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.
Ancient conceptions of metals as solid, fusible and malleable substances can be found in Plato's Timaeus (c. 360 BCE) and Aristotle's Meteorology. [2] [3]
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. [4] [5] [6]
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. [7] The malleability of mercury then became known, and it was included amongst the metals. [4]
In 1789 Fourcroy [8] highlighted the weakness of this distinction between metals and semimetals. He said it was evident from the fact that
This idea of a semimetal, as a brittle (and thereby imperfect) [9] [10] metal, was gradually discarded after 1789 with the publication of Lavoisier's 'revolutionary' [11] Elementary Treatise on Chemistry. [12] [n 1]
In 1800, Pinkerton used the word metalloid, in its literal sense, to describe a mineral variety of pyroxene 'with metallic splendour.' [14]
In 1808, Erman and Simon suggested using the term metalloid to refer to the newly discovered elements sodium and potassium. [15] 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'. [16] Their suggestion was ignored by the chemical community. [17]
In 1811, Berzelius referred to nonmetallic elements as metalloids, [17] [18] in reference to their ability to form oxyanions. [19] [20] 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 [17] although it was subsequently regarded by some commentators as counterintuitive, [20] misapplied, [12] incorrect [21] or invalid. [22] In 1825, in a revised German edition of his Textbook of Chemistry, [23] [24] 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). [25]
In 1844, Jackson gave the meaning of 'metalloid' as 'like metals, but wanting some of their properties.' [26] 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. [27]
In 1864, calling nonmetals 'metalloids' was still sanctioned 'by the best authorities' [28] 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. [28]
By as early as 1866 some authors were instead using the term nonmetal, rather than metalloid, to refer to nonmetallic elements. [29] 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.' [30]
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. [31]
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. [32]
Beach, writing in 1911, explained it this way: [33]
In or around 1917, the Missouri Board of Pharmacy wrote [34] that:
We may notice that the distinction between metals and non-metals is not a sharply defined one—the one class shades off into the other. In the case of copper and carbon the balance of evidence is clear enough, but in some cases (e.g. arsenic) there is just about as much to be said on one side as on the other, and it is impossible to decided whether the element is a metal or non-metal. Such elements are often called metalloids.
Littler W 1935, Elementary Chemistry, G Bell and Sons, London, p. 337
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 [35] 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. [36]
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. [17]
In 1947, Pauling included a reference to metalloids in his classic [37] and influential [38] 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.' [39]
In 1959 the International Union of Pure and Applied Chemistry (IUPAC) recommended that '[t]he word metalloid should not be used to denote nonmetals' [40] although it was still being used in this sense (around that time) by, for example, the French. [41]
In 1969 the classic [42] and authoritative [43] Hackh's Chemical Dictionary included entries for both 'metalloid' and 'semimetal'. The latter term was described as obsolete. [44]
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. [41] [45] Despite this recommendation, use of the term 'metalloid' increased dramatically. [17] 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'. [46]
The most recent IUPAC publications on chemical nomenclature (the "Red Book", 2005) [47] and terminology (the "Gold Book", 2006–) [48] 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'. [49] 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. [50] [51] 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. [52] In contrast, the room-temperature concentration of electrons in metals usually exceeds 1022 cm−3. [53]
References to 'metalloid' as being outdated have also been described as 'nonsense' noting that 'it accurately describes these weird in-between elements'. [54]
A chemical element is a chemical substance that cannot be broken down into other substances by chemical reactions. The basic particle that constitutes a chemical element is the atom. Elements are identified by the number of protons in their nucleus, known as the element's atomic number. For example, oxygen has an atomic number of 8, meaning each oxygen atom has 8 protons in its nucleus. Atoms of the same element can have different numbers of neutrons in their nuclei, known as isotopes of the element. Two or more atoms can combine to form molecules. Chemical compounds are molecules made of atoms of different elements, while mixtures contain atoms of different elements not necessarily combined as molecules. Atoms can be transformed into different elements in nuclear reactions, which change an atom's atomic number.
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-containing compounds such as alkanes and its derivatives are universally considered organic, but many others are sometimes considered inorganic, such as halides of carbon without carbon-hydrogen and carbon-carbon bonds, and certain compounds of carbon with nitrogen and oxygen.
The periodic table, also known as the periodic table of the elements, is an ordered arrangement of the chemical elements into rows ("periods") and columns ("groups"). It is an icon of chemistry and is widely used in physics and other sciences. It is a depiction of the periodic law, which states that when the elements are arranged in order of their atomic numbers an approximate recurrence of their properties is evident. The table is divided into four roughly rectangular areas called blocks. Elements in the same group tend to show similar chemical characteristics.
A metalloid is a chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature.
In the context of the periodic table a nonmetal is a chemical element that mostly lacks distinctive metallic properties. They range from colorless gases like hydrogen to shiny crystals like iodine. Physically, they are usually lighter than elements that form metals and are often poor conductors of heat and electricity. Chemically, nonmetals have relatively high electronegativity or usually attract electrons in a chemical bond with another element, and their oxides tend to be acidic.
A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals are most often so classified. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur.
A pnictogen is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the elements nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and moscovium (Mc).
A semimetal is a material with a very small overlap between the bottom of the conduction band and the top of the valence band. According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. In insulators and semiconductors the filled valence band is separated from an empty conduction band by a band gap. For insulators, the magnitude of the band gap is larger than that of a semiconductor. Because of the slight overlap between the conduction and valence bands, semimetals have no band gap and a negligible density of states at the Fermi level. A metal, by contrast, has an appreciable density of states at the Fermi level because the conduction band is partially filled.
Chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC).
A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combined without reacting, they may form a chemical mixture. If a mixture is separated to isolate one chemical substance to a desired degree, the resulting substance is said to be chemically pure.
There are currently 118 known chemical elements with a wide range of physical and chemical properties. Amongst this diversity, scientists have found it useful to apply names for various sets of elements that have similar properties, to varying degrees. Many of these sets are formally recognized by the standards body IUPAC.
This is a list of 194 sources that list elements classified as metalloids. The sources are listed in chronological order. Lists of metalloids differ since there is no rigorous widely accepted definition of metalloid. Individual lists share common ground, with variations occurring at the margins. The elements most often regarded as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. Other sources may subtract from this list, add a varying number of other elements, or both.
The chemical elements can be broadly divided into metals, metalloids, and nonmetals according to their shared physical and chemical properties. All elemental metals have a shiny appearance ; are good conductors of heat and electricity; form alloys with other metallic elements; and have at least one basic oxide. Metalloids are metallic-looking, often brittle solids that are either semiconductors or exist in semiconducting forms, and have amphoteric or weakly acidic oxides. Typical elemental nonmetals have a dull, coloured or colourless appearance; are often brittle when solid; are poor conductors of heat and electricity; and have acidic oxides. Most or some elements in each category share a range of other properties; a few elements have properties that are either anomalous given their category, or otherwise extraordinary.
The dividing line between metals and nonmetals can be found, in varying configurations, on some representations of the periodic table of the elements. 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 lie just below the line.
The metallic elements in the periodic table located between the transition metals to their left and the chemically weak nonmetallic metalloids to their right have received many names in the literature, such as post-transition metals, poor metals, other metals, p-block metals and chemically weak metals. The most common name, post-transition metals, is generally used in this article.
Heavy metals are metallic elements with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published, none of which have been widely accepted. The definitions surveyed in this article encompass up to 96 out of the 118 known chemical elements; only mercury, lead and bismuth meet all of them. Despite this lack of agreement, the term is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
Nonmetals show more variability in their properties than do metals. Metalloids are included here since they behave predominately as chemically weak nonmetals.