The HNH angle value is higher than HPH, HAsH and HSbH angles. Why?

The difference in the bond angles is based on the electronegativity and the size of the central atom. For example, nitrogen is the smallest in size with maximum elctronegativity (3.0). The electron density is very hgh around nitrogen which also means strong repulsions in the electron pairs around it resulting in maximum bond angle (107°). As we move down the group, the atomic sizes increase and the electronegativities decrease. As a result, there is a gradual decrease in the electron density on the central atom resulting in decreased bond angles in the same order. Thus, the HNH angle value is higher than HPH, HAs H and HSbH angles.