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 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 considered 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 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 periodic table of the chemical elements is a tabular method of displaying the chemical elements.
Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869.
Mendeleev intended the table to illustrate recurring ('periodic') trends in the properties of the elements.
The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.
The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior.
The table has also found wide application in physics, biology, engineering, and industry. The current standard table contains 117 confirmed elements as of October 16, 2006 (while element 118 has been synthesized, element 117 has not).
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