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

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]. With the increase in the size of atoms, the strength pπ– pπ bonds decreases. Thus pπ– pπ bonds are weaker for heavier atoms. Therefore, phosphorus [like other heavier metals] exists in the P₄ state, as shown below.