This set of molecular orbitals and the energy-level diagram are valid for all diatomic molecules made from second-row atoms, from Li, through CO, to
F and the nonexistent Ne. Although CO has two different atoms, the energy levels of the atoms are so similar that we can treat the molecule like we do
N. Filling diagrams for most of these molecules are shown on the next page. Li has two bonding electrons, which occupy the lowest molecular orbital. Be has another two electrons, which fill the antibonding orbital, cancel the bonding of the first two electrons, and cause the molecule to fall apart. B has two more electrons, and these go into the and bonding orbitals. Because of the repulsion between electrons, one goes into each orbital, giving B two impaired electrons. These two levels are filled by two more electrons in C, and all of the bonding orbitals are full in N and CO. In 0 one electron is added to each of the and antibonding states, in F these states are filled, and the final antilionding orbital is filled in Ne.