Communication pubs.acs.org/jchemeduc
Which Elements Are Metalloids? René E. Vernon* Charles Sturt University, Locked Bag 588, Wagga Wagga, New South Wales 2678, Australia ABSTRACT: The results of a recent survey of 194 metalloid lists are consistent with a three-criterion description of metalloids published over 35 years ago. The classifications of selenium, polonium, and astatine, and other metalloid-like elements, are briefly reviewed in this light. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Inorganic Chemistry, Physical Chemistry, Misconceptions/Discrepant Events, Metalloids/Semimetals, Periodicity/Periodic Table that metalloids are typically semiconductors, “although antimony and arsenic [being semimetals in the physics-based sense] have electrical conductivities which approach those of metals”.13 The applicable elements and criterion values are listed in Table 1.
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ast issues of this Journal have reviewed the concept of a metalloid,1 investigated the most easily defensible list of metalloids,2 and challenged the basis for classifying polonium and astatine as metalloids.3,4 The concept of a metalloidas a member of a class of elements intermediate between metals and nonmetalsis a relatively old one, straddling the establishment of the periodic law in 1869.5 Even so, this concept has only been reasonably well accepted since 1940−1960, and has yet to be more clearly defined, even if only to establish a common or core set of metalloids. The related purpose of this communication is to draw attention to a recent survey of 194 metalloid lists and to an older (little known) description of metalloids based on three criteria. It transpires that the elements most commonly recognized as metalloids in the survey are the same as the elements whose properties were used to inform the three-criterion description. The classifications of selenium, polonium, and astatine, as well as gray tin and other metalloid-like elements, are briefly reviewed in this light.
Table 1. The Six Elements Commonly Recognized as Metalloids12,14−16
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IP/ (kcal/mol)
IP/ (kJ/mol)
ENa
ENb
Electronic Band Structurec
β-Boron α-Silicon α-Germanium α-Arsenic α-Antimony α-Tellurium Average
193 189 184 228 201 210 201
800 786 762 944 830 869 832
2.0 1.8 1.8 2.0 1.9 2.1 1.9
2.04 1.90 2.01 2.18 2.05 2.10 2.05
semiconductor semiconductor semiconductor semimetal semimetal semiconductor -
a
Original Pauling electronegativity value. bRevised Pauling value. cFor the element in its standard state, as shown in the first column.
SURVEY OF ELEMENTS COMMONLY RECOGNIZED AS METALLOIDS By way of a literature search, the author compiled a list of 194 metalloid lists, dating from 1947 onward.6 The list includes 26 different elements identified as metalloids, across 67 different list configurations, with an average of just over seven elements per list. The percentage appearance frequencies of the elements most frequently identified as metalloids are boron (86), silicon (95), germanium (96), arsenic (100), selenium (23), antimony (88), tellurium (98), polonium (49), and astatine (40). Of the menagerie of other elements occasionally identified as metalloids, carbon (9), aluminum (9), and bismuth (6) are the front runners. The survey results are consistent with the assertions made by other authors that six elements are commonly recognized as metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium.1,2,7−11
Masterton and Slowinski’s description is remarkable for its use of these three more-or-less clearly defined criteria. In contrast, metalloids tend to be collectively characterized in terms of generalities or a few broadly indicative physical or chemical properties,1 with a single quantitative criterion or attribute (such as electrical conductivity,17 packing efficiency,18 or the acid−base character of group oxides19) being mentioned only occasionally.20 Their description can be expressed more formally, in SI units, to give the following semiquantitative working definition: A metalloid is a chemical element that, in its standard state, has (a) the electronic band structure of a semiconductor or a semimetal, (b) an intermediate first ionization potential (say, 750−1,000 kJ/mol), and (c) an intermediate electronegativity (1.9−2.2, revised Pauling).
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SELENIUM, POLONIUM, AND ASTATINE Neither selenium nor polonium satisfies the working definition of a metalloid noted above. Selenium has a first ionization potential of 941 kJ mol−1 and is sometimes described as a
THREE-CRITERION DESCRIPTION The elements commonly recognized as metalloids in the survey had also been identified and described as such in 1977 by Masterton and Slowinski.12 They wrote that metalloids have first ionization potentials (IP) clustering around 200 kcal/mol and electronegativity (EN) values close to 2.0. They also said © XXXX American Chemical Society and Division of Chemical Education, Inc.
Element
A
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As well, the question of which elements would be counted as insulators rather than semiconductors was not addressed. There may be some merit in adapting Adler’s suggestion by distinguishing between the notion of a “metalloid”, and that of a “near metalloid”. Metalloids are those elements commonly recognized as such, which fall within the scope of the definition set out in this article. A near metalloid then becomes any other chemical element having the electronic band structure of a semimetal or a semiconductor, with the latter (for this purpose) being taken as an element having a band gap less than or equal to the visible spectrum cutoff of 1.8 eV.48 A semiconductor with such a relatively narrow band gap49 has a metallic or black appearance,50,51 and metalloids have traditionally been regarded as looking like metals52 (semiconductors with wider band gaps, and insulators, appear colored, white, or transparent).53,54 This definition would include graphitic carbon, black phosphorus, gray selenium, gray tin, iodine,55 and bismuth. As noted, these are all elements described in the literature as being metalloidlike, or words to that effect. Craig,56 for example, refers to the “near metalloidal” status of selenium; Schroers57 refers to carbon and phosphorus as near metalloids. Although aluminum and polonium are also mentioned in the literature as showing some metalloid character, both elements have the electronic band structures of metals and, arguably, show at least a reasonable preponderance of metallic character. They are both better regarded as poor metals or chemically weak metals.37,38
semiconductor. However its electronegativity of 2.55 is too high. Polonium has an ionization potential of 812.1 kJ mol−1 and an electronegativity of 2.0, but has a metallic band structure.21,22 Hawkes2,3,23 also concluded that polonium was better classified as a metal. Astatine has a first ionization potential24 of 899 kJ·mol−1 and an electronegativity of 2.2. However, its electronic band structure is not known with any great degree of certainty. Batsanov25 gave a calculated band gap energy for diatomic astatine of 0.7 eV, a figure which could reasonably be associated with semiconductor status. Evidence for the existence of diatomic astatine is, however, sparse and inconclusive.26 Edwards and Sienko27 speculate, on the basis of the Goldhammer−Herzfeld criterion for metallicity, that astatine is probably a metalloid. Siekierski and Burgess28 contend or presume that astatine would be a metal if it could form a condensed phase but do not give a specific basis for their claim. Hawkes2,3,23 initially concluded that astatine should be classified as a nonmetal but has subsequently agreed, upon further reviewing its mixed and intermediate properties (known and extrapolated),13 that astatine is better classified as a metalloid.29 This development is consistent with other references to the character of astatine. Bresler30 commented that “the properties of the metal and the halogen are curiously combined in this element”. Rössler31 similarly highlighted “the chemical ambiguity of astatine between halogen and metal character”. More recently, Restrepo et al.32 reported that astatine appeared to share more in common with polonium (a metal) than it did with the established halogens. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.
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SUMMARY There are six elements commonly recognized as metalloids. Thirty-five years ago, Masterton and Slowinksi published a three-criterion description of metalloids. Their description provides a useful basis to establish a more formal definition of a metalloid. In this light, selenium is better classified as a nonmetal, and polonium as a metal. Astatine may or may not be a metalloid according to this definition. However, on the basis of its currently known and extrapolated properties, astatine is better classified as a metalloid. If it subsequently transpires that the band structure of astatine is found to be metallic, then the working definition of a metalloid suggested here may need to be revisited. Gray tin and other elements with semimetal or semiconductor band structures, such as graphite and bismuth, while not falling within the scope of the metalloid definition suggested in this communication, may be usefully regarded as near metalloids, in recognition of their metalloid-like character.
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GRAY TIN AND OTHER METALLOID-LIKE ELEMENTS Gray tin is the stable form of tin below around 55 °F (13 °C). As tin, it has a first ionization potential14 of 708 kJ/mol and an electronegativity of 1.96 revised Pauling. Unlike ordinary tin, which has the electronic band structure of a metal, gray tin has the electronic band structure of a semimetal.33 With its relatively low first ionization potential, gray tin does not satisfy the definition of a metalloid suggested here. Cohen and Chelikowsky34 regard gray tin as a very poor metal; Pauling35 says it has the properties of a metalloid; Ebbing and Gammon,36 who distinguish between metals, metalloids, and nonmetals, treat gray tin as a nonmetal. Physically, gray tin could be viewed as a metalloid (silvery metallic appearance; brittle; moderate conductivity, increasing with temperature; semimetal band structure). Chemically, however, gray tin falls marginally on the metal side of the line given tin is ordinarily regarded as chemically weak metal rather than a chemically weak nonmetal.37,39 Similar to gray tin, there are a number of other elements that are mentioned in the literature as being “near metalloidal”, showing metalloid(al) character, or having metalloid-like or some metallic characteristics. Examples include graphitic carbon40,41 (a semimetal along its basal plane; a semiconductor along its edge plane), black phosphorus42−44 (a semiconductor), and bismuth45,46 (a semimetal). An interesting and elegant solution for accommodating these metalloid-like elements was provided by Adler,47 who suggested defining a metalloid simply as a semiconductor or a semimetal. His suggestion can be viewed as a step too far; few other authors have regarded these additional elements as metalloids.
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I gratefully acknowledge the opportunity to put questions about metalloids to, and discuss related matters with, Stephen Hawkes, at Oregon State University; William Jensen, of the Department of Chemistry, University of Cincinnati; David Johnson, editor of Metals and Chemical Change (2002);58 Peter Nelson, of the Department of Chemistry, University of Hull; and Alan Russell, lead author of Structure−Property Relations in Nonferrous Metals (2005).59 The approach taken in this communication does not necessarily reflect the views of B
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(18) Suresh, C. H.; Koga, N. A consistent approach toward atomic radii. J. Phys. Chem. A 2001, 105 (24), 5940−5944 (5942−5943). (19) Hutton, W.; Dickerson, R. E. A Study Guide to Chemical Principles; Benjamin: New York, 1970; p 162. (20) Explaining the role of classification in science, Jones (2010) however writes that ‘Classes are usually defined by more than two attributes...’ Jones, B. W. Pluto: Sentinel of the Outer Solar System; Cambridge University: Cambridge, U.K., 2010; p 169. (21) Kraig, R. E.; Roundy, D.; Cohen, M. L. A study of the mechanical and structural properties of polonium. Solid State Commun. 2004, 129 (6), 411−413 (12). (22) Alloul, H. Introduction to the Physics of Electrons in Solids; Springer-Verlag: Berlin, 2010; p 83. (23) Hawkes, S. J. Polonium and astatine are not semimetals. Chem 13 News 1999, 27 (Feb), p 14. (24) Rothe, S.; Andreyev, A. N.; Antalic, S.; Borschevsky, A.; Capponi, L.; Cocolios, T. E.; De Witte, H.; Eliav, E.; Fedorov, D. V.; Fedosseev, V. N.; Fink, D. A.; Fritzsche, S.; Ghys, L.; Huyse, M.; Imai, N.; Kaldor, U.; Kudryavtsev, Y.; Köster, U.; Lane, J. F. W.; Lassen, J.; Liberati, V.; Lynch, K. M.; Marsh, B. A.; Nishio, K.; Pauwels, D.; Pershina, V.; Popescu, L.; Procter, T. J.; Radulov, D.; Raeder, S.; Rajabali, M. M.; Rapisarda, E.; Rossel, R. E.; Sandhu, K.; Seliverstov, M. D.; Sjödin, A. M.; Van den Bergh, P.; Van Duppen, P.; Venhart, M.; Wakabayashi, Y.; Wendt, K. D. A. Measurement of the first ionization potential of astatine by laser ionization spectroscopy. Nat. Commun. 2013, 4, 1835. (25) Batsanov, S. S. Quantitative characteristics of bond metallicity in crystals. J. Struct. Chem. 1971, 12 (5), 809−813 (811). (26) Main page for astatine. http://en.wikipedia.org/wiki/Astatine (accessed Oct 2013). (27) Edwards, P. P.; Sienko, M. J. On the occurrence of metallic character in the periodic table of the elements. J. Chem. Educ. 1983, 60 (9), 691−696. “The Goldhammer−Herzfeld criterion is a ratio that compares the force holding an individual atom’s valence electrons in place with 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. Metallic behaviour is then predicted. Otherwise nonmetallic behaviour is anticipated.” (Main page for metalloids. http://en.wikipedia.org/wiki/Metalloid (accessed Oct 2013). (28) Siekierski, S.; Burgess, J. Concise Chemistry of the Elements; Horwood: Chichester, 2002; pp 65, 122. (29) Hawkes S. J. Personal communication, 29 Oct 2010. (30) Bresler, S. E. Radioactive Elements, 2nd ed.; State Technical Theoretical Press: Moscow, 1952. In Korenman, I. M. Regularities in properties of thallium. Russ. J. Gen. Chem. 1959, English translation, Consultants Bureau: New York, 29 (2), 1366−1390 (1368). (31) Rössler, K. Handling of astatine. In Kugler, H. K.; Keller, C., Eds.; Gmelin Handbook of Inorganic Chemistry, At Astatine, 8th ed.; Springer-Verlag: Berlin, 1985; pp 140−156. (143) (32) Restrepo, G.; Llanos, E. J.; Mesa, H. Topological space of the chemical elements and its properties. J. Math. Chem. 2006, 39, (2), 401−416 (408, 411, 413). In a 2004 paper with the same lead author, and using a smaller set of properties, the authors comment that about 1.6 per cent of missing experimental values were interpolated; I presume the same general approach was taken in the 2006 paper (in the absence of any information therein on this point). Restrepo, G.; Mesa, H.; Llanos, E. J.; Villaveces, J. L. Topological study of the periodic system. J. Chem. Inf. Model. 2004, 44 (1), 68−75 (69). (33) Lovett, D. R. Semimetals & Narrow-Bandgap Semi-Conductors; Pion: London, 1977; p 101. (34) Cohen, M. L.; Chelikowsky, J. R. Electronic Structure and Optical Properties of Semiconductors; Springer Verlag: Berlin, 1988; p 99. (35) Pauling, L. General Chemistry; Dover Publications: New York, 1988; p 577. (36) Ebbing, D. D; Gammon, S. D. General Chemistry, 9th ed. enhanced; Brooks/Cole: Belmont, CA, 2010; p 891.
these authors. I thank anonymous reviewers for their helpful comments and suggestions.
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(1) Goldsmith, R. H. Metalloids. J. Chem. Educ. 1982, 59 (6), 526− 527. (2) Hawkes, S. J. Semimetallicity. J. Chem. Educ. 2001, 78 (12), 1686−1687. (3) Hawkes, S. J. Polonium and astatine are not semimetals. J. Chem. Educ. 2010, 87 (8), 783. (4) Although Holt, Rinehart, and Wilson reached the same conclusion as Hawkes on the classification of polonium and astatine, their analysis is less thorough, in terms of breadth and depth, than that of the latter author. Accordingly, their work is not further considered here. Holt A.; Rinehart B.; Wilson C. Why polonium and astatine are not metalloids in HRW texts. c. 2007. http://go.hrw.com/resources/ go_sc/periodic/Po_At_Metalloids.pdf (accessed Oct 2013). (5) For example, Brande and Cauvin (1845) classified the elements as gases (O, H, and N); halogens; metalloids (S, P, C, B, and Si?) “resemble the metals in certain respects, but are in others wildly different”; and metals, predating Mendeléeff’s announcement of the discovery of periodic law in 1869. As another example, Tilden (1876) subsequently divided the elements into (“basigenic”) true metals, metalloids (“imperfect metals”), and (“oxigenic”) nonmetals. He counted H, As, Sb, Bi, and Te as metalloids, along with Ti(?), Zr, V, Nb(?), Ta(?), Mo, W, and U. Mendeléeff (1891) similarly regarded tellurium as forming “a transition from the metals to the non-metals”. Brande, W. T.; Cauvin, J. A Dictionary of Science, Literature and Art; Harper & Brothers: New York, 1845; p 223; Mendeléeff, D. I. The Principles of Chemistry, 5th ed.; Kamensky, G. (Translator); Greenaway, A. J., Ed.; Longmans, Green & Co.: London, 1891; Vol. 2, p 264; Tilden, W. A. Introduction to the Study of Chemical Philosophy; D. Appleton and Co.: New York, 1876; pp 171−173. (6) Main page for the list of metalloid lists. http://en.wikipedia.org/ wiki/List_of_metalloid_lists (accessed Oct 2013), as compiled, created and edited by the author. (7) Boylan, P. J. Elements of Chemistry; Allyn & Bacon: Boston, MA, 1962; p 493. (8) Sherman, E.; Weston, G. J. Chemistry of the Non-Metallic Elements; Pergamon Press: New York, 1966; p 64. (9) Wulfsberg, G. Principles of Descriptive Inorganic Chemistry; Brooks/Cole: Monterey, CA, 1991; p 201. (10) Kotz, J. C.; Treichel, P.; Weaver, G. C. Chemistry & Chemical Reactivity, 7th ed.; Brooks/Cole: Belmont, CA, 2009; p 62. (11) Mann et al. in fact refer to these elements as “the recognized metalloids”: Mann, J. B.; Meek, T. L.; Allen, L. C. Configuration energies of the main group elements. J. Am. Chem. Soc. 2000, 122 (12), 2780−2783 (2783). (12) Masterton, W. L.; Slowinski, E. J. Chemical Principles, 4th ed.; W. B. Saunders: Philadelphia, PA, 1977; p 160. Masterton and Slowinski also comment that polonium and astatine are ordinarily classified as metalloids, but add that “since very little is known about their chemical and physical properities, any such classification must be rather arbitrary”. (13) Main page for metalloids. http://en.wikipedia.org/wiki/ Metalloid (accessed Oct 2013). (14) Martin, W. C.; Musgrove, A.; Kotochigova, S.; Sansonetti, J. E. Ground Levels and Ionization Energies for Neutral Atoms [online]; National Institute of Standards and Technology (NIST): Gaithersburg, MD, 2010. http://www.nist.gov/pml/data/ion_energy.cfm (accessed Oct 2013). Values shown in the table have been converted from the NIST values, which are given in eV. (15) Berger, L. I. Semiconductor Materials; CRC Press: Boca Raton, FL, 1997. (16) Lovett, D. R. Semimetals & Narrow-Bandgap Semi-Conductors; Pion: London, 1977; p 3. (17) Hill, G.; Holman, J. Chemistry in Context, 5th ed.; Nelson Thornes: Cheltenham, 2000; p 41. C
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