`PCl_(5)` is known but `NCl_(5)` is not known.
Or Nitrogen does not form pentahalide.

Explanation

Electronic configuration of P is `1s^(2)2s^(2)s^(6)3s^(2)" "3p_(x)^(1)3p_(y)^(1)3p_(z)^(1)3d^(0)`. Thus, P has empty 3d orbitals to

which the 3s electron can be excited to have five half-filled orbitals needed for formation of `PCl_(5)`. Thus `PCl_(5)` is known.
In contrast, electronic configuration of N is `1s^(2)2s^(2)" "2p_(x)^(1)2p_(y)^(1)2p_(z)^(1)`. Since the valence shell of nitrogen has n=2, therefore, it cannot have d-orbitals. However, if one of the 2s electrons is excited to 3s orbital, five

half-filled orbitals needed to form `NCl_(5)` can still be obained. But such an excitation is thermodynamically not favoureble since the energy needed for excitation is more than the energy expected to be generated during the formation of two additionl P-Cl bonds. Therefore, nitrogen does not form `NCl_(5)`. In other words, `NCl_(5)` is unknown.