JEE Main · 2004 · 24 Apr 2004 · Offline · Classification of Elements and Periodicity

Fluorine's standing as the strongest oxidising halogen rests chiefly on its unusually weak $\mathrm{F}-\mathrm{F}$ bond, whose low dissociation energy comes from the strong repulsion between the two small, electron-crowded fluorine atoms. Breaking that bond costs little, and together with the large hydration enthalpy of the small $\mathrm{F}{}^{-}$ ion it makes the overall reduction highly favourable. Electron affinity and hydration help, but it is the low bond dissociation energy that counts most.