Nitrogen has an electronegativity value \(3.0\), which is much higher than that of \(\mathrm{H}(2 \cdot 1)\). As a result, \(\mathrm{N}-\mathrm{H}\) bond is quite polar and hence \(\mathrm{NH}_3\) undergoes intermolecular \(\mathrm{H}\) - bonding.


Phosphorus have an electronegativity value \(2 \cdot 1\). Thus, \(\mathrm{P}-\mathrm{H}\) bond is not polar and hence \(\mathrm{PH}_3\) does not undergo \(\mathrm{H}\) - bonding.