As we move down in Group 15, we can see the inert pair effect. This effect impacts the penetration effect of ‘s orbitals. These orbitals penetrate and come closer to the nucleus and enjoy the greater nuclear attraction. Also, these orbitals, then, don’t participate in bonding. So, p electrons can be easily removed but not s electrons. This is the reason why the stability of the +3 oxidation state increases in heavier elements of Group 15 in comparison with the +5 oxidation state.