The most reactive among the halogens is

Fluorine is the most reactive halogen because it is the smallest atom in its group, which allows its nucleus to exert a very strong pull on electrons. It has the highest electronegativity and a low bond dissociation energy, meaning it can easily break bonds and attract new electrons to complete its outer shell. As you move down the group from fluorine to iodine, the atomic size increases and the chemical reactivity decreases.