Homoatomic single bonds

In the case of dinitrogen and dioxygen, the multiple bonds were homoatomic, and so there is no permanent polarisation across the central bond. In the case of diimide, exactly the same atoms appear on each side of the central bond, and so again there is no permanent charge separation. However, temporary and induced polarisations may occur in each of these cases as may occur in homoatomic single bonds. 

Calculation of A requires the bond energy Da-b of the actual single bond and the bond energies Da-a and Db-b of the homoatomic single bonds A—A and B—B. 

For all other elements, the homoatomic single bond energies cannot be obtained directly from thermochemical data. In order to obtain such bond energies, Pauling has made the following assumption which will be referred to as the second Pauling postulate. 

There is no general criterion for the apphcabiUty of the second postulate to compounds AB for which only one of the homoatomic single bond energies Da-a and Db-b is available. A necessary but not... 

Table 2.1 lists a few typical single-bond energies for homoatomic (A—A or B—B) and heteroatomic (A—B) bonds. 

For elements which form diatomic molecules A2 with single bonds, Da-a is equal to the energy of dissociation. This applies to H2, the halogens, the alkali metals, silver and copper. A few other elements form homoatomic molecules A . In the P4—AS4—and Sb4-molecules, for instance, each element is bonded to three other atoms, as in compounds of type AB3. As there are six A—A bonds in the A4-molecules, the bond energy Da-a is one sixth of the energy of dissociation for the reaction... 

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