Homonuclear diatomic molecules, electron dissociation energy

The isoelectronic equivalence is the simplest procedure for estimating electron affinities. It was applied to H2 and I2 and to the atomic electron affinities. Species with the same outer electronic configuration should have similar electron affinities and bond dissociation energies. This results in the relative constancy of the electron affinities of a given family of atoms. The equivalence of the bond dissociation energies for the X2( ) and Rg2(+) ions is also based on this principle. The systematic variation of the electron affinities of the homonuclear diatomic molecules is another example. 

Figure 9.1 Electron affinities of the elements, electron affinities, and bond dissociation energies of the homonuclear diatomic molecules in the form of a Periodic Table [1],...

Two branches are found. Dissociation energies are less than limyv oo 7V> total number of electrons is uneven. They are larger than lim y oo 7V > total number of electrons is even. The first property is due to the occupation of a nonbonding orbital, the second property corresponds with the experimental observation that the bond energies of homonuclear diatomic molecules are usually stronger than the energy of an atom-atom bond in the bulk. The atom-atom distance also usually proves smaller in a homonuclear molecule than in the bulk. Both properties arise from the weaker metal-metal bond in the bulk of a metal. This is due to delocalized nature of metal electrons, as we will explain in more detail later. 

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