Give reason, why elemental nitrogen exists as a diatomic molecule, whereas elemental phosphorus is a tetratomic molecule.

Explanation

Due to smalll size and high `EN` of `N` atom, nitrogen is a diatomic gaseous molecule at ordinary temperature. The existence of nitrogen as diatomic molecule `(N -= N)` is due to its ability to form `(p pi - p pi)` multiple bonds.
The triple bond in nitrogen molecule is very stable as the dissociation enegry is very high `(225 Kcal mol^-1)`. Phosphorus, arsenic and antimony all exist as discrete tetratomic tetrahedral molecules, as these are not capable of forming multiple bonds due to the repulsion between the non-bonded electrons of the inner core.