Question

Nitrogen exists as diatomic molecule and phosphorus as $P_4$ why?

Answer 1

Due to its small size and high electronegativity, nitrogen atom forms $p\pi -p\pi$ multiple bonds with itself (triple bond); so it exists as a discrete diatomic molecule in elemental form. On moving down a group, the tendency to form pπ-pπ bonds decreases (because of the large size of heavier elements). The phosphorus atom has a large size and less electronegativity so it forms single bond instead of $p\pi -p\pi$ multiple bonds. Therefore, phosphorus (like other heavier metals) exists as $P_4$ molecule in its elemental form.