Nitrogen exists as diatomic molecule and phosphorus as `P_(4)`. Why ?

Explanation

Nitrogen because of its small size and high electronegativity forms `p pi - p pi`-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 large size and lower electronegativity usually does not form `p pi - p pi` multiple bonds with itself. Instead it prefers to form P-P single bonds and hence it exists as tetrahedral `P_(4)` molecules.