(a) Why elemental phosphorous does not exist as `P_2` like `N_2` ?
(b) `NCl_3` gets easily hydrolysed, while `NF_3` does not. Why ?

Explanation

(a) Nitrogen due to its small size has a tendency to form multiple bond and thus exists as diatomic molecule. Phosphorous, on the other hand, has comparatively large size, therefore, extent of overlap of `(3p - 3p)` which can result in `pi-bond` formation is less, and phosphorous has no tendency to form multiple bond with itself. Hence phosphours does not exist as `P_2` molecule.
(b) In `NCl_3, N` is more electronegative than `Cl`, hence `N` has `delta -` charge and `Cl` has `delta +` charge, moreover due to presence of vacant d-orbital on `Cl`, it accepts a pair of electron from `O` atom of `H_2 O` molecule. Thus hydrolysis is possible. On the other hand, in `NF_3` due to high `N-F` bond strength, `NF_3` molecule is inert and does not undergo hydrolysis.