N-N single bond is weaker than P-P bond due to smaller size of N as compared to P . Smaller size of N leads to smaller N-N bond length. As a result, the lone pair of electrons on the both the N atoms repel each other leading to unstability or weakening of N-N bond. Because of larger size of P atom, P-P bond length is more and lone pair-lone pair repulsion between P atoms is less which makes the P-P bond stronger than N-N bond.