Why does NH3 form hydrogen bond but PH3 does not?

Nitrogen is highly electronegative as compared to phosphorus. This causes a greater attraction of electrons towards nitrogen in NH₃ than towards phosphorus in PH₃. Hence, the extent of hydrogen bonding in PH₃ is very less as compared to NH_3.

The electronegativity of N (3.0) is much higher than that of FI (2.1). As a result, N – H bond is quite polar and hence NH₃ undergoes intermolecular H-bonding. In contrast, both P and H have an electronegativity of 2.1. Therefore P – H bond is non-polar and hence PH₃ does not undergo H – bonding.

In NH₃,the nitrogen atom forms hydrogen bond because of the following reasons :

• Small size of nitrogen 
• High electronegativity (3.0) of nitrogen

N-H bond is polar forming hydrogen bond. 

P-H bond is almost purely covalent due to larger size and lesser electronegativity.

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.