the photoelectron spectrum is consistent with the need to accommodate eight electrons in the orbitals. The electrons enter the molecular orbitals in increasing order of energy, starting with the orbital of lowers energy, and talking note of the requirement of the exclusion principle that no more than two eletron can occupy any one orbital. the first two electrons enter 1a and fill it. The next four enter the doubly degenerate 1e orbitals and fill them. the last two enter the 2a orbital, which calculations show is almost nonbonding and localized on the N atom. The resulting overall ground-state electron configuration is therefoe 1a2 1e4 2a2. No antibonding orbitals are occupied, so the molecule has a lowe energy than separated atoms. the conventional description of NH3 as a molecule with a lone pair is also mirrored in the configuration the HOMO is 2a, which is largely confined to the N atom and makes only a small contribution to the bonding. We have already seen that lone pair electrons play a considerable role in determinig the shapes of molecules. the extensive vibrational structure in the 11 eV band of the photoelectron spectrum is consistent with this observation, for photoejection of a 2a electron removes the effectiveness of the lone pair and the shape of the ionized molecule is considerably different from that of the NH3 itself. Photoionization therefore results in extensive vibrational structure in the spectrum.