Due to larger atomic size $\text{P}$ is unable to form pi bond and so it is tetra-atomic $\left({\mathrm{P}}_4\right)$ in which each $\text{P}$ atom is linked with $\text{3}$ other $\text{P}$ atoms by $\text{3}$ sigma bonds. But, due to smaller atomic size $\text{N}$ forms $\text{1}$ sigma and $\text{2}$ pi bonds i.e. triple bonds with other $\text{N}$ atom and exists as diatomic molecule $\left({\mathrm{N}}_2\right)$ .