Bond dissociation energy depends on bond strength. Bond strength depends on the attractive and repulsion forces present in a molecule. Due to higher nuclear mass of D2, the attraction between nucleus and bond pair in D−D is stronger than in H−H. This results in greater bond strength and higher bond dissociation enthalpy. Thus, the bond dissociation enthalpy of D−D is higher than that of H−H.
The bond dissociation enthalpy of F−F is minimum as the repulsion between the bond pair and lone pairs of F is strong. Hence, the increasing order of bond dissociation enthalpy is F−F<H−H<D−D.