Periodicity and Peculiarity in 120 First- and Second-Row Diatomic Molecules

The ground and very low-lying excited states of all 120 first and second row diatomic molecules are surveyed. Three quarters of these molecules 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 atoms are also presented. The Aufbau principle, combined with standard energy ordering for the valence molecular orbitals, is able to properly account for the ground-state term symbols of all but twenty of the diatomics studies. The twenty exceptions produce higher than expected ground state spin multiplicity and arise when there are 4-5 or 7-8 valence electrons and group 3, 4, or 5 (but not group 6 or 7) atoms are involved.