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- or both 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. As a result, among heavier elements of group 15, the electron pair in s-orbital of the valence shell shows reluctance 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.