic[93] non-metallic
Ionization energy relatively low intermediate ionization energies,[94] usually falling between those of metals and nonmetals[95] high
Electronegativity low have electronegativity values close to 2[96] (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)[97] high
Ion formation tend to form cations have a reduced tendency to form anions in water, when compared to ordinary nonmetals[98]
solution chemistry is dominated by the formation and reactions of oxyanions[99][100]
tend to form anions
Bonds seldom form covalent can form salts as well as covalent compounds[101] form many covalent
Oxidation number nearly always positive positive or negative[102] positive or negative
+Metals give alloys can form alloys[58][101][103] ionic or interstitial compounds formed
Oxides lower oxides are ionic and basic
higher oxides are increasingly covalent and acidic
very few glass formers[104]
polymeric in structure;[105] tend to be amphoteric or weakly acidic[54][106]
are glass formers (B, Si, Ge, As, Sb, Te)[107]
covalent, acidic
few glass formers (P, S, Se)[108]
Halides, esp. chlorides ionic
water soluble (not hydrolysed)
covalent, volatile[109][110]
some partly reversibly hydrolysed[111]
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[112] covalent, gaseous or liquid hydrides
Organometallic compounds many form such can form[113] not formed

Distinctive

Of the above physical and chemical properties, brittleness[114][115] or semiconductivity[116] or both[117] 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—has also been cited as a benchmark criterion.[118]

Although metalloids are all reckoned to be solid[119] as well as showing metallic lustre, their other properties vary from element to element.[120] Noting metallic character is a combination of several properties, Hawkes[49] 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 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. 'Semiconductor' is a physics-based concept referring to the electronic properties of materials (including elements and compounds).[121] Not all elements classified in the literature as metalloids necessarily exhibit semiconductivity, although most do.[122]

Applicable elements

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,[123] for example, recognised only four metalloids: germanium, arsenic, antimony and tellurium. Selwood,[124] 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.[125]

In any event, individual metalloid classification arrangements tend to share common ground, with most variations occurring around the (indistinct)[126][127] margins.

Common metalloids

Consistent with the list of metalloid lists, the following elements are commonly classified as metalloids:[48][49][128][129][130][131]

One or more from among selenium, polonium or astatine are sometimes added to the list.[49][132][133] Boron is sometimes excluded from the list, by itself or together with silicon.[134][135] Tellurium is sometimes not regarded as a metalloid;[136] the inclusion of antimony, polonium and astatine as metalloids has also been questioned.[49][137][138]

Selenium, polonium and astatine

Selenium shows borderline metalloid or non-metal behaviour.[139][140][n 8]

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.[143] The metallic character of selenium is further indicated by its lustre;[144] its crystalline structure, which is thought to include weakly 'metallic' interchain bonding;[145] its capacity, when molten, to be drawn into thin threads;[146] its reluctance to acquire 'the high positive oxidation numbers characteristic of nonmetals';[147] and the existence of a hydrolysed cationic salt in the form of trihydroxoselenium (IV) perchlorate Se(OH)3+ClO4.[148][149]

The non-metallic character of selenium is indicated by its brittleness;[144] its band structure, which is that of a semiconductor;[149] its low electrical conductivity which, at ~10−9 to 10−12 S·cm−1 when highly purified,[67][68][150] is comparable to or less than that of bromine (7.95×10–12 S·cm−1),[151] a nonmetal; its relatively high[152] electronegativity (2.55 revised Pauling); the retention of its semiconducting properties in liquid form;[149] and its reaction chemistry, which is mainly that of its nonmetallic anionic forms Se2–, SeO2−
3
and SeO2−
4
,[153] although it shares with sulfur and tellurium the capacity to form cyclic polycations (such as Se2+
8
) when dissolved in oleums.[154]

Polonium is 'distinctly metallic' in some ways,[155] as indicated by the many salts it forms,[156] the presence of the rose-coloured Po2+ cation in aqueous solution,[157] and the metallic conductivity of both of its allotropic forms.[155] However, it also shows nonmetallic character by forming numerous metal polonides containing the Po2– anion.[158]

Astatine may be a non-metal or a metalloid;[159] it is ordinarily classified as a non-metal,[137][138][160][161] but has some 'marked' metallic properties.[162] Immediately following its production in 1940, early investigators considered it to be a metal.[163] 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,[164] and in 1950 as being a halogen and (therefore) an active non-metal.[165]

In terms of non-metallic indicators:

  • Batsanov gives a calculated band gap energy of 0.7 eV,[166] this being consistent with nonmetals (in physics) having separated valence and conduction bands and thereby being either semiconductors or insulators;[86][167]
  • it has the narrow liquid range ordinarily associated with non-metals,[168] 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;[169] and
  • most of its known compounds, which include astatides (XAt), astatates (XAtO3), and monovalent interhalogen compounds, are analogous to those of iodine,[160] which is a halogen and a nonmetal.[170][171]

In terms of metallic indicators:

  • Samsonov[172] 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[173] 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[82] note that elements with an enthalpy of vaporization (EoV) greater than ~42 kJ/mol are metallic in the liquid state. Such elements include boron,[n 9] silicon, germanium, antimony, selenium and tellurium. Vásaros & Berei[177] 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[178] falls just short of the threshold figure.
  • Champion et al.[179] 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;[180] a visible piece of astatine would be immediately and completely vaporized due to the heat generated by its intense radioactivity.[181]

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);[182] electronegativities (revised Pauling); and electronic band structures[183][184] (most thermodynamically stable forms under ambient conditions).

Masterton and Slowinski[185] 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.[n 10]

In other quantitative terms, the common metalloids show packing efficiencies of between 34 to 41 per cent (boron 38; silicon and germanium 34; arsenic 38.5; antimony 41; tellurium 36.4).[189][190][191] 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,[192][n 11] but higher than those of elements ostensibly classified as non-metals, such as graphite (17 per cent),[195] sulphur (19.2),[196] iodine (23.9),[196] selenium (24.2),[196] and black phosphorus (28.5).[191]

The common metalloids also have Goldhammer-Herzfeld criterion ratios of between ~0.85 to 1.1 (average 1.0).[91][92]

Other metalloids

The lack of an agreed definition of a metalloid has meant that hydrogen,[197][198][199] beryllium,[200] carbon,[201][202][203] nitrogen,[204] aluminium,[205][206] phosphorus,[203][207] sulfur,[203][208][209] zinc,[210] gallium,[211] tin, iodine,[204][212] lead,[213] bismuth[136] and radon[214][215][216] are occasionally classified as metalloids.[217]

The term metalloid has also been used to refer to:

  • elements that exhibit metallic lustre and electrical conductivity and that are also amphoteric, such as arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin, lead and aluminium;[218]
  • elements that are otherwise sometimes referred to as poor metals;[219] and
  • non-metallic elements (for example, nitrogen; carbon) that can form alloys with,[220][221][222] or modify the properties of,[223] 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,[224] these are sometimes used as a basis to classify aluminium as a metalloid:

  • its crystalline structure shows some evidence of directional bonding[225][226][227]
  • although it forms an Al3+ cation in some compounds, it bonds covalently in most others[228][229][230]
  • its oxide is amphoteric, and a conditional glass-former[108]
  • it forms anionic aluminates,[224] such behaviour being considered non-metallic in character.[231]

Stott[232] labels aluminium as weak metal, having the physical properties of a good metal but some of the chemical properties of a non-metal. Steele[233] 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[234][235][236] 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.[138][n 12]

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[238] 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.[1]

In 1954, Szabó & Lakatos[239] included beryllium and aluminium in their list of metalloids, together with boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine.

In 1957, Sanderson[240][n 13] 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[244] 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,[245][246] or the like.

Metals falling into this loose category—aluminium, tin and bismuth, for example—tend to show 'odd' packing structures,[247] marked covalent chemistry (molecular or polymeric),[248] and amphoteric behaviour.[249][250] They are also referred to as (chemically) weak metals,[251][252] poor metals,[253][254] post-transition metals,[255][256][n 14] 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,[257][258] phosphorus,[259][260][261][262][263] selenium[140][264] and iodine,[265][266][267] exhibit metallic lustre, semiconducting properties (for example, intermediate electrical conductivity;[268] a relatively narrow band gap;[269][270] and light sensitivity[268]) and bonding or valence bands with delocalized character, in their most thermodynamically stable forms under ambient conditions (carbon as graphite; phosphorus as black phosphorus;[n 15] 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 allotrope of carbon is clearly non-metallic, but the graphite allotrope displays limited electrical conductivity more characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth 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.[274]

This line has been called the metal-nonmetal line,[275] the metalloid line,[276][277] the semimetal line,[278] the Zintl border [279] or the Zintl line.[280][281][n 16] The latter two terms also refer to a vertical line sometimes drawn between groups 13 and 14, which was christened by Laves in 1941,[283] and used to differentiate intermetallic compounds 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.[284]

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.[285]

In 1891, Walker published a periodic 'tabulation' with a diagonal straight line drawn between the metals and the non-metals.[286]

In 1906, Alexander Smith included a periodic table with a zigzag line separating the nonmetals from the rest of elements, in his highly influential[287] textbook, Introduction to General Inorganic Chemistry.[288]

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.[289][n 17] 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.[290][291][292]

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'.[231] A binary classification can also facilitate the establishment of some simple rules for determining bond types between metals and/or nonmetals.[39]

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'.[293]

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.[293][294]

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[295] or diffuse region,[296] 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.'[297]

Several other sources note confusion or ambiguity as to the location of the dividing line;[298][299] suggest its apparent arbitrariness[300] provides grounds for refuting its validity;[39] and comment as to its misleading, contentious or approximate nature.[49][301][302] Deming himself noted that the line could not be drawn very accurately.[303]

Typical applications

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

Common metalloids, such as arsenic and antimony,[304] 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.[305] These compounds have found or continue to find practical uses in chemical, domestic and industrial glassware[306][307] and optics (especially Ge and Te).[308][309]

Alloys

In 1914 Desch[310] 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[311] later noted that compounds of silicon, germanium, arsenic and antimony with the poor metals, 'are probably best classed as alloys'.

Boron can form intermetallic compounds and alloys with transition metals, of the composition MnB, if n > 2.[312]

Sanderson commented that silicon 'is metalloid in nature, appearing quite metallic in its ability to alloy with metals.'[313]

Germanium forms a considerable number of alloys, most importantly with the coinage metals.[314]

Arsenic can form alloys with metals, including platinum and copper.[315]

Antimony is well known as an alloy former, as exemplified by type metal (a lead alloy with up to 25 per cent, by weight, antimony) and pewter (a tin alloy with up to 20 per cent antimony).[316]

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).[317]

Semiconductors and electronics

All the common metalloids or their compounds have found application in the semiconductor or solid-state electronic industries.[318][319] 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.[320][321]

Nomenclature origin and usage

Pre-1800

Ancient conceptions of metals as solid, fusible and malleable substances can be found in Plato's Timaeus (c. 360 BCE) and Aristotle’s Meteorology.[322][323]

At an early date, attempts were made by Pseudo-Geber (c. 1310), Basil Valentine[n 18] (Conclusiones), Paracelsus (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.[325][326]

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 (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 in 1783,[327] the malleability of mercury became known, and it was included amongst the metals.[325]

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)[328][329] metal, was gradually discarded following the publication, in 1789, of Lavoisier's 'revolutionary'[330] Elementary Treatise on Chemistry.[331]

1800–1950s

In 1807, possibly '[in] an attempt to revive this old distinction between metals and substances resembling metals',[332] 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.[48]

In 1811[48] or 1812, Berzelius 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, SO2−
4
, a property likewise exhibited by many of the metals, such as chromium, by way of the chromate ion, CrO2−
4
).[333][334] The terminology of Berzelius was widely adopted[48] although it was subsequently regarded by some commentators as counterintuitive,[334] misapplied,[331] incorrect[335] or invalid.[46]

In 1825, in a revised German edition of his Textbook of Chemistry,[336][337] 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).[338]

In 1844, Jackson[339] 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.[340]

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.[341]

By as early as 1866 some authors were instead using the term non-metal, rather than metalloid, to refer to non-metallic elements.[342]

In 1876, Tilden[343] 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.[344]

Beach, writing in 1911, explained it this way:[345]

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[346] 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[347] defined 'metalloid' as an old, obsolescent term for 'non-metal'[n 19] whereas in Webster's New International Dictionary[348] 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.[48]

In 1947, Pauling included a reference to metalloids in his classic[349] and influential[350] 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.'[351]

In 1959 the International Union of Pure and Applied Chemistry (IUPAC) recommended that '[t]he word metalloid should not be used to denote non-metals'[352] even though it was still being used in this sense (around that time) by, for example, the French.[212]

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.[212][353] Notwithstanding this recommendation, use of the term 'metalloid' increased dramatically.[48] 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;[354] 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'.[355] References to the term 'metalloid' as being outdated have also been described as 'nonsense' noting that 'it accurately describes these weird in-between elements'.[356]

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.[184][357] 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 3×1018 cm−3, 2 ×1020 cm−3, 5×1019 cm−3 and 3×1017 cm−3 respectively.[358] In contrast, the room-temperature concentration of electrons in metals usually exceeds 1022 cm−3.[359]

Notes

  1. ^ Not all elements with mixed or intermediate properties are necessarily hard to characterize. Gold, for example, has mixed properties but is still recognized as 'king of metals.' In addition to metallic behaviour (such as high electrical conductivity, and cation formation), gold also shows marked non-metallic behaviour in the form of the most positive electrode potential; an electronegativity of 2.54 (highest among the metals) that exceeds that of some non-metals (hydrogen 2.2; phosphorus 2.19; radon 2.2); the most negative electron affinity; and the highest ionization energy (but for zinc and mercury). It also forms the Au auride anion thereby behaving analogously to the halogens; and it sometimes has a tendency, known as 'aurophilicity', to bond to itself.[6] On halogen character, see also Belpassi et al.[7] who conclude that in the aurides MAu (M = Li–Cs) gold 'behaves as a halogen, intermediate between Br and I'. On aurophilicity, see also.[8]
  2. ^ On the fuzziness of metalloids see for example Rouvray;[11] Cobb & Fetterolf;[12] and Fellet.[13] For the 'buffer zone' terminology see Rochow.[14] For examples of the application of a single criterion to classify metalloids see Mahan and Myers,[15] who use electrical conductivity; Miessler and Tarr,[16] who use electronegativity; and Hutton and Dickerson,[17] who rely on the acid-base behaviour of group oxides. Kneen, Rogers and Simpson[18] further suggest the use of such individual criteria as the structure of the elements, or their reactions with acids. For an example of the use of multiple criteria see Masterton and Slowinski,[19] who characterize metalloids on the concurrent basis of ionization energy, electronegativity and electrical behaviour.
  3. ^ Oderberg[47] argues on ontological grounds that anything that is not a metal, is a non-metal and that this includes semi-metals (i.e. metalloids).
  4. ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[61][62]
  5. ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[64][65] If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.[66][67][68]
  6. ^ Non-metals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 103 in graphite.[69][70]
  7. ^ The Goldhammer-Herzfeld criterion is a measure of the ratio of the force holding an individual atom's valence electrons in place as compared to the forces, acting on the same electrons, arising from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than or equal to the atomic force, valence electron itinerancy is indicated and metallic behaviour is predicted;[88][89] otherwise non-metallic behaviour is anticipated. Although based on classical arguments[90] the Herzfeld criterion nevertheless offers a relatively simple first order rationalization for the occurrence of metallic character amongst the elements.[91]
  8. ^ Rochow (1957, p. 224),[141] who would later write his 1966 monograph The metalloids,[142] commented that, 'In some respects selenium acts like a metalloid and tellurium certainly does.'
  9. ^ The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.[174] found that liquid boron behaved like a metal; Glorieux et al [175] characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity; Millot et al.[176] reported that the emissivity of liquid boron was not consistent with that of a liquid metal.
  10. ^ Selenium has an IE of ~226 kcal/mol and is sometimes described as a semiconductor, but has a relatively high 2.55 EN. Polonium has an IE of ~196 kcal/mol and a 2.0 EN, but has a metallic band structure.[186][187] Astatine has an estimated IE of ~210±10 kcal/mol[188] and an EN of 2.2, but its electronic band structure is not known with any great degree of certainty.
  11. ^ Gallium is unusual (for a metal) in having a packing efficiency of just 39 per cent.[193] Other notable values are 42.9 for bismuth[191] and 58.5 for liquid mercury.[194]
  12. ^ A mnemonic which captures the common metalloids goes: Up, up-down, up-down, up…are the metalloids! [237]
  13. ^ Sanderson proposed a simple rule for distinguishing between metals and non-metals: 'With the single exception of hydrogen, all elements are metals if the number of electrons in the outermost shell of their atoms is equal to or less than the period number of the element (which is the same as the principal quantum number of that shell). Hydrogen and all other elements are nonmetals, but if the number of electrons in the outermost shell is one (or two) greater than their principal quantum number, they may show some metallic characteristics.' Radon was left out of his list of somewhat metallic elements despite its apparent eligibility (principle quantum number = 6; outermost shell electrons = 8); at that time, the noble gases were still considered to be incapable of forming chemical compounds. Following the synthesis of the first noble gas compound in 1962, references to cationic behaviour by radon appear from as early as 1969 (Stein 1969;[241] Pitzer 1975;[242] Schrobilgen 2011[243]).
  14. ^ Aluminium sometimes is[255] or is not[256] counted as a post-transition metal.
  15. ^ White phosphorus is the most common, industrially important,[271] and easily reproducible allotrope and, for those reasons, is the standard state of the element.[272] Paradoxically, it is also thermodynamically the least stable, as well as the most volatile and reactive form.[273]
  16. ^ Sacks[282] described the dividing line as, 'A jagged line, like Hadrian's Wall...[separating] the metals from the rest, with a few "semimetals," metallloids—arsenic, selenium—straddling the wall.'
  17. ^ The dividing line on the latter Mendeleev table starts off stepped as it travels past B, Si, P and As but then becomes serrated as it threads back up around Cr, back down past Se and Te, back up around Mn, then down past Br, I and a placeholder for eka-iodine.
  18. ^ Allegedly born c. 1394[324]
  19. ^ Couch also commented (p. 128) that there was, 'no sharp line of demarcation between metals and non-metals as many of the latter class possess some metallic properties' [italics added].

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  293. ^ a b Brown & Holme 2006, p. 57
  294. ^ Swenson 2005
  295. ^ Simple Memory Art c. 2005
  296. ^ Chedd 1969, pp. 12–13
  297. ^ Mendeléeff 1897, p. 23
  298. ^ Mackay & Mackay 1989, p. 24
  299. ^ Norman 1997, p. 31
  300. ^ Whitten, Davis & Peck 2003, p. 1140
  301. ^ Kotz, Treichel & Weaver 2005, pp. 79–80
  302. ^ Housecroft & Constable 2006, p. 322
  303. ^ Deming 1923, p. 381
  304. ^ Russell & Lee 2005, pp. 421, 423
  305. ^ Kaminow & Li 2002, p. 118
  306. ^ Deming 1925, pp. 330 (As2O3), 418 (B2O3; SiO2; Sb2O3)
  307. ^ Witt & Gatos 1968, p. 242 (GeO2)
  308. ^ Eagleson 1994, p. 421 (GeO2)
  309. ^ Rothenberg 1976, 56, 118‒119 (TeO2)
  310. ^ Desch 1914, p. 86
  311. ^ Phillips & Williams 1965, p. 620
  312. ^ Van der Put 1998, p. 123
  313. ^ Sanderson 1960, p. 83
  314. ^ Klug & Brasted 1958, p. 199
  315. ^ Good et al. 1813
  316. ^ Russell & Lee 2005, pp. 423‒4; 405‒6
  317. ^ Davidson & Lakin 1973, p. 627
  318. ^ Berger 1997, p. 91
  319. ^ Hampel 1968, passim
  320. ^ Rochow 1966, p. 41
  321. ^ Berger 1997, pp. 42‒43
  322. ^ Cornford 1937, pp. 249–50
  323. ^ Obrist 1990, pp. 163–64
  324. ^ Thomson 1830, p. 44
  325. ^ a b Paul 1865, p. 933
  326. ^ Roscoe & Schorlemmer 1894, pp. 3–4
  327. ^ Jungnickel & McCormmach 1996, p. 279–281
  328. ^ Craig 1849
  329. ^ Roscoe & Schorlemmer 1894, pp. 1–2
  330. ^ Strathern 2000, p. 239
  331. ^ a b Roscoe & Schormlemmer 1894, p. 4
  332. ^ Tweney & Shirshov 1935
  333. ^ Partington 1964, p. 168
  334. ^ a b Bache 1832, p. 250
  335. ^ Glinka 1958, p. 76
  336. ^ Partington 1964, pp. 145, 168
  337. ^ Jorpes 1970, p. 95
  338. ^ Berzelius 1825, p. 168
  339. ^ Jackson 1844, p. 368
  340. ^ Brande & Cauvin 1945, p. 223
  341. ^ The Chemical News and Journal of Physical Science 1864
  342. ^ Oxford English Dictionary 1989, 'non-metal'
  343. ^ Tilden 1876, p. 198
  344. ^ The Chemical News and Journal of Physical Science 1888
  345. ^ Beach 1911
  346. ^ Mayo 1917, p. 55
  347. ^ Couch 1920, p. 128
  348. ^ Webster's New International Dictionary 1926, p. 1359
  349. ^ Lundgren & Bensaude-Vincent 2000, p. 409
  350. ^ Greenberg 2007, p. 562
  351. ^ Pauling 1947, p. 65
  352. ^ IUPAC 1959, p. 10
  353. ^ IUPAC 1971, p. 11
  354. ^ Google Ngram, viewed 11 February 2011
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  356. ^ Gray 2010
  357. ^ Wilson 1939, pp. 21–22
  358. ^ Feng & Jin 2005, p. 324
  359. ^ Sólyom 2008, p. 91

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  • metalloid — [met′ə loid΄] n. 1. NONMETAL 2. an element having some of, but not all, the properties of metals, as arsenic or silicon adj. 1. like a metal in appearance 2. of, or having the nature of, a metalloid …   English World dictionary

  • Metalloid — Met al*loid, n. [L. metallum metal + oid: cf. F. m[ e]tallo[ i]de.] (a) Formerly, the metallic base of a fixed alkali, or alkaline earth; applied by Sir Humphrey Davy to sodium, potassium, and some other metallic substances whose metallic… …   The Collaborative International Dictionary of English

  • Metalloid — Met al*loid, a. 1. Having the appearance of a metal. [1913 Webster] 2. (Chem.) Having the properties of a nonmetal; nonmetallic; acid; negative. [1913 Webster] …   The Collaborative International Dictionary of English

  • metalloid — UK [ˈmetəlɔɪd] / US noun [countable] Word forms metalloid : singular metalloid plural metalloids chemistry a chemical element such as silicon that is not a metal but that has some of the qualities that a metal has Derived word: metalloid UK / US… …   English dictionary

  • metalloid — Resembling a metal in at least one amphoteric form; e.g., silicon and germanium as semiconductors. [metal + G. eidos, resemblance] * * * met·al·loid met əl .ȯid n an element (as boron, silicon, or arsenic) intermediate in properties between the… …   Medical dictionary

  • Metalloid — metaloidas statusas T sritis fizika atitikmenys: angl. metalloid vok. Metalloid, n rus. металлоид, m pranc. métalloïde, m …   Fizikos terminų žodynas

  • metalloid — metaloidas statusas T sritis fizika atitikmenys: angl. metalloid vok. Metalloid, n rus. металлоид, m pranc. métalloïde, m …   Fizikos terminų žodynas

  • metalloid — I. noun Date: 1813 1. an element intermediate in properties between the typical metals and nonmetals 2. a nonmetal that can combine with a metal to form an alloy II. adjective also metalloidal Date: 1836 1. resembling a metal 2. of, relating to,… …   New Collegiate Dictionary

  • metalloid — /met l oyd /, n. 1. a nonmetal that in combination with a metal forms an alloy. 2. an element that has both metallic and nonmetallic properties, as arsenic, silicon, or boron. adj. 3. of or pertaining to a metalloid. 4. resembling both a metal… …   Universalium

  • metalloid — met·al·loid || metÉ™lɔɪd n. element which is both metallic and nonmetallic; nonmetal adj. of metalloid (element which is both metallic and non metallic); resembling both a metal and a nonmetal …   English contemporary dictionary


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