I>Br>Cl>F→ atomic radii F, Cl, Br, I belongs to the same group orderly. Atomic radii go on increasing as the nuclear charge increases in preceeding downwards in a group. The decreasing order of bond length C−I>C−Br>C−Cl>C−F. The order of bond dissociation energy R−F>R−Cl>R−Br>R−I. During dehydrohalogenation C−I bond breaks more easily than C−F bond. So reactivity order of halides is, R−I>R−Br>R−Cl>R−F.