The stability of `+5` oxidation state decreases down the group `15` of the periodic table.Explain this observation giving appropriate reasons.

Explanation

All the elements of group `15` have five electrons in the valence shell, two in the `s`-subshell and three in the `p`-subshell.Therefore, among others, they can show positive oxidation states of `+3` (due to participation of only `p`-electrons) and `+5` (due to the participation of both `s-` and `p-`electrons of the valence shell).Now `N` does not have `d`- or `f`-electrons but all other elements have either `d`- and `f`-electrons in the inner shells. Since `d` and `f` electrons shield the `s`-electrons of the valence shell more effectively than the `p`-electrons, therefore, on moving down the group, tendency of `s`-electrons to participate in bond formation decreases while that of `p`-electrons increases.In other words, among heavier elements of group `15`, the electron pair in `s`-orbital of the valence shell bonds shows reductance to participate in bond formation.In other words, among heavier elements, only `p`-electrons participate in bond formation and hence show an oxidation state of `+3`.This is called inert pair effect.Further, as the number of `d-` and `f-` electrons increases down the group, the inert pair effect becomes more and more pronounced. In other words, down the group from `As` to `Bi`, the stability of `+3` oxidation state increases while that of `+5` oxidation state decreases.