The term pnictogen is more recent. The name was devised by van Arkel in the 1950s [24]. International Union of Pure and Applied Chemistry (IUPAC) originally rejected pnictogens as the name for Group 15. In 1970, it was pronounced by IUPAC that if group names were needed, they should be triels for Group 13, tetrels for Group 14, and pentels for Group 15 [25]. Fernelius et al. explained [26]:
There is strong, though not unanimous, sentiment within the IUPAC Commission on the Nomenclature of Inorganic Chemistry to adopt some systematic method for arriving at group names. The following has been suggested.
Family
B, Al, Ga, In, Tl — triels
C, Si, Ge, Sn, Pb — tetrels
N, P, As, Sb, Bi — pentels
Pentels never seemed to be adopted; instead pnictogens (mentioned earlier) became the commonly used term for Group 15 elements. Nevertheless, triels and tetrels have gained in popularity in recent years to describe the Group 13 and Group 14 elements, respectively [27], even in Chemistry International, IUPAC’s own magazine [28].
d-Block Metal Appellations
There have been numerous names associated with parts of the d-block elements. The issue of which are encompassed by the term “transition metals” (formerly the “transitional metals”) will be discussed in Chapter 8. Here are some selected examples that have a specific purpose.
Heavy Transition Metals
Though aficionados of hard rock music may be able to define the term “heavy metal,” there is no consensus on the term in the chemical context. In a comprehensive review, Duffus showed that there were four totally different criteria used for the term “heavy metal”: high density; atomic mass; atomic number; or toxicity [29]. There was no agreement on cutoff values for any of these criteria. As a result of the ambiguity, Duffus recommended that the term have no official recognition. Hawkes argued that “heavy metal” was a useful descriptor, suggesting that it encompass all metals in Groups 3 to 16 in the 4th Period and beyond [30]. However, this block does seem to include a very large number of disparate metals; and without serving any clear purpose.
By contrast, the similar sounding heavy transition metals are well defined [31]. These are the dense large transition metals of the 5th and 6th Periods (Figure 5.5). As will be discussed in Chapter 8, there are many similarities between the lower pair of elements in each transition metal group. The “heavy” term differentiates them from the smaller and significantly lower density transition metals of the 4th Period. Curiously, the companion term “light transition metals” does not seem to have the same degree of popularity.
Figure 5.5 The heavy transition metals.
Figure 5.6 The refractory metals.
Refractory Metals
Of significant importance in metallurgy and materials science are the refractory metals. The metals fitting this category, shown in Figure 5.6, are niobium and tantalum of Group 5, molybdenum and tungsten of Group 6, and rhenium of Group 7 (if technetium had stable isotopes, it would also fit this category). These metals have melting points above 2000°C and are very hard. In addition, they are chemically inert and have a relatively high density.
Noble Metals
The chemical classification of a noble metal is a metal that is highly resistant to oxidation. This category is usually accepted as consisting of the elements: ruthenium and osmium of Group 8; rhodium and iridium of Group 9; palladium and platinum of Group 10; and gold of Group 11 (Figure 5.7).
Figure 5.7 The noble metals.
The synthesis of a compound containing a gold–xenon cation, [AuXe4]2+ [32], ignited an interest in the formation of “noble metal–noble gas” species. However, this raised the issue of which exactly were the noble metals. In a subsequent article on gold–xenon compounds, an analogous mercury(II)–xenon compound was synthesized. This suggested that mercury should also be considered as a noble metal [33]. In another source, it was stated that the noble metals consisted of the platinum metals plus the coinage metals of silver and gold [34]. However, to this Author, the Ru–Os–Rh–Ir–Pd–Pt–Au cluster seems to make the most sense as comprising the “noble metals.”
Other Appellations
There are many other terms that have appeared in the context of the chemical elements. In this Author’s view, there are three worthy of permanent status; one well known, and two lesser known.
Figure 5.8 The rare earth metals.
Rare Earth Metals
In Chapter 4, the controversy over the membership of Group 3 was described. Fortunately, membership of the rare earth metals (also called the rare earth elements) is unambiguous. These are the 15 lanthanoid metals plus scandium and yttrium of Group 3 as shown in Figure 5.8. Though they are far from “rare,” their ores are found in significant concentrations only in certain specific locations. All of the rare earth metals are characterized by a predominance — though not exclusivity — of the +3 oxidation state, as will be discussed in Chapter 12. Restrepo has shown that in terms of an ordered hypergraph, the rare earth elements do indeed form a complete and unique set of elements [35].
Superheavy Elements
Though the term “heavy metals” is largely meaningless unless defined in context, the use of the term superheavy elements has become IUPAC recommended [36]. Commonly abbreviated SHE, the term is given to the postactinoid elements, that is, the elements commencing with rutherfordium, element 104 and continuing to element 126. This limit was chosen presumably in the context of the “magic number” of 126 (see Chapter 1). For any element 127
