Q.Why fluorine is more reactive than other halogens?

Step 1. Consider the F-F bond strength.

Despite being the smallest halogen, fluorine has an anomalously weak F-F bond (about 159 kJ/mol, weaker even than Cl-Cl and Br-Br) because the very short bond forces the non-bonding lone pairs on the two fluorine atoms into significant mutual repulsion. This low bond dissociation energy means very little energy is needed to generate reactive fluorine atoms, giving reactions of F2 a low activation energy and hence a fast rate.

Step 2. Consider the strength of the bonds fluorine forms once reacted.

Because fluorine is the most electronegative element, the bonds it forms with almost any other element (X-F bonds) are exceptionally strong and release a large amount of energy on formation, making fluorine's reactions strongly exothermic overall, even after accounting for the (small) energy cost of breaking F-F.

Step 3. Consider fluorine's small size and high electron affinity.

Fluorine's small atomic size lets it approach and attack other atoms/bonds very closely, and its very high electron affinity/electronegativity gives it a powerful thermodynamic drive to pull electron density towards itself in any reaction.

Step 4. Combine into an overall explanation. …

Separate the kinetic factor (weak F-F bond, low activation energy) from the thermodynamic factor (very strong bonds formed with other elements) to bu …

Explaining fluorine's reactivity only by 'it is very electronegative' without also mentioning the anomalously weak F-F bond …