Tutoring chemistry, back bonding might come up.

Back bonding is a concept from molecular orbitals. In a molecule with a central 3rd-period nonmetal (such as sulfur or phosphorus), double bonding is possible so that the central atom exceeds an octet. The phenomenon is back bonding.

The reason back bonding is possible is that the central atom, being a third-period nonmetal, has (vacant) d-orbitals. It is thought that a peripheral atom bonding with it can form a second bond by merging one of its p-orbitals with one of the central atom’s vacant d-orbitals. The peripheral atom then contributes two electrons to this p-d hybrid; the result is a pπ-dπ bond, aka back bonding.

Back bonding is thought to explain shorter-than-expected P-O bonds in phosphoric acid and S-O bonds in sulfuric acid.

Source:

Mortimer, Charles E. Chemistry, sixth edition. Belmont: Wadsworth, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.