In NH3, the nitrogen atom forms hydrogen bond because of the following reasons :
- Small size of nitrogen
- High electronegativity (3.0) of nitrogen
Due to more difference of electronegativity between N and H atom the N-H bond is polar forming hydrogen bond. On the contrary, in PH3 the P-H bond is almost purely covalent due to larger size and lesser electronegativity (2.11) of phosphorus and hence does not form hydrogen bond.