Group 17 Halogen Reactivity

The reactivity of a halogen depends on several energy terms, not just electron gain enthalpy. Although chlorine has a slightly more negative electron gain enthalpy, fluorine is the most reactive halogen because of two factors. First, the F-F bond dissociation enthalpy is unusually low owing to strong repulsion between the lone pairs on the two small, close fluorine atoms, so the molecule splits into atoms very easily. Second, the small fluoride ion has a very high hydration enthalpy, releasing large energy when it is solvated. Together these make fluorine's overall reactions highly exothermic and favourable. The option of high electron gain enthalpy alone is misleading, since chlorine's is actually more negative. The option of large atomic size is false, as fluorine is the smallest halogen. The option of metallic character is incorrect, as fluorine is a reactive non-metal. This multi-factor reactivity is a standard NCERT analysis. It is worth emphasising that this is not a special case but a representative example of how group 17 halogen reactivity operates throughout p-block elements. Plausibility check: the weak F-F bond and strong fluoride hydration together outweigh the modest electron gain enthalpy, confirming fluorine's supreme reactivity.