What is the general trend in reactivity of halogens down the group?

The reactivity of halogens decreases down the group due to the increasing atomic radius and the corresponding reduction in the ability of the outer electrons to attract additional electrons. As you move down the group from fluorine to iodine, the size of the atoms increases significantly because additional electron shells are added. This increased distance reduces the effective nuclear charge experienced by the valence electrons and makes it harder for these atoms to gain an extra electron, which is essential for their reactivity.

Additionally, the shielding effect also plays a role. As more inner electron shells are added, they shield the outer electrons from the full positive charge of the nucleus, further diminishing the attraction for incoming electrons. Therefore, while fluorine is highly reactive, as we proceed down the halogen group to chlorine, bromine, and eventually iodine, their ability to attract and gain electrons lessens, leading to reduced reactivity.