The stability of +5 oxidation state decrease down the group 15.while +3 state increase why? Also explain inert pair effect.

In group 15, the stability of the +5 oxidation state decreases from P to Bi. Because neutral covalent compounds of the trivalent group 15 elements have a lone pair of electrons on the central atom, they tend to be Lewis bases.

The inert pair effect is the tendency of the two electrons in the outermost atomic s-orbital to remain unshared in compounds of post-transition metals. ... The term "inert pair" was first proposed by Nevil Sidgwick