Chlorine

Chlorine is a <a href="/facts/Chemical_element/0jducgWD">chemical element</a>; it has <a href="/facts/Symbol_(chemistry)/2Fe1VAL0">symbol</a> Cl and <a href="/facts/Atomic_number/bgAr581S">atomic number</a> 17. The second-lightest of the <a href="/facts/Halogen/YeFgWHeA">halogens</a>, it appears between <a href="/facts/Fluorine/YUV0Fb8m">fluorine</a> and <a href="/facts/Bromine/oLfj3C7D">bromine</a> 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 <a href="/facts/Oxidizing_agent/1FQzB8pQ">oxidising agent</a>: among the elements, it has the highest <a href="/facts/Electron_affinity/1mf4A396">electron affinity</a> and the third-highest <a href="/facts/Electronegativity/H7Vq7Uah">electronegativity</a> on the revised <a href="/facts/Electronegativity/H7Vq7Uah">Pauling scale</a>, behind only <a href="/facts/Oxygen/acyn6o8r">oxygen</a> and fluorine.
Chlorine played an important role in the experiments conducted by medieval <a href="/facts/Alchemy/CEx0U1Em">alchemists</a>, which commonly involved the heating of chloride <a href="/facts/Salt_(chemistry)/lsSKwGpn">salts</a> like <a href="/facts/Ammonium_chloride/SfXdntlv">ammonium chloride</a> (<a href="/facts/Sal_ammoniac/1XD61gHP">sal ammoniac</a>) and <a href="/facts/Sodium_chloride/MWcXl3Mb">sodium chloride</a> (<a href="/facts/Common_salt/SCplalQ3">common salt</a>), producing various chemical substances containing chlorine such as <a href="/facts/Hydrogen_chloride/7Wzkhomd">hydrogen chloride</a>, <a href="/facts/Mercury(II)_chloride/aUAKsq6a">mercury(II) chloride</a> (corrosive sublimate), and <a href="/facts/Aqua_regia/AHr1mobH">aqua regia</a>. However, the nature of free chlorine gas as a separate substance was only recognised around 1630 by <a href="/facts/Jan_Baptist_van_Helmont/3P0mC614">Jan Baptist van Helmont</a>. <a href="/facts/Carl_Wilhelm_Scheele/kcKBwyXa">Carl Wilhelm Scheele</a> wrote a description of chlorine gas in 1774, supposing it to be an <a href="/facts/Oxide/DXtoVe1C">oxide</a> of a new element. In 1809, chemists suggested that the gas might be a pure element, and this was confirmed by <a href="/facts/Sir_Humphry_Davy/aDNRfNou">Sir Humphry Davy</a> in 1810, who named it after the <a href="/facts/Ancient_Greek_language/LIK7NlUh">Ancient Greek</a> 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 <a href="/facts/Ion/3bePpDna">ionic</a> <a href="/facts/Chloride/m9NUaUDR">chloride</a> compounds, which includes table salt. It is the <a href="/facts/Abundance_of_elements_in_Earth%2527s_crust/UDSVHH56">second-most abundant</a> halogen (after fluorine) and 20th most abundant element in Earth's crust. These crystal deposits are nevertheless dwarfed by the huge reserves of chloride in seawater.
Elemental chlorine is commercially produced from <a href="/facts/Brine/xsN2GcLF">brine</a> by <a href="/facts/Electrolysis/pU75nCDI">electrolysis</a>, predominantly in the <a href="/facts/Chloralkali_process/5aAamEP9">chloralkali process</a>. The high oxidising potential of elemental chlorine led to the development of commercial <a href="/facts/Bleach_(chemical)/QPxAysPl">bleaches</a> and <a href="/facts/Disinfectant/33fF2QzG">disinfectants</a>, and a <a href="/facts/Reagent/c3GLInCu">reagent</a> for many processes 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 <a href="/facts/Polyvinyl_chloride/9VkegJvr">polyvinyl chloride</a> (PVC), many intermediates for the production of <a href="/facts/Plastics/Kla5QItn">plastics</a>, 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 <a href="/facts/Swimming_pool/32DUFtdN">swimming pools</a> to keep them <a href="/facts/Swimming_pool_sanitation/PI5VtBx7">sanitary</a>. Elemental chlorine at high <a href="/facts/Concentration/wqBmz5od">concentration</a> is extremely dangerous, and <a href="/facts/Poisonous/XIQjhpsw">poisonous</a> to most living organisms. As a <a href="/facts/Chemical_warfare/zT4t2DGk">chemical warfare</a> agent, chlorine was first used in <a href="/facts/World_War_I/vITTUanQ">World War I</a> as a <a href="/facts/Poison_gas/JUtjyv9C">poison gas</a> weapon.
In the form of chloride <a href="/facts/Ions/wrbwhF3z">ions</a>, chlorine 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 <a href="/facts/Upper_atmosphere/5yH29VP8">upper atmosphere</a>, chlorine-containing organic molecules such as <a href="/facts/Chlorofluorocarbon/uLApxm7K">chlorofluorocarbons</a> have been implicated in <a href="/facts/Ozone_depletion/ssGWpcx3">ozone depletion</a>. Small quantities of elemental chlorine are generated by oxidation of chloride ions in <a href="/facts/Neutrophil/KL6oTt8b">neutrophils</a> as part of an <a href="/facts/Immune_system/HRCTf1pb">immune system</a> response against bacteria.

Chlorine open-in-new

Chlorine