Oxidising Power Down Group 15

The relative stability of oxidation states down Group 15 is explained by the inert pair effect. As the group is descended from phosphorus to bismuth, the outer ns^2 electron pair becomes increasingly reluctant to take part in bonding because of poor shielding by the intervening d and f electrons, which holds the s pair more tightly. Consequently the +5 state, which requires the use of all five valence electrons including the s pair, becomes less stable, while the +3 state, which uses only the three p electrons, becomes more stable down the group. This is why bismuth is most stable in the +3 state and BiF_5 is a strong oxidiser, whereas phosphorus readily shows +5. The option of increasing electronegativity is wrong, as electronegativity decreases down the group. The option of decreasing atomic size is false, since size increases. The option of increasing non-metallic character is incorrect, as metallic character increases down the group. This inert pair trend is a key JEE Advanced concept. Plausibility check: the strong oxidising power of Bi(V) compared with stable Bi(III) confirms the decreasing stability of the +5 state.