Explanation:
We know that Ionic radius increases on going downwards in any group. In group 15, the last elements Sb and Bi have ionic radii lesser than the rest of its elements i.e N,P and As.
This is due to the fact that Sb and Bi have completely filled d and f sub-shells which have least shielding effect.
Thus, they do not guard the valence electrons to a greater extent due to which the valence electrons feel almost whole nuclear charge.
This, results into attraction of valence electrons towards nucleus and hence size decreases.
