Homopolar compounds

Carbon tetrachloride was cited earlier as an example of a homopolar compound here, however, it is treated as a heteropolar compound to see how far the theory of the heteropolar bond can be applied. Only when difficulties are encountered need we look for another explanation. 

Penetration by the hydrogen ion certainly alters the character of an ionic compound, and there is much to be said for considering the altered compound as a homopolar compound, which will be dealt with in greater detail later, but treating HC1 as a true homopolar compound unfortunately does not enable us to predict any of its properties. It is, therefore, instructive to consider it as a distorted ionic structure, and to attempt to predict its properties in a qualitative manner. HG1 always behaves as a normal ionic compound in its reactions with oxides, which will be referred to again in Section 27. 

These results may be generalized in a Principle of Correspondence There exists an isomorphism between the ion-packing model of heteropolar compounds and the electron-pair model of homopolar compounds. This correspondence is summarized in Table I. 

Zinc forms a bipositive ion with an 18 electron structure and a moderately small radius (0.74 A). The large ion (1.84 A) is considerably polarised by it and the two forms of ZnS are predominantly homopolar, since both atoms can adopt sp hybridisation and pool their valence electrons. Blende is cubic (Fig. 89), wurtzite hexagonal (Fig. 90), there being an average of 4 electrons per atom for bond formation as in diamond. Homopolar compounds of this type are formed by many pairs of elements whose valency electrons total 8. They are called adamantine compounds and have either a blende or wurtzite structure (see Table 22). 

The A2 metals and the elements of the earlier B subgroups (Bj metals) form the electron compounds already discussed. With the metals of the later B subgroups the A2 metals, like the Aj, tend to form intermetallic phases more akin to simple homopolar compounds, with structures quite different from those of the pure metals. The nickel arsenide structure has, like typical alloys, the property of taking up in solid solution a considerable excess of the transition metal. From Table 29.12... 

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