It is because
(i) radius of anion ( anionic radius ) is greater , and
(ii) radius of cation ( cationic radius ) is lesser
than that of its parent neutral atom.
As bond order increases bond dissociation energy also increases .
Thus
(i) the triple bonded with bond order
= 3
has the highest bond dissociation energy .
(ii) The doubly bonded with bond order
= 2
has lesser bond dissociation energy than
(iii) and both are single bonded having the same bond order
= 1
but , the electron - electron repulsive force ( e - e repulsion ) is maximum in a molecule due to its small size. This accounts for the lower bond dissociation energy of as compared to