the molecular orbitals of a molecule are strictly non-bonding in nature. However, in the context of localized molecular orbitals, the concept of a filled, non-bonding orbital tends to correspond to electrons described in Lewis structure terms as "lone pairs."

There are several symbols used to represent unoccupied non-bonding orbitals. Occasionally, n* is used, in analogy to ?* and ?*, but this usage is rare. Often, the atomic orbital symbol is used, most often p for p