Ionic-bond model table

Knowledge of the interatomic distances allows the experimental valence of an atom to be calculated using tables of bond-valence parameters. If the apparent valence of an atom is higher than that expected for an ionic bonding model, say... 

The type of bonding found in metals is quite different from that in other crystals. As we compare the various main group and transition metals in the periodic table we see only small differences in electronegativity. So, there is little tendency for ionic bonding in metals. The electronic configurations of metal atoms, even in the transition metals, do not have nearly-filled subshells, so there is little tendency to form covalent bonds by sharing electrons to achieve a stable octet. The familiar classical models of chemical bonding (see Chapter 3) do not extend to metals. 

To test our bonding model, as before, separate calculations must be made for the purely ionic case and the purely covalent case. However, from the outset we will assume that CN4 means covalent, and CN6 means ionic. The main test will be to see if we can match the values of in Table 5.5 with theoretical results, but... 

Accordingly, we identify 12.4 eV as the value of Eg from our bonding model. This closely agrees with Eg = 12.2 eV, found from the dielectric constant. Table 5.6 contains comparisons of the same kind for ionic compounds. The agreement is surprisingly good, since the excited states contributing to the polarizability need not be the same as the lowest excited states in the UV spectrum. 

There are also molecules that are exceptions to the octet rule because one of the atoms has fewer, rather than more than, eight electrons in its valence shell in the Lewis structure (Figure 1.19). These molecules are formed by the elements on the left-hand side of the periodic table that have only one, two, or three electrons in their valence shells and cannot therefore attain an octet by using each of their electrons to form a covalent bond. The molecules LiF, BeCl2, BF3, and AIC13 would be examples. However, as we have seen and as we will discuss in detail in Chapters 8 and 9, these molecules are predominately ionic. In terms of a fully ionic model, each atom has a completed shell, and the anions obey the octet rule. Only if they are regarded as covalent can they be considered to be exceptions to the octet rule. Covalent descriptions of the bonding in BF3 and related molecules have therefore... 

When we apply the initial model to double compounds with ionic-covalent and metallic bonds, the calculations were made based on the equation (2) for 45 binary structures in the assumption of paired inter-atomic interaction. The results of some of them are given in table... 

In a rough model one might expect that, if the UOl group is surrounded by more electronegative cations (i.e. if the bonding in UOe is relatively ionic), the emission is more towards the yeUow. Examples in Table 10 show that there is some experimental evidence for such a model. Independently of the question whether this model is correct or not, it is clear that the position of the c. t. transi-... 

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