Because of its small size and high electronegativity nitrogen 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, has large size and lower electronegativity and 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.