Why are pentahalides more covalent than trihalides in the members of the nitrogen family ?

Explanation

The electronic configuration of the elements of nitrogen family (group 15) is `ns^(2)p^(3)`. Because of the inert pair effect, the valence s-electrons cannot be released easily for the bond formation. This means that the elements can form trivalent cation `(E^(3+))` by releasing valence p-electrons while it is difficult to form pentavalent cation `(E^(5+))`. Under the circumstances, if all the five valence electrons are to be involved in the bond formation, the compounds showing pentavalency or `+5` oxidation state must be of covalent nature. This is particularly the case in the higher members (Sb,Bi) of the family where the inert pair effect is quite prominent.