`PCl_5` is known but `PI5` is not known. Why?

Explanation

Nitrogen (N) has seven electrons and phosphorus has fifteen electrons. Their electronic configuration is as follows :
`N (Z = 7) = `1s^(2) 2s^(2) 2p^(3)`, P (Z = 15) = `1s^(2)` 2s^(2) 2p^(6) 3s^(2) 3p^(3)`
In case of nitrogen, there is absence of d- orbitals. Hence, N cannot extend its covalency beyond three. While in case of phosphorus, vacant 3d-orbitals are present. Therefore, P can increase its covalency upto five. Therefore, `PCl_(5)` can exist but `NCl_(5)` cannot.