Question

Stability of +5 oxidation state decrease from N to Bi. Why?

Answer 1

You have observed a manifestation of the inert pair effect.

Explanation:

The inert pair effect relates to the increasing stability of an oxidation state that is `II` LOWER than the Group number, and is observed in Groups 13, 14, 15, and 16, for the heavier Group members. Nitrogen and phosphorus, as Group 15 elements, can be oxidized up to `N(V+)` to `P(V+)`, the Group oxidation states; lower elements of Group 15, TEND to display stable `+III` oxidation states, (the which is certainly true of phosphorus).

Note that while we have described the condition, we have not really rationalized it effectively. I would check the references on the link, your text, and your lecture notes for their explanation.