Question

bond angle in NH3 is greater than in PH3 why

Answer 1

The electronegativity of Nitrogen is much more than phosphorous. Thus the N-H bonds in NH3 are polar and the Hydrogen atoms of NH3 acquire a partially positive charge while the H atoms of PH3 remain neutral.
The partially charged H atoms of NH3 repels each other and thus Bond Angle increases. While PH3 lacks this effect. Thus bond angle of NH3 is greater than PH3.

Answer 2

The bond angle in NH3 is larger than, in PH3 because the P−H bonds are longer and the lower electronegativity of P permits electron-density to be displaced towards hydrogen to a greater extent than in the case of NH3. Both of these effects diminish the repulsion due to bond-pairs.

Answer 3

according to drago's rule there is no hybridization in the orbitals of ph3 , cause of this  phosphorus(P) valence shell p orbitals present mutually perpendicular to each other.so in the PH3  angle between p-h bonds is approximately 93°. while in the NH3 hybridization takes place because of this it makes pyramidale structure and angle between the N-H bond is approximately 107°48'.