Why does Nitrogen form N₂ (and not N₄) while but Phosphorus forms P₄ (and not P₂)?
Due to larger atomic size $\ce{P}$ is unable to form pi bonds and so it is tetra-atomic in which each $\ce{P}$ atom is linked with 3 other $\ce{P}$ atoms by 3 sigma bond. But, due to smaller atomic size $\ce{N}$ forms 1 sigma and 2 pi bonds i.e. triple bonds with other $\ce{N}$ atom and exists as diatomic molecule.






Reference: http://philmintz.tripod.com/Chemistry/page4.html