Nitrogen is diatomic where as phosphorus is polyatomic.  It is because         

Nitrogen exists as diatomic molecule (N₂​)  with triple bond between two N atoms and phosphorus as P₄​. Nitrogen can form pπ−pπ multiple bonds. Phosphorous cannot form such bonds due to repulsion between non bonded electrons of inner core due to its large size. In case of nitrogen, there is no such repulsion as it has only 1s electrons in inner shell so that the overlap of p orbitals to form pi bonds is easy.