31. Sodium chloride is typically thought of as an ionic compound, but one can draw a molecular orbital diagram for NaCl, as well. Use the provided MO diagram and answer the following questions: a) label the orbitals as σ, σ*, π, π*, or nb; b) show the electron occupation in the molecular orbitals; c) sketch the highest energy orbital, showing both shape and phase; d) is this MO diagram consistent with NaCl being ionic? Why or why not?
All of the electron density from the valence electrons is located in orbitals primarily centered on the Cl atom. Thus, the charge on the Cl atom is close to –1, as expected for an ionic substance. The Na centered orbitals are unfilled, leaving the charge on the Na close to +1. Finally, there is little overlap between the Cl orbitals and Na orbitals (because of the substantial energy difference in atomic orbital energies) so there is essentially no sharing of electron density. These are the requirements for an ionic compound, so this MO diagram is consistent with traditional views of bonding in NaCl.