Halogen/ˈhælədʒən/ Definition 1 of 2Pro Tip 1/3Chemical group Halogen refers to a group in the periodic table, not a single element. Fluorine and iodine belong to the halogen group. Pro Tip 2/3Reactivity context Halogen elements are highly reactive, often discussed in chemistry reactions. The reactivity of halogens decreases down the group. Pro Tip 3/3Common salts Halogen elements combine with metals to form salts, like table salt. Sodium chloride is a salt formed with the halogen chlorine.