Chlorine

n. gaseous chemical element

Chlorine (, , meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VIIa or VIIb). As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its common elemental form (Cl2 or "dichlorine") under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. As a common disinfectant, it is used in swimming pools to keep them clean. In the upper atmosphere, chlorine based molecules have been implicated in the destruction of the ozone layer.
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Noun
1. 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)
(synonym) Cl, atomic number 17
(hypernym) chemical element, element
(hyponym) radiochlorine
(substance-holonym) sodium chloride, common salt
(derivation) chlorinate

(n.)
One of the elementary substances, commonly isolated as a greenish yellow gas, two and one half times as heavy as air, of an intensely disagreeable suffocating odor, and exceedingly poisonous. It is abundant in nature, the most important compound being common salt. It is powerful oxidizing, bleaching, and disinfecting agent. Symbol Cl. Atomic weight, 35.4.
  

A chemical used to disinfect water and as a bleach.