(c) The building-up principle for molecules
We use the building-up principle in conjunction with the energy level diagram in the same way as for atoms. The order of occupation of the orbitals is the order of increasing energy as depicted in Figs 3.14 or 3.18. Each orbital can accommodate up to two paired electrons. If more than one orbital is available for occupation (because they happen to have identical energies, as in the case of pairs of π orbitals), then the orbitals are occupied separately. In that case, the electrons in the half-filled orbitals adopt parallel spins (||), just as is required by Hund's rule for atoms.
With very few exceptions, these rules lead to the actual ground-state configuration of the Period 2 diatomic molecules. For example, the electron configuration of N2, with 10 valence electrons, is
N2 :………………
Molecular orbital configurations are written like those for atoms: the orbitals are listed in order of increasing energy, and the number of electrons in each one is indicated by a superscript. Note that π^4 is shorthand for the occupation of two different π orbitals.