Valence Compounds and the Ionic Model

A valence compound is defined as one in which every atom is uniquely labeled as either a cation or an anion, and every bond has a cation at one end and an anion at the other, i.e., there are no bonds between two cations or between two anions. 

A bond network with this property is said to have a bipartite graph, and a corollary to this definition is that the bond network of a valence compound crmtains only even-membered rings since an odd-membered ring necessarily contains a homoionic bond. 

The valence shell of the cation carries a small charge and is linked to its core by a weak electric field. Consequently it lies far from the nucleus. On the other hand, the valence shell of the anion carries a large charge and is held closer to the nucleus. As a result, the bond overlap between the cation and the anion occurs closer to the anion as shown in Fig. 4. Quantum mechanics places restrictions on the number of electrons that can be accommodated in the overlap region, and while the nature of these restrictions is complex, it is conveniendy summarized by the octet rule (11) which states thaP  

All the valence electrons of the cations must be accommodated within the valence shells of the anions. 

This leads to an interesting extension of the core-and-valence-shell picture. Where the valence of a bond was previously defined as the flux linking the cation core to the electrons it contributes to the bond, in the ionic model it is defined by the same flux which now links the cation to the anion. If the positions of the atoms in the array are known from experiment, this flux can be directly calculated. The calculation involves extensive computation, but Preiser et al. [17] have shown that in stmctures in equilibrium, the correlation between the bond flux and bond length is the same as the correlation that had previously been observed between the bond valence and bond length, showing that the electrostatic flux and bond valence are indeed the same. 

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