LITMIR.BIZ

Скачать книгу

number that — like other measures in the applied sciences — has a complex referential background. It is conceptually rooted in the realm of chemical reactivity on the one hand, but it is supplied by the physics of isolated particles on the other.

       Electronegativity Scales

Though electronegativity is commonly associated with Pauling and his scale [10], there are many other scales, including the widely used Allred–Rochow [11]. All numerical ranges share a common definition:

      Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) toward itself.

If electronegativity is such a vague concept with disparate definitions, why is it so important, and why has it not been replaced by a clearly quantifiable atomic parameter? It was Rodebush in 1924 who provided an elegant answer [12]:

       I had hoped that we might be able to substitute electron affinity or ionizing potential for the wretched term electronegativity, but these quantities are measured for the gaseous state and our ordinary chemical properties are concerned with the condensed phases.

Even in the 21st century, the nature of electronegativity is a continuing topic of discourse and debate [13, 14].

       The Sanderson Electronegativity Scale

In addition to the well-known scales, in an oft-overlooked series of articles, Sanderson developed and applied his own electronegativity scale [15]. Though his value system did not gain acceptance, the plot that he generated provided a useful qualitative overview of comparative electronegativities (Figure 2.1).

      Sanderson indicated in his plot that the “secondary” linkages progressed from the 3rd Period to the 4th Period. Of particular note in the context of Chapter 9, he high-lighted in the article the closer resemblance in electronegativity (and electron configuration) of aluminum with the Group 3 elements, and of silicon with the Group 4 elements. The plot also showed the “saw-tooth” pattern in electronegativities that are observed when descending groups in the later main group elements.

      Figure 2.1 Patterns in the Sanderson electronegativities of the elements (adapted from Ref. [15]).

       The van Arkel–Ketelaar Bond Triangle

One of the many applications of the electronegativity concept is that of the bond triangle [16]. Devised by van Arkel, then improved upon by Ketelaar, the triangle categorizes elements, alloys, and compounds according to their degrees of ionic, covalent, and metallic character [17]. The triangle was originally more conceptual than detailed. However, using electronegativity values, Jensen semiquantified the diagram as shown in Figure 2.2 [18].

Sproul and his coresearchers have refined and updated the bond triangle, both from a practical and theoretical perspective [19]. The triangle has become popular as a means of assigning bonding in newly synthesized solid-state compounds, such as Y₃AlC [20] and the AgXY₂ family where X is a Group 13 element, and Y is a Group 16 element [21]. In addition, Sproul has attempted to revive the triangle in the context of teaching bond type at university level [22].

      Figure 2.2 A semiquantitative bond triangle (adapted from Ref. [15]).

In Britain, an advanced high school qualification, called the Pre-U has been devised by the University of Cambridge International Examinations. A key part of the chemistry Pre-U is devoted to the van Arkel–Ketelaar Triangle as a basis for the bonding discussion [23].

       Oxidation State

Another rather nebulous — but very useful — property is oxidation state [24]. To use the deductive approach as will be shown shortly, the value is determined upon the basis of comparative electronegativities.

According to Jensen, the first use of the term was by Talbot and Blanchard in 1907 [25]. Actually, there are two terms: “oxidation state” and “oxidation number.” In 1990, the International Union of Pure and Applied Chemistry (IUPAC) provided a set of recommended lengthy rules by which the “oxidation state” of any element could be calculated (given in Arabic numerals). IUPAC reserved the term “oxidation number” for the central atom in a coordination compound (given in Roman numerals) [26]. This latter usage corresponded to the use of Roman numerals in the Stock system of inorganic nomenclature.

The 1990 IUPAC definition of oxidation state was critiqued by Loock, who pointed out the significant shortcoming that the IUPAC method would only give an average oxidation state if there were two atoms in a compound in dissimilar environments. Instead, he proposed a succinct definition based on Pauling’s use of the term [27]:

      The oxidation state of an atom in a compound is given by the hypothetical charge of the corresponding atom ion that is obtained by heterolytically cleaving its bonds such that the atom with the higher electronegativity in a bond is allocated all electrons in this bond. Bonds between like atoms (having the same formal charge) are cleaved homolytically.

Jensen strongly supported the Pauling/Loock definition, adding a summary of the difference in the approaches [28]:

       …the memorized IUPAC rules …are ultimately traceable to an attempt to assign oxidation values based solely on the use of a species’ compositional formula, whereas the Pauling[/Loock] approach requires instead a knowledge of the species’ electronic bonding topology as represented by a Lewis diagram.

A variation of the Pauling/Loock approach — the “exploded structure method” — was devised by Kauffman [29], though its shortcomings were described by Woolf [30]. IUPAC subsequently reversed their view, changing from a mechanical rule-based approach (still widely used in textbooks) to the electronegativity–Lewis structure approach of Pauling/Loock. However,

Скачать книгу