Explain why both N and Bi do not form pentahalides while phosphorus does.

Explanation

N does not form a pentahalide because it does not have d-orbitals to which 2s electron may be excited to make available five half-filled orbitals needed for the formation of five N-halogen bonds. Refer to Ans. To Q .1, page 11//142.
In contrast, Bi in principle, can form a pentahlide because it has vacant 6d-orbitals to which 6s-electron can be excited to make available five half orbitals needed for the formation of five Bi - halogen bonds.

But due to inert pair effect, the energy needed for excitation of 6s-electron to 6d-orbital is much more than the energy released when two additional Bi-halogen bonds are formed. Thus, bismuth does not form a pentahalide (excpt `BiF_(5)`) but forms only a trihalide.