Nitrogen forms N₂ , but phosphorus is converted into P₄ from P₂. The reason for this is

Nitrogen (N) has a small size and has high electronegativity compared to phosphorus. Due to small size, the lateral overlapping, which is required in the formation of $\mathrm{p\pi }-\mathrm{p\pi }$ bonding is very effective. Therefore, N atom exists as N₂.

End-to-end overlapping is more effective than lateral overlapping in phosphorus because, it has a large size. This indicates that $\mathrm{p\pi }-\mathrm{p\pi }$ in phosphorus is quite weak. As a result, instead of formation pi bonds, one P atom interacts with the other three P atoms to form four sigma bonds through end-to-end overlapping. Hence, P exists as P₄ not P₂.