(a) Though nitrogen exhibits `+5` oxidation state, it does not form pentahalide. Give reason.
(b) `PH_3` has lower boiling point than `NH_3`. Why ?

Explanation

(a) Electronic configuration of nitrogen is `1 s^2 2 s^2 2 p^3`. Die to absence of d-orbitals in its valence shell, nitrogen cannot expand its coordination number beyond four. That is why nitrogen does not form pentahalides.
(b) Nitrogen is more electronegative than phosphorous and hence capable of forming hydrogen bonds. Due to association of `NH_3` molecules by hydrogen bonding. ammonia has higher boiling point. On the other hand, due to absence of hydrogen bonding in `PH_3` molecules are not associated and `PH_3` has lower boiling point than `NH_3`.