Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly halfway between them. Chlorine is a yellow-green gas at room temperature. It is a very reactive element and a strong oxidising agent: among the elements, it has the highest electron attraction and the third-highest electronegativity on the Pauling scale, behind only oxygen and fluorine.
Chlorine played an important role in the experiments managed and done by medieval alchemists, which commonly involved the heating of chloride salts like ammonium chloride sal ammonic and sodium chloride (common salt), producing different chemical substances containing chlorine such as hydrogen chloride, mercury(II) chloride (corrosive sublimate), and hydrochloric acid (in the form of aqua regia). However, the nature of free chlorine gas as a separate substance was only recognised around 1630 by Jan Baptist van Helmont. Carl Wilhelm Scheele wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element. In 1809, chemists suggested that the gas might be a (completely/complete, with nothing else mixed in) element, and this was proven true by Sir Humphry Davy in 1810, who named it from Very old Greek: ÏÎ»ÏÏÏÏ, romanized: khloros, lit. ‘pale green’ based on its colour.
Because of its great reactivity, all chlorine in the Earth’s crust is in the form of ionic chloride compounds, which includes table salt. It is the second-most plentiful halogen (after fluorine) and twenty-first most plentiful chemical element in Earth’s crust. These crustal deposits are anyway dwarfed by the huge reserves of chloride in seawater.
Elemental chlorine is commercially produced from salt water by electrolysis, mostly in the chlor-alkali process. The high oxidising potential of elemental chlorine led to the development of commercial strong whitening chemicals and disinfectants, and a reagent for many processes in the chemical industry. Chlorine is used in the manufacture of a wide range of person of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride (PVC), many intermediates for the production of plastics, and other end products which do not contain the element. As a common disinfectant, elemental chlorine and chlorine-creating compounds are used more directly in swimming pools to keep them sanitary. Elemental chlorine at high concentration is very dangerous, and poisonous to most living (living things). As a chemical war fighting agent, chlorine was first used in World War I as a poison gas weapon.
In the form of chloride ions, chlorine is necessary to all known species of life. Other types of chlorine compounds are rare in living things, and synthetically produced chlorinated organics range from inert to poisonous. In the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been involved in ozone depletion. Small amounts of elemental chlorine are created by oxidation of chloride to hypochlorite in neutrophils as part of a disease-fighting system response against bacteria.