Nitrogen exists as diatomic molecule and phosphorus as P4. Why?
Nitrogen has small atomic size. Two nitrogen atoms can be linked to each other by three covalent bonds to form :N≡N:.
The octet of both N atoms is complete. P has large atomic size and little tendency to form triple bonds. It can complete its octet by sharing valency electrons with three other P atoms to form tetra-atomic P4 molecule.