Nitrogen form ${\mathrm{\text{N}}}_2$ (i.e. $\mathrm{\text{N}}\equiv \mathrm{\text{N}}$ ) but phosphorus form ${\mathrm{\text{P}}}_4$ , because in ${\mathrm{\text{P}}}_2$ , $\text{p}\text{π}-\text{p}\text{π}$ bonding is present which is a weaker bonding.

Nitrogen is smaller in size hence it forms multiple bond with other nitrogen atom $\left(\text{N}\equiv \text{N}\right)$

Nitrogen has unique ability to form $\text{p}\text{π}-\text{p}\text{π}$ multiple bonds with itself and with other elements having small size and high electron negativity.

However heavier elements of this group do not form $\text{p}\text{π}-\text{p}\text{π}$ bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping. Thus nitrogen exist as diatomic molecule with a triple bonds.

The single P - P bond is stronger and has higher catenation tendency