Applications of the valence matching principle

It is instructive, for example, to compare the structural chemistry of and Zn + which have identical charges and similar sizes. Mg + is hard and rarely occurs in other than six coordination (bonding strength of 0.33 vu), but, because Zn + has a filled d shell which can readily mix with the electrons of the valence shell, it is soft and is found equally often in four- and six-coordination (bonding strength between 0.33 and 0.50, average 0.40 vu, see Section 6.5). A more complete discussion of the behaviour of soft cations is given in Chapter 8. 

Many examples of the valence matching principle will be found in the following chapters, but a couple of examples are given here to illustrate its power. 

We often ask the question Why does such and such a compound exist but more rarely do we ask the question Why does such and such a compound not exist , yet the answer to the second question may be more revealing than the answer to the first. A number of years ago, Dent-Glasser (1979) published a paper entitled Non-existent silicates in which she noted that there were many stoichiometrically allowed silicate structures that had never been observed. In particular she pointed out that the alkali metals rarely formed orthosilicates such as Na4Si04, and transition metals rarely formed highly condensed silicates such as NiSi40g. Dent-Glasser s list of known silicates is shown in Fig. 4.6 

It is now time to show how the ideas developed in the previous chapters can be applied to real chemical systems. Apart from a few simple gases, the materials we come across in everyday life are either solids or liquids. A proper understanding of the chemistry of the solid state requires some appreciation of the role of symmetry in crystals and is therefore deferred to Part III. This chapter explores the use of bond valences to understand the simpler chemistry of liquids. Most of this chapter is devoted to the chemistry of aqueous solutions because water is not only the solvent of choice for polar systems but also the most common solvent in our environment. 

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