Among the halogens (F, Cl, Br, I), bond dissociation enthalpy is **lowest** for:

Down group 17: bond enthalpy is Cl-Cl (242) > Br-Br (192) > F-F (158) > I-I (151) kJ/mol approximately. The surprise is **F-F**, which is *lower* than Cl-Cl despite the trend. Two fluorine atoms are very small and the lone pairs on each repel strongly when they bond — that lone-pair repulsion weakens the F-F bond unusually. Flowing down (Cl → Br → I), bond enthalpy decreases steadily as atomic size increases (orbital overlap weakens). The fluorine anomaly explains its outsize reactivity.