RELATION BETWEEN COVALENT AND IONIC BONDS

There can be resonance between covalent and ionic states. In the molecule H H a complete shift of the electron pair to the left would have the effect to make the left-hand H atom a negative ion, leaving the right-hand one as a positive ion. Next to the state H H there will be two others, H H+ and H+H , which closely resemble the electrostatic model for the H2 molecule. Since the three states are resonating, the states H H+ and H+H will make a contribution to the bonding energy, too in this case, however, their contribution will be relatively small, because the energy of the covalent state H H certainly is much lower than that of the ionic states. H H+ and H+H-. 

The existence of the states H H+ and H+H is not in disagreement with the properties of the hydrogen molecule, e.g. the absence of a dipole moment. Since the states H+H and H H+ certainly have the same energy, they are equally frequent and the total state 

Now let us consider the compound HF, for which again there are different states, H F, H+F and H F+. Since hydrogen has a smaller electron affinity than fluorine, and a higher ionization energy, 

In the preceding sections it was found that the octet configuration, either with single or multiple bonds, is very often observed in the compounds of the elements in the end of the periods. Octet configurations, however, can occur, as well, in the first subgroups, where 

Yet octets are not the only stable electron configurations. It was remarked before that d electrons probably contribute to the bond formation of molecules like IF5, TeFe, etc. see Section 54). Electronic configurations, in which d electrons play a part, are of great importance in the bond formation of all elements between the aluminium and the copper group, all of which elements have deficient d shells. 

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