Chemical bond model inorganic solids

The set of all observed structures is necessarily a highly biased selection of all conceivable structures, but any proper model of chemical bonding in inorganic solids should be able to account for the structures that do not exist as well as for those that do. 

A promising simplification has been proposed by Bader (1990) who has shown that the electron density in a molecule can be uniquely partitioned into atomic fragments that behave as open quantum systems. Using a topological analysis of the electron density, he has been able to trace the paths of chemical bonds. This approach has recently been applied to the electron density in inorganic crystals by Pendas et al. (1997, 1998) and Luana et al. (1997). While this analysis holds great promise, the bond paths of the electron density in inorganic solids are not the same as the more traditional chemical bonds and, for reasons discussed in Section 14.8, the electron density model is difficult to compare with the traditional chemical bond models. 

How must the rules of the chemical bond model be modified when used to describe inorganic solids In both versions of the model, each atom is assumed to have an atomic valence which corresponds to the number of electrons (positive valence) or holes (negative valence) that are available in the valence shell of the neutral atom. In the original bond model the valence simply represented the number of bonds that the atom formed, i.e. its coordination number, but compounds were later discovered that contained double and triple bonds. In these compounds the coordination number was different from the atomic valence and each bond was associated with more than one unit of valence. However, in all cases the sum of the valences (or strengths) associated with the bonds around each atom was found to be equal to the valence of the atom. This is the principal rule of the chemical bond model and is known as the valence sum rule. ... 

This book is divided into four parts. Part I provides a theoretical derivation of the bond valence model. The concept of a localized ionic bond appears naturally in this development which can be used to derive many of its properties. The remaining properties, those dependent on quantum mechanics, are, as in the traditional ionic model, fitted empirically. Part II describes how the model provides a natural approach to understanding inorganic chemistry while Part 111 shows how the limitations of three-dimensional space lead to new and unexpected properties appearing in the inorganic chemistry of solids. Finally, Part IV explores applications of the model in disciplines as different as condensed matter physics and biology. The final chapter examines the relationship between the bond valence model and other models of chemical bonding. 

An approach to chemical bonding that is currently attracting attention is that based on an analysis of electron densities calculated from quantum mechanics or measured using X-ray diffraction. Since the electron density shows how the electrons are distributed, it gives a better physical picture of the nature of chemical bonding than other models. It has been admirably described by Bader (1990) and, for inorganic solids, by Pendas et al. (1997, 1998) and Luana et al. (1997), but it is only necessary here to give a brief account of the approach to show why it is difficult to relate its concepts to those of the bond valence model. 

The retention model developed by Eon and Guiochon [7,8] to describe the adsorption effects at both gas-liquid and liquid-solid interfaces, which was later modified by Mdckel et al. [6] to account for the retention at chemically bonded reversed-phase materials in HPLC, is not applicable to ion chromatography. But if the dependence of the capacity factors of various inorganic anions on the column temperature is studied, certain parallels with HPLC are observed. The linear dependences shown in Fig. 3-2 are obtained for the ions bromide and nitrate when the In k values are plotted versus the reciprocal temperature (van t Hoff plot). However, in the case of fluoride, chloride, nitrite, orthophosphate, and sulfate, the k values were found to be constant within experimental error limits in the temperature range investigated. Upon linear regression of the values in Table 3-1, the following relations are derived for bromide and nitrate ... 

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