After considering several columns in the periodic table containing both metals and nonmetals, we finally return to a homogeneous group. All of the halogens are nonmetals, although iodine's slight metallic sheen indicates some mobility of electrons in the crystalline solid. Radioactive astatine, if it were more than just a fleeting element seen in cyclotron bombardments, presumably would be even more metallic. (It has been estimated that the total amount of astatine in the Earth's crust at any one time, produced by radioactive decay of other elements, is approximately one ounce.)

The halogens all achieve a -1 oxidation state by gaining one electron per atom to complete their outer electron shell. Cl, Br, and I also occur in positive oxidation states as in perchloric acid, , but fluorine is too electronegative for this to occur. The most common halogen compounds are salts with metal cations, and the name halogen itself means "salt-former." The hydroacids HF, HCI, HBr, and III increase in acid strength as the halogen atom becomes larger and its proton becomes more weakly held. All but HIP are completely dissociated in water.