11 - Second row homonuclear diatomics

Summary

For many years chemists have considered that an understanding of the theory of the bonding of the homonuclear diatomic molecules from the second row of the periodic table is central to understanding all of bonding, and we consider these stable molecules first from our VB point of view. The stable molecules with interesting multiple bonds are B₂, C₂, N₂, O₂, and F₂. Of course, F₂ has only a single bond by ordinary bonding rules, but we include it in our discussion. Li₂ is stable, but, qualitatively, is similar to H₂. The question of the existence of Be₂ is also interesting, but is really a different sort of problem from that of the other molecules. Of the five molecules we do consider, B₂ and C₂ are known only spectroscopically, while the other three exist at room temperature all around us or in the laboratory.

Atomic properties

Before we launch into the discussion of the molecules, we examine the nature of the atoms we are dealing with. As we should expect, this has a profound effect on the structure of the molecules we obtain. We show in Fig. 11.1 the first few energy levels of B through F with the ground state taken at zero energy. The L-S term symbols are also shown. The ground configurations of B and F each support just one term, P, but the other three support three terms. All of these are at energies below ≈4.2 eV (relative to their ground state energies).