Nitrogen exists as diatomic molecule and phosphorus as P4

Nitrogen exists as diatomic molecule and phosphorus as P₄. Why? 

Answer

Nitrogen owing to its small size has a tendency to form pπ−pπ multiple bonds with itself. Nitrogen thus forms a very stable diatomic molecule, N₂. On moving down a group, the tendency to form pπ−pπ bonds decreases (because of the large size of heavier elements). Therefore, phosphorus (like other heavier metals) exists in the P₄ state.