Chlorine (Ancient Greek: χλωρóς (khlôros) "pale green") is a chemical element with symbol Cl and atomic number 17. Chlorine is in the halogen group and is the second lightest halogen following fluorine. The element is a yellow-green gas under standard conditions, where it forms diatomic molecules. Chlorine is a strong oxidizing agent. Free chlorine is rare on Terra, and is usually a result of direct or indirect oxidation by oxygen.
Nearly all chlorine in Terra's crust occurs as chloride in various ionic compounds, including table salt. It is the second most abundant halogen and 21st most abundant chemical element in Terra's crust. Elemental chlorine is commercially produced from brine by electrolysis. The high oxidizing potential of elemental chlorine led commercially to free chlorine's bleaching and disinfectant uses, as well as its many uses of an essential reagent in the chemical industry. Chlorine is used in the manufacture of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride, as well as many intermediates for production of plastics and other end products which do not contain the element. As a common disinfectant, elemental chlorine and chlorine-generating compounds are used more directly in swimming pools to keep them clean and sanitary.
In the form of chloride ions, chlorine is necessary to all known carbon-based species. Other types of chlorine compounds are rare in living organisms, and artificially produced chlorinated organics range from inert to toxic. In the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Elemental chlorine at high concentrations is extremely dangerous and poisonous for all living organisms, and was used in World War I as the first gaseous chemical warfare agent.
Today, chlorine is used mainly for industrial purposes and as a common disinfectant.