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 and onward, the cumbersome term beyond superheavy elements was recommended.
Ephemeral Elements
Overlapping — but not identical to — the superheavy elements are the ephemeral elements [37]. The concept of ephemerality derives from the Greek word “ephemeros,” literally meaning “lasting only one day.” The term “ephemeral elements” is not widely used, but it is a useful concept to indicate elements for which there is no known isotope with a half-life of more than a day (hence, following the Greek meaning). In this way, we can roughly identify those elements for which the chemistry is either theoretical, or else based upon the study of individual atoms. At the time of writing, the ephemeral elements are those shown in Figure 5.9.
An isotope of dubnium, element 105, 268Db, has a half-life of 28 hours, thus dubnium is excluded from the ephemeral element list. This list may diminish if longer lived isotopes of any of these elements are synthesized. On the other hand, it will increase if elements beyond z = 118 are discovered. Confusingly, the term “ephemeral elements” has been used to describe claims for new elements that were subsequently shown to be false [38].
Figure 5.9 The ephemeral elements (at the date of writing).
Commentary
It is quite remarkable that well into the 21st century there are still disagreements on which elements fit into which categories — and the terminology. Hopefully, this chapter has provided some “food for thought.”
References
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Chapter 6
Isoelectronicity
Isoelectronicity is a concept widely used in chemistry — or often misused. It is difficult to think of a more ambiguous term. This chapter commences with a summary of the various ways in which isoelectronic has been used, together with the modifiers that have been applied to delineate its meaning in a specific context. After providing a consistent series of definitions, the richness of isoelectronic relationships, in all their forms, will be explored.
The term isoelectronic is ubiquitous in chemistry. But what precisely does a writer mean when they use the term? There is no better way to start the discourse than the immortal words of Humpty Dumpty to Alice, in Alice Through the Looking Glass, as shown in Figure 6.1 [1]:
Figure 6.1 One of the classic exchanges in Alice Through the Looking Glass (from Ref. [1]).
Chemists have shown a strong propensity to use the Humpty Dumpty approach to the word “isoelectronic.” Often is the word used, rarely is the word defined. The Reader is left to their own intuition as to which sort of isoelectronicity the author of a specific research paper intended. In this chapter, clarity will be provided by the use of modifiers to remove the need to: “. . . make words mean so many different things.”
Historical Definitions of Isoelectronic
The earliest discussion of the isoelectronic concept seems to date from 1919. Langmuir, in his 66-page discourse on the arrangement of electrons in atoms and molecules, commented on patterns among compounds [2]. Though some of the terms he used were archaic, the discussions clearly laid the foundation of the isoelectronic concept. Langmuir employed the symbol, N, to denote the total number of electrons, while E was used to represent the outer valence electrons. He noted the resemblance of carbon monoxide to dinitrogen, and extended the similarity to hydrogen cyanide as they all have N = 14 and E = 10. Langmuir noted many other isoelectronic linkages, such as that of cyanogen, (CN)2, and dichlorine. Of specific note, he saw the predictive ability of the isoelectronic concept [3]:
For example, since phosphorus and nitrogen atoms contain the same number of electrons in their shells, the simple octet theory represented by Equation 2, indicates that nitrogen compounds corresponding to all known phosphorus compounds could exist and vice versa.
In 1933, Penney and Sutherland [4] made the isoelectronic concept the focus of their work on the shapes of triatomic species, though their definition was somewhat circuituitous: one simply counts the number of electrons in the molecule not in closed electron shells. The authors showed that all triatomics with 16 “molecular or valence” electrons, as they called them, had a linear geometry about the central atom while the 18-electron species were “wide-angled.”
Use of the term isoelectronic principle seems to have become prevalent in the 1950s and 1960s. Moody [5] used the principle to explain the identical structures of all the following highest oxidation-state fluoride species:
Moody cited Penney and Sutherland for his own definition of isoelectronic:
Simple structures containing the same number of valency electrons are represented by the same bond diagram.
Coulson, in his classic book, Valence, described the principle as being [6]:
Two molecules with the same number of valence electrons are isoelectronic: and the principle states that such systems will have similar molecular orbitals.
Brown had a narrower definition [7]:
Two molecular species with the same number of atoms and the same total number of valency electrons are said to be iso-electronic, and the isoelectronic principle states that such molecular species will have similar molecular orbitals and molecular structures.
While companion used isoelectronic in an all-encompassing manner to indicate any cluster of atoms whose total electrons added up to the same value [8]:
The noble-gas atom Ne and the molecules HF, H2O, NH3, and CH4 all have the same number of electrons (are isoelectronic).
Bent had an equally broad definition [9]:
As a general rule, or principle, molecules are isoelectronic with each other when they have the same number of electrons and the same number of heavy-atoms.
Modern Definitions of Isoelectronic
Most first-year university texts use the term “isoelectronic” (without definition) to justify the formation of ions by the main group elements. Specifically, cations are formed that are isoelectronic with the preceding noble gas; anions are formed that are isoelectronic with the following noble gas.
However, there does seem to be some convergence on isoelectronic definitions. For example, Housecroft and Sharpe [10] provided the restrictive definition of:
Two species are isoelectronic if they possess the same total number of electrons.
They continued:
The word isoelectronic is often used in the context of meaning “the same number of valence electrons,” although strictly such usage should always be qualified; e.g. HF, HCl, and HBr are isoelectronic with respect to their valence electrons.
While Massey [11] gave a similar but subtly different definition of:
Species that have identical ligands and the same number of electrons on the central atom are said to be isoelectronic and almost invariably they have the same molecular structure.
He, too, looked upon the valence electron counting as a different case:
The principle can often be exte
nded to include species that are not strictly isoelectronic but in which the central atoms have the same number of outer electrons rather than the same total number. The and ions are pseudo-isoelectronic with tetrahedral SiCl4 and can be expected to have the same structure.
Proposed Definition
Throughout science, the prefix iso- means “the same.” Thus, strictly speaking, the term “isoelectronic” should simply mean the same number of electrons, period. This all-encompassing meaning needs to be narrowed down if isoelectronic is to have a useful role in identifying chemical patterns and trends. For true (or exact) isoelectronic status, the most logical definition would be the following:
Species (atoms, molecules, ions) are isoelectronic with each other if they have the same total number of electrons and of valence electrons together with the same number and connectivity of atoms.
This definition will be used in the following sections of this chapter. One of many more unusual examples of truly isoelectronic species are [In(NO3)4]− and [Sn(NO3)4].
Of course, a new term is needed to describe species that have the same number of valence electrons but not necessarily the same total number of electrons. Almost 50 years ago, Gillis had proposed homoelectronic [12] while Massey favored the term pseudo-isoelectronic [11]. Neither of these terms explicitly identifies their meaning. Elliott and Boldyrev [13] have used the term valence-isoelectronic. This would seem the most appropriate term as the Reader is immediately aware of the meaning without need to resort to a chemical dictionary. To provide examples for clarity, OCO and NCO− are isoelectronic, while OCO, OCS, and SCS are valence-isoelectronic. Thus a definition of valence-isoelectronic is:
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