pπ−pπ bonding is weak bonding. Due to larger atomic size of phosphorus. It is unstable to form π−π bond and so, it is tetra-atomic in which each p-aom is linked with 3 others P-atom by 3σ-bonds. In nitrogen, due to smaller atomicsize N forms 1σ and 2π-bonds i.e. triple bonds with other N atom and exists as diatomic molecules. Hence, the bonding is responsible for that. Phosphorus from P4 because in P2,pπ−pπ bond is weaker.