The correct order ofbond angle in NH3,H2O,PH3,H2S with best explanation please as i was poor in physical chemistry

 When the central atom gets smaller and smaller, steric (repulsive) effects between the outer atoms prevent them from "touching" each other. So, smaller central atoms (N in NH3) give rise to larger bond angles. So, the H-N-H bond angle should be significantly larger than the H-P-H angle.

When the central atom is big enough that the H-H distance is large compared to the size of the H atom, the steric effects grow vanishingly small. Hence, the decrease in bond angle from say, AsH3 to SbH3 will be negligible.

 

NH3 >> PH3 > AsH3 > SbH3 (and all angles are less than 109.5 deg)