Why + 3 oxidation state is stable in heavier element of group 15?

Explanation

(1) All the elements of group 15 show oxdiation state of `+3` and `+5`
(2) Down the group, the stability of +5 oxidation state decreases while that of+3 state increases due to inert pair effect of `ns^(2)` electrons.
(3) Large amount of energy is needed to remove all five electrons (`ns^(2), np^(3)`) from the valence shell, hence stable +5 oxidation state cannot be formed by heavier element like Bi of group 15 having large atomic size.