Antibonding, Nonbonding and Bonding orbitals

Recently I came across nonbonding orbitals. I know something about bonding and antibonding orbitals and their formation, but I have no idea about nonbonding orbitals. Could someone explain their formation and the difference when compared with the other two?

• Could you edit your question to include where you came across nonbonding orbitals? Could you also describe your understanding of bonding and antibonding orbitals to give an indication what level of theory would be helpful as an answer? – Karsten Theis Jul 19 '19 at 19:58

Consider hydrogen fluoride. The 1s orbital on hydrogen has the same symmetry as the $$p_z$$ orbital on fluorine. These mix into a bonding and antibonding orbital. Hydrogen however does not offer any other occupied atomic orbitals with matching symmetry for the $$p_x$$ and $$p_y$$ atomic orbitals on fluorine. Therefore, these orbitals remain as non-bonding. Furthermore, the 2s fluorine atomic orbitals are much lower in energy than hydrogen's 1s orbital, so this does not mix appreciably either, and can therefore also be treated as non-bonding.