halogen, any of the six nonmetallic elements that constitute Group 17 (Group VIIa) of the periodic table. The halogen elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). They were given the name halogen, from the Greek roots hal- (“salt”) and -gen (“to produce”), because they all produce sodium salts of similar properties, of which sodium chloride—table salt, or halite—is best known.

Because of their great reactivity, the free halogen elements are not found in nature. In combined form, fluorine is the most abundant of the halogens in Earth’s crust. The percentages of the halogens in the igneous rocks of Earth’s crust are 0.06 fluorine, 0.031 chlorine, 0.00016 bromine, and 0.00003 iodine. Astatine and tennessine do not occur in nature, because they consist of only short-lived radioactive isotopes.

The halogen elements show great resemblances to one another in their general chemical behaviour and in the properties of their compounds with other elements. There is, however, a progressive change in properties from fluorine through chlorine, bromine, and iodine to astatine—the difference between two successive elements being most pronounced with fluorine and chlorine. Fluorine is the most reactive of the halogens and, in fact, of all elements, and it has certain other properties that set it apart from the other halogens.

Chlorine is the best known of the halogen elements. The free element is widely used as a water-purification agent, and it is employed in a number of chemical processes. Table salt, sodium chloride, of course, is one of the most familiar chemical compounds. Fluorides are known chiefly for their addition to public water supplies to prevent tooth decay, but organic fluorides are also used as refrigerants and lubricants. Iodine is most familiar as an antiseptic, and bromine is used chiefly to prepare bromine compounds that are used in flame retardants and as general pesticides. In the past ethylene dibromide was extensively used as an additive in leaded gasoline.

Oxidation

Probably the most important generalization that can be made about the halogen elements is that they are all oxidizing agents; i.e., they raise the oxidation state, or oxidation number, of other elements—a property that used to be equated with combination with oxygen but that is now interpreted in terms of transfer of electrons from one atom to another. In oxidizing another element, a halogen is itself reduced; i.e., the oxidation number 0 of the free element is reduced to −1. The halogens can combine with other elements to form compounds known as halides—namely, fluorides, chlorides, bromides, iodides, and astatides. Many of the halides may be considered to be salts of the respective hydrogen halides, which are colourless gases at room temperature and atmospheric pressure and (except for hydrogen fluoride) form strong acids in aqueous solution. Indeed, the general term salt is derived from rock salt, or table salt (sodium chloride). The tendency of the halogen elements to form saltlike (i.e., highly ionic) compounds increases in the following order: astatine < iodine < bromine < chlorine < fluorine. Fluorides are usually more stable than the corresponding chlorides, bromides, or iodides. (Often astatine is omitted from general discussions of the halogens because less is known about it than about the other elements.)

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The oxidizing strength of the halogens increases in the same order—i.e., from astatine to fluorine. Therefore, of the halogen elements, elemental fluorine is prepared with the greatest difficulty and iodine with the least. As a class, the halogen elements are nonmetals, but astatine shows certain properties resembling those of the metals.

Electronic structure

The chemical behaviour of the halogen elements can be discussed most conveniently in terms of their position in the periodic table of the elements. In the periodic table the halogens make up Group 17 (according to the numbering system adopted by the International Union of Pure and Applied Chemistry), the group immediately preceding the noble gases. The halogen atoms carry seven valence electrons in their outermost electron shell. These seven outermost electrons are in two different kinds of orbitals, designated s (with two electrons) and p (with five). Potentially, a halogen atom could hold one more electron (in a p orbital), which would give the resulting halide ion the same arrangement (configuration) as that of the noble gas next to it in the periodic table. These electron configurations are exceptionally stable. This pronounced tendency of the halogens to acquire an additional electron renders them strong oxidizers.

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At room temperature and atmospheric pressure the halogen elements in their free states exist as diatomic molecules. In molecular fluorine (F₂) the atoms are held together by a bond made from the union of a p orbital from each atom, with such a bond being classed as a sigma bond. It should be mentioned that the dissociation energy for fluorine (the energy necessary to break the F―F bond) is over 30 percent smaller than that of chlorine but is similar to that of iodine (I₂). The weakness of the F―F single bond compared with chlorine can be ascribed to the small size of fluorine resulting in a decreased overlap of bonding orbitals and an increased repulsion of the nonbonding orbitals. In iodine, however, the p orbitals are more diffuse, which means the bond becomes weaker than in chlorine or bromine.