A halogen is a chemical element that forms a salt when it reacts with metal. Halogen lamps are illuminated by bulbs that contain a halogen and an inert gas.

There are five halogens in the periodic table of chemical elements: fluorine, chlorine, bromine, iodine, and astatine. The halogens are all highly reactive, which means they're quick to form bonds with other elements. What they often end up becoming is some kind of salt, including common table salt. In fact, the literal translation of halogen is "salt-producer," from Greek roots hals, "salt," and gen, "giving birth to."

Definitions of halogen
  1. noun
    any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions
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    At, astatine, atomic number 85
    a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium
    Br, atomic number 35, bromine
    a nonmetallic heavy volatile corrosive dark brown liquid element belonging to the halogens; found in sea water
    Cl, atomic number 17, chlorine
    a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water)
    F, atomic number 9, fluorine
    a nonmetallic univalent element belonging to the halogens; usually a yellow irritating toxic flammable gas; a powerful oxidizing agent; recovered from fluorite or cryolite or fluorapatite
    I, atomic number 53, iodin, iodine
    a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)
    a radioactive isotope of chlorine
    heavy radioactive isotope of iodine with a half-life of 8 days; used in a sodium salt to diagnose thyroid disease and to treat goiter
    light radioactive isotope of iodine with a half-life of 60 days; used as a tracer in thyroid studies and as a treatment for hyperthyroidism
    type of:
    group, grouping
    any number of entities (members) considered as a unit
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