Q1. Why is the bond angle of PH3 molecule lesser than that in NH3 molecule?
NH3, there are 4 electron pairs (3 bonding pairs and one lone pair) in the outer most shell of N. The repulsion between lone pair and a bond pair of electrons always exceeds that between two bond pairs. Thus the bond angles are reduced from 109027' to 107048'.
PH3 would be expected to be similar. But, the bond pairs of electrons are much further away from the central atom due to its larger size than they are in NH3. Thus the lone pair causes even greater distortion in PH3. The bond angle decrease to 91018'
Thus, the bond angle of PH3 molecule is lesser than that in NH3 molecule.