The observed bond angle in Ammonia is $107°48\text{'}$ , the expected bond angle in HPH is

Nitrogen, N and phosphorus, P belongs to VA group. In both ${\text{NH}}_3{\text{\hspace{0.17em}and\hspace{0.17em}PH}}_3$, central atoms N and P have same hybridization ${\text{sp}}^3$ with 3 B.P and 1 L.P electrons. According to VSEPR theory, we can predict the geometry and bond angle of molecules. $\text{E.N\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}}\text{'}\text{N'}>\text{E.N\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}}\text{'}\text{P'}$ . Hence, the bond angle in HPH ($<107°48°$) should be less than HNH ($107°48°$).