Atomicity of nitrogen is two, while that of phosphorus is four because

In ${\mathrm{N}}_2$ molecule, there is triple bond between two N atoms because size of N is small and there is less repulsion is faced by inner core electrons.  Therefore, its atomicity is 2. Whereas phosphorus cannot form ${\mathrm{N}}_2$ like bond because of its large size and repulsion due to inner core electrons. Therefore, P exists as ${\mathrm{P}}_4,$ and its atomicity becomes 4.