PERIODICITY AND PECULIARITY IN 120 FIRST-ROW AND SECOND-ROW DIATOMIC-MOLECULES

The ground and very low-lying excited states of all 120 first- and sec ond-row diatomic molecules are surveyed. Three quarters of these molec ules have had their ground state term symbols reliably experimentally determined. However, one quarter remain predicted only theoretically. For all 120 species, the best available experimental (where known) and theoretical values for the dissociation energies to ground state atom s are also presented. The Aufbau principle, combined with standard ene rgy ordering for the valence molecular orbitals, is able to properly a ccount for the ground state term symbols of all but 20 of the diatomic s studies. The 20 exceptions produce higher than expected ground state spin multiplicity and arise when there are 4-5 or 7-8 valence electro ns and, group 3, 4, or 5 (but not group 6 or 7) atoms are involved.