Single N−N bond is weaker than the single P−P bond. This is because of:
(a) larger N−N bond length in comparison to P−P bond length
(b) high inter electronic repulsion of the non-bonding electrons, owing to the small N−N bond length in comparison to that in P−P single bond
(c) higher electronegativity of NN in comparison to PP
(d) smaller atomic size of NN as compared to that of P