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

Nitrogen because of its small size and high electronegativity forms pπ – pπ-multiple bonds. Therefore, it exists as a diatomic molecule having a triple bond between the two N-atoms. Phosphorus, on the other hand, due to its larger size and lower electronegativity usually does not form pπ – pπ multiple bonds with itself. Instead it prefers to form P—P single bonds and hence it exists as tetrahedral P₄ molecules.