Nitrogen and Phosphorus belong to group 15 of the periodic table. Moving down a group, the atomic size of elements increases This is because new shells are added as we go down a group. Nitrogen has a smaller atomic size than Phosphorus.

As a result of the smaller atomic size of Nitrogen, it has greater ionization enthalpy than phosphorus. Thus, Nitrogen has a greater tendency to retain electrons. Nitrogen- nitrogen bonds are much stronger than Phosphorus-Phosphorus bonds. Due to this, Phosphorus exist as P4 molecules whereas nitrogen exist as N2 molecules. Due to the presence of multiple bonds between Nitrogen atoms, the N2 molecule is quite inert. As a result, N2 exists as a gas at room temperature. On the other hand, Phosphorus forms bonds with itself resulting in the formation of P4 which is a polymeric molecule and hence has a high melting point and exist as a solid at room temperature.