AskDefine | Define halogen

Dictionary Definition

halogen n : any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions

User Contributed Dictionary



From Greek ἅλς ‘salt’ + -gen ‘produce’.




  1. Any element of group 7, i.e. fluorine, chlorine, bromine, iodine and astatine, which form a salt by direct union with a metal.


any element of group 7

Extensive Definition

The halogens or halogen elements are a series of nonmetal elements from Group 17 (old-style: VII or VIIA; Group 7 IUPAC Style) of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At. The undiscovered element 117, temporarily named ununseptium, may also be a halogen.
The group of halogens is the only group which contains elements in all three familiar states of matter at standard temperature and pressure.


Owing to their high reactivity, the halogens are found in the environment only in compounds or as ions. Halide ions and oxoanions such as iodate (IO3−) can be found in many minerals and in seawater. Halogenated organic compounds can also be found as natural products in living organisms. In their elemental forms, the halogens exist as diatomic molecules, but these only have a fleeting existence in nature and are much more common in the laboratory and in industry. At room temperature and pressure, fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids; Group 17 is therefore the only periodic table group exhibiting all three states of matter at room temperature.


The term halogen originates from 18th century scientific French nomenclature based on adaptations of Greek roots: hals (sea) or halas (salt), and gen- (to generate) — referring to elements which produce a salt in union with a metal.


The halogens show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, and increasing melting and boiling point.
Halogen Standard Atomic Weight (u) Melting Point (K) Boiling Point (K) Electronegativity (Pauling)
Fluorine 18.998 53.53 85.03 3.98 Chlorine 35.453 171.6 239.11 3.16 Bromine 79.904 265.8 332.0 2.96 Iodine 126.904 386.85 457.4 2.66 Astatine (210) 575 610 ? 2.2

Diatomic halogen molecules

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