GOC (Bond Dissociation energy)
Why is the Bond Dissociation energy of I2 is the least?
Well, bond dissociation energy refers to the energy required to break bond between 2 atoms. Its value depends on the strength of the bond.(more strong bond requires high energy to break).As size of atom increases, its bond strength and dissociation energy decreases . So dissociation energy decreases down the group as size increases.
In the halogen family,size of F is very small, so it faces a strong electronic repulsion between its electrons , which makes it bong weak thus reducing its dissociation energy.(the effect is so strong that it's dissociation energy is less than Br).
That's why the order is Cl2>Br2>F2>I2