The halogens are a group of chemical elements that includes fluorine, chlorine, bromine, iodine, and astatine. Halogen comes from Greek terms meaning to produce sea salt. None of the halogens occur naturally in the form of elements, but, except for astatine, they are very widespread and abundant in chemical compounds where they are combined with other elements.Sodium chloride, common table salt, is the most widely known.

All of the halogens exist as diatomic molecules when pure elements. Fluorine and chlorine are gases. Bromine is one of only two liquid elements, and iodine is a solid. Astatine atoms exist only for a short time and then decay radioactively. Fluorine is the most reactive of all known elements. Chemical activity, the tendency to form chemical compounds, decreases with atomic number, from fluorine through iodine. Simple compounds of these elements are called halides. When one of the elements becomes part...