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14.G. Leigh, “Periodic Tables and IUPAC,” Chem. Int. 31(1), 1–2 (2009).
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Chapter 5
Categorizations of the Elements
As will become apparent throughout this book, chemists can be very casual about the use or misuse of chemical terms. This lack of clear definitions even applies to the categorization of the elements themselves. In this chapter, a range of types of categorizations will be introduced and their meanings clarified.
Is polonium a metalloid? What is a weak metal? Which are the noble metals? So many questions, and to many of them, no established definitive answers. In this chapter, definitive proposals will be given as to which elements belong to which categories.
Nonmetals, Metals, and “In-Betweens”
In beginning chemistry, elements are classified as “metals” or “nonmetals.” This is a considerable oversimplification, as will be shown in the following. Each element is an individual, but it is convenient for chemists to impose systems of categorization. However, all terminology needs clear definition. Following from the definition, there should be clear reasoning behind assignment of an element to a particular category.
Nonmetals
The nonmetals are a “motley crew.” Essentially, they are every element that does not fit into another category. They include all the elements that are in Group 18 and Group 17, plus the top two elements of Group 16 and Group 15, the top element of Group 14, and hydrogen as can be seen in Figure 5.1. The only contentious element placed here in this category is astatine. Often identified as a metalloid or even a metal, the predominance of evidence is that astatine is a nonmetal [1, 2].
Figure 5.1 Elements classified as nonmetals.
Metalloids
The realization that there was no specific line of demarcation between metals and nonmetals dates back to the late 19th century (even though many commercial Periodic Tables continue to show one) [3]. It was in the 1890s that Newth declared that there were elements with intermediate properties: the metalloids. That was the easy part. The hard part was deciding which elements should be classified as metalloids.
The name “metalloid” was traditionally used for these in-between elements, then the term “semimetal” became preferred. However, the term “semimetal” was subsequently appropriated to be defined in terms of semiconductor materials in general, not simply chemical elements. As a result, “metalloid” has regained its meaning as specifically pertaining to certain chemical elements to the left of the nonmetals in the Periodic Table.
Vernon compiled all the elements identified as metalloids in sources from 1947 until 2012 [4]. The most popular elements cited with their frequency in parentheses were the following nine elements: antimony (88%); arsenic (100%); astatine (40%); boron (86%); germanium (96%); polonium (49%); selenium (23%); silicon (95%); and tellurium (98%). Adapting previously suggested criteria, Vernon devised the following definition for a metalloid:
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).
According to Vernon, there were six elements only which fitted his criteria for metalloid classification: antimony, arsenic, boron, germanium, silicon, and tellurium.
Hawkes plotted the electrical conductivity (as log10 in S⋅m−1) of the proposed metalloids together with a selection of metals and nonmetals along a scale (Figure 5.2). He observed that there were five elements whose electrical conductivity fitted into the gap between metals and nonmetals [5]. These elements were arsenic, boron, germanium, silicon, and tellurium.
Figure 5.2 A scale of electrical conductivity indicating the placement of some elements (adapted from Ref. [5]).
Figure 5.3 The commonly accepted metalloids.
It is this Author’s preference to adopt the Hawkes list of five elements as metalloids (Figure 5.3). Antimony, which made Vernon’s list but not Hawkes’s list, would seem better accommodated in the category in the next subsection (in the following).
Though the categorization earlier is the one that will be adopted here, there is one other candidate for consideration as a metalloid: radon. It has been argued by Stein that radon behaves as an ionic cation in aqueous solution and therefore should be treated as metalloid [6]. An interesting proposal, but one that has not garnered any significant support.
Chemically Weak Metals
To reiterate the point, there are no rigid boundaries in properties across the Periodic Table. Just as we have “invented” an additional category of metalloids for those elements that have properties between metal and nonmetals, so there is now a need for a category between metalloids and “true” metals. A “true” metal has essentially cationic behavior, these metals can also be found as parts of polyatomic
