Origin and use of the term metalloid
The origin and usage of the term metalloid is convoluted. Its origin lies in attempts, dating from antiquity, to describe metals and to distinguish between typical and less typical forms. It was first applied to metals that floated on water (lithium, sodium and potassium), and then more popularly to nonmetals. Only recently, since the mid-20th century, has it been widely used to refer to elements with intermediate or borderline properties between metals and nonmetals.
More sophisticated classification arrangements were proposed by Pseudo-Geber (in the Geber corpus, c. 1310), Paracelsus (De Natura Rerum libri nonem, 1525–6; and later works), Basil Valentine (Conclusiones, 1624), and Boerhaave (Elementa Chemiæ, 1733). They attempted to separate the more characteristic metals from substances having those characteristics to a lesser degree. Such substances included zinc, antimony, bismuth, stibnite, pyrite and galena. These were all then called semimetals or bastard metals.
In 1735 Brandt proposed to make the presence or absence of malleability the principle of this classification. On that basis he separated mercury from the metals. The same view was adopted by Vogel (1755, Institutiones Chemiæ) and Buffon (1785, Histoire Naturelle des Minéraux). In the interim, Braun had observed the solidification of mercury by cold in 1759–60. This was confirmed by Hutchins and Cavendish in 1783. The malleability of mercury then became known, and it was included amongst the metals.
- between the extreme malleability of gold and the singular fragility of arsenic, other metals presented only imperceptible gradations of this character, and because there was probably no greater difference between the malleability of gold and that of lead, which was considered to be a metal, than there was between lead and zinc, which was classed among semi-metals, while in the substances intermediate between zinc and arsenic the differences were slight.
This idea of a semimetal, as a brittle (and thereby imperfect) metal, was gradually discarded after 1789 with the publication of Lavoisier's 'revolutionary'  Elementary Treatise on Chemistry.[n 1]
In 1808, Erman and Simon suggested using the term metalloid to refer to the newly discovered elements sodium and potassium. These elements were lighter than water and many chemists did not regard them as proper metals. Erman and Simon's proposal may have been made '[in] an attempt to revive this old distinction between metals and substances resembling metals'. Their suggestion was ignored by the chemical community.
In 1811, Berzelius referred to nonmetallic elements as metalloids, in reference to their ability to form oxyanions. A common oxyanion of sulfur, for example, is the sulfate ion SO2−
4. Many metals can do the same. Chromium, for instance, can form the chromate ion CrO2−
4. Berzelius' terminology was widely adopted although it was subsequently regarded by some commentators as counterintuitive, misapplied, incorrect or invalid. In 1825, in a revised German edition of his Textbook of Chemistry, Berzelius subdivided the metalloids into three classes. These were: constantly gaseous 'gazolyta' (hydrogen, nitrogen, oxygen); real metalloids (sulfur, phosphorus, carbon, boron, silicon); and salt-forming 'halogenia' (fluorine, chlorine, bromine, iodine).
In 1844, Jackson gave the meaning of 'metalloid' as 'like metals, but wanting some of their properties.' In 1845, in A dictionary of science, literature and art, Berzelius' classification of the elementary bodies was represented as: I. gazolytes; II. halogens; III. metalloids ('resemble the metals in certain aspects, but are in others widely different'); and IV. metals.
In 1864, calling nonmetals 'metalloids' was still sanctioned 'by the best authorities' even though this did not always seem appropriate. The greater propriety of applying the word metalloid to other elements, such as arsenic, had been considered.
By as early as 1866 some authors were instead using the term nonmetal, rather than metalloid, to refer to nonmetallic elements. In 1875, Kemshead observed that the elements had been subdivided into two classes—'non-metals or metalloids, and metals.' He added that '[t]he former term, although not so convenient, because a compound word, is more correct, and is now universally employed.'
In 1876, Tilden protested against, 'the [still] too common though illogical practice of giving the name metalloid to such bodies as oxygen, chlorine or fluorine'. He instead divided the elements into ('basigenic') true metals, metalloids ('imperfect metals') and ('oxigenic') nonmetals.
As late as 1888, classifying the elements into metals, metalloids, and nonmetals, rather than metals and metalloids, was still regarded as peculiar and potentially confusing.
Beach, writing in 1911, explained it this way:
- Metalloid (Gr. "metal-like"), in chemistry, any nonmetallic element. There are 13, namely, sulfur, phosphorus, fluorin[e], chlorin[e], 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 nonmetals for the very reason that there is no hard and fast line between metals and nonmetals, so that "metal-like" or "resembling metals" is a better description of the class than the purely negative "nonmetals". Originally it was applied to the nonmetals 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 'nonmetal.' [n 2] In contrast, Webster's New International Dictionary noted that use of the term metalloid to refer to nonmetals was the norm. Its application to elements resembling the typical metals in some way only, such as arsenic, antimony and tellurium, was recorded merely on a 'sometimes' basis.
Use of the term metalloid subsequently underwent a period of great flux up to 1940. Consensus as to its application to intermediate or borderline elements did not occur until the ensuing years, between 1940 and 1960.
In 1947, Pauling included a reference to metalloids in his classic and influential textbook, General chemistry: An introduction to descriptive chemistry and modern chemical theory. He described them as 'elements with intermediate properties ... occupy[ing] a diagonal region [on the periodic table], which includes boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.'
In 1959 the International Union of Pure and Applied Chemistry (IUPAC) recommended that '[t]he word metalloid should not be used to denote nonmetals' although it was still being used in this sense (around that time) by, for example, the French.
In 1970 IUPAC recommended abandoning the term metalloid because of its continuing inconsistent use in different languages. They suggested using the terms metal, semimetal and nonmetal instead. Despite this recommendation, use of the term 'metalloid' increased dramatically. Google Ngram Viewer showed a fourfold increase in the use of the word 'metalloid' (as compared to 'semimetal') in the American English corpus from 1972 to 1983. There was a sixfold increase in the British English corpus from 1976 to 1983. As at 2011, the difference in usage across the English corpus was around 4:1 in favour of 'metalloid'.
The most recent IUPAC publications on chemical nomenclature (the "Red Book", 2005)  and terminology (the "Gold Book", 2006–)  do not include any recommendations as to the usage or non-usage of the terms metalloid or semimetal.[n 3]
Use of the term semimetal, rather than metalloid, has recently been discouraged. This is because the former term 'has a well defined and quite distinct meaning in physics'. In physics, a semimetal is an element or a compound in which the valence band marginally (rather than substantially) overlaps the conduction band. This results in only a small number of effective charge carriers. Thus, the densities of charge carriers in the elemental semimetals carbon (as graphite, in the direction of its planes), arsenic, antimony and bismuth are 3×1018 cm−3, 2 ×1020 cm−3, 5×1019 cm−3 and 3×1017 cm−3 respectively. In contrast, the room-temperature concentration of electrons in metals usually exceeds 1022 cm−3.
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 its first seventeen years, Lavoisier's work was republished in twenty-three editions and six languages, and carried his 'new chemistry' across Europe and America.
- Couch also commented that there was, 'no sharp line of demarcation between metals and nonmetals as many of the latter class possess some metallic properties' [italics added].
- The "Gold Book"  contains one reference to semimetals in the physics-based sense (see 'semiconductor-metal transition') and one reference in the chemistry based sense (see 'organometallic compounds'). The latter entry notes that 'traditional metals and semi-metals' can form such compounds, as can 'boron, silicon, arsenic and selenium'.
- Partington 1961, p. 148
- Cornford 1937, pp. 249–50
- Obrist 1990, pp. 163–64
- Paul 1865, p. 933
- Roscoe & Schorlemmer 1894, pp. 3–4
- Partington 1961, pp. 148, 192–193
- Jungnickel & McCormmach 1996, p. 279–281
- Fourcroy, p. 380
- Craig 1849
- Roscoe & Schorlemmer 1894, pp. 1–2
- Strathern 2000, p. 239
- Roscoe & Schormlemmer 1894, p. 4
- Salzberg 1991, p. 204
- Pinkerton 1800, p. 81
- Erman and Simon (1808) "3. Dritter Bericht des Hrn. Prof. Erman und des Geh. Oberbauraths Simon über ihre gemeinschaftlichen Versuche" (Third report of Prof. Erman and State Architect Simon on their joint experiments), Annalen der Physik, 28 (3) : 347-367. After studying the new metallic elements that Humphry Davy had isolated, physicist Paul Erman and architect Paul Ludwig Simon concluded that potassium and sodium didn't conform to the traditional criteria of metals and therefore should be classified as "metalloids". From p. 347: "Hierin liegt aber eine sehr grosse Schwierigkeit, weil veilleicht keine der bekannten Substanzen einen so hohen Grad von Oxydabilität besitzt, als diese beiden Metalloide, wie man sie wohl an füglichsten vor der Hand nennen sollte." (However, here lies a very great difficulty because perhaps none of the known substances possess so high a degree of oxidizability as these two metalloids, as one should perhaps call those [that are] most conveniently available.)
- Tweney & Shirshov 1935
- Goldsmith 1982, p. 526
- Berzelius 1811, p. 258
- Partington 1964, p. 168
- Bache 1832, p. 250
- Glinka 1959, p. 76
- Hérold 2006, pp. 149–150
- Partington 1964, pp. 145, 168
- Jorpes 1970, p. 95
- Berzelius 1825, p. 168
- Jackson 1844, p. 368
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- Kemshead 1875, p. 13
- Tilden 1876, pp. 171–3; 198. He listed as metalloids: H; Ti(?), Zr; V, Nb(?), Ta(?); Mo, W, U; As, Sb, Bi; and Te. Non-metals included B, C, Si and Se. Graphitic C and Si were however noted as furnishing the only examples among the non-metals of electric conductivity.
- The Chemical News and Journal of Physical Science 1888
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- Lundgren & Bensaude-Vincent 2000, p. 409
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- IUPAC 1959, p. 10
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- American Institute of Chemists 1969, p. 485
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- Grant 1969, pp. 422, 604: 'metalloid.—(1) having the physical properties of metals and the chemical properties of nonmetals, e.g., As. (2) a nonmetal (incorrect usage) ... semimetal.—an element midway in properties between metals and nonmetals, as arsenic (obsolete).'
- IUPAC 1971, p. 11
- Google Ngram, viewed 11 February 2011
- IUPAC 2005
- IUPAC 2006–
- Atkins 2010 et al., p. 20
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