Question

Why $+3$ oxidation state is stable in heavier elements of Group 15?

Answer 1

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 greater nuclear attraction. Also, these orbitals, then, don’t participate in bonding. This is the reason why the stability of $+3$ oxidation state increases in heavier elements of Group 15 in comparison with $+5$ oxidation state.