Chlorine is a chemical element with the symbol Cl and atomic number The second-lightest of the halogensit appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature.
It is an extremely reactive element and a strong oxidising agent : among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised Pauling scalebehind only oxygen and fluorine. Chlorine played an important role in the experiments conducted by medieval alchemistswhich commonly involved the heating of chloride salts like ammonium chloride sal ammoniac and sodium chloride common saltproducing various chemical substances containing chlorine such as hydrogen chloridemercury II chloride corrosive sublimateand hydrochloric acid in the form of aqua regia.
However, the nature of free chlorine gas as a separate substance was only recognised around by Jan Baptist van Helmont. Carl Wilhelm Scheele wrote a description of chlorine gas insupposing it to be an oxide of a new element. Inchemists suggested that the gas might be a pure element, and this was confirmed by Sir Humphry Davy inwho named it after the Ancient Greek χλωρός khlōrós"pale green" because of 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 abundant halogen after fluorine and twenty-first most abundant chemical element in Earth's crust.
These crustal deposits are nevertheless dwarfed by the huge reserves of chloride in seawater. Elemental chlorine is commercially produced from brine by electrolysispredominantly in the chlor-alkali process. The high oxidising potential of elemental chlorine led to the development of commercial bleaches and disinfectantsand a reagent for many processes in the chemical industry.
Chlorine is used in the Vikt och hälsa of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride PVCmany intermediates for the production of plasticsand 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 sanitary. Elemental chlorine at high concentration is extremely dangerous, and poisonous to most living organisms.
As a chemical warfare agent, chlorine was first used in World War I as a poison gas weapon. In the form of chloride ionschlorine is necessary to all known species of life. Other types of chlorine compounds are rare in living organisms, and artificially produced chlorinated organics range from inert to toxic.
In the upper atmospherechlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Small quantities of elemental chlorine are generated by oxidation of chloride ions in neutrophils as part of an immune system response against klorin fett.
The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as BC and brine as early as BC. The element was first studied in detail in by Swedish chemist Carl Wilhelm Scheeleand he is credited with the discovery.
Scheele observed several of the properties of chlorine: the bleaching effect on litmusthe deadly effect on insects, the yellow-green color, and the smell similar to aqua regia. Common chemical theory at that time held that an acid is a compound that contains oxygen remnants of this survive in the German and Dutch names of oxygen : sauerstoff or zuurstofboth translating into English as acid substanceso a number of chemists, including Claude Bertholletsuggested that Scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum.