Explain :
`NCl_5` does not exists but `PCl_5` exists.

Explanation

The valence shell electronic configuration of both N and P are follows :

Since nitrogen atom has no vacant 2d orbitalx, it cannot extend its ovalence to five. Therefore, a molecule of `NCl_(5)` does not exist. But in case of phosphours, an electron can be3 promoted from 3s filled orbital to 3d vacant orbitals. Therefore, phosphorus can extend its covalency to fivel. Thus, a molecule of `PCl_(5)` can exist.