• # question_answer A molecule of$PC{{l}_{5}}$ exists while that of$NC{{l}_{5}}$  does not.

The valence shell electronic configurations of both N and P are as follows : Since nitrogen atom has no vacant 2d orbitals, it cannot extend its covalency to five. Therefore, a molecule of$NC{{l}_{5}}$does not exist. But in case of phosphorus, an electron can be promoted from 3s filled orbital to 3d vacant orbitals. Therefore, phosphorus can extend its covalency to five. Thus, a molecule of $PC{{l}_{5}}$ can exist.