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 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 Pauling scale, behind only oxygen and fluorine. Nitrogen is also more electronegative in many other scales such as Mulliken-Jaffe, Allred-Rochow, Noorizadeh-Shakerzadeh, Nagle, Martynov-Batsanov and Allen electronegativity scales.
|Pronunciation||/ - /, |
|Appearance||pale yellow-green gas|
|Standard atomic weight Ar, std(Cl)||[35.446, 35.457] conventional: 35.45|
|Chlorine in the periodic table|
|Atomic number (Z)||17|
|Group||group 17 (halogens)|
|Electron configuration||[Ne] 3s2 3p5|
|Electrons per shell||2, 8, 7|
|Phase at STP||gas|
|Melting point||(Cl2) 171.6 K (−101.5 °C, −150.7 °F)|
|Boiling point||(Cl2) 239.11 K (−34.04 °C, −29.27 °F)|
|Density (at STP)||3.2 g/L|
|when liquid (at b.p.)||1.5625 g/cm3|
|Critical point||416.9 K, 7.991 MPa|
|Heat of fusion||(Cl2) 6.406 kJ/mol|
|Heat of vaporisation||(Cl2) 20.41 kJ/mol|
|Molar heat capacity||(Cl2)|
|Oxidation states||−1, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)|
|Electronegativity||Pauling scale: 3.16|
|Covalent radius||102±4 pm|
|Van der Waals radius||175 pm|
|Spectral lines of chlorine|
|Speed of sound||206 m/s (gas, at 0 °C)|
|Thermal conductivity||8.9×10−3 W/(m⋅K)|
|Electrical resistivity||>10 Ω⋅m (at 20 °C)|
|Molar magnetic susceptibility||−40.5×10−6 cm3/mol|
|CAS Number||Cl2: 7782-50-5|
|Discovery and first isolation||Carl Wilhelm Scheele (1774)|
|Recognized as an element by||Humphry Davy (1808)|
|Main isotopes of chlorine|
Chlorine played an important role in the experiments conducted by medieval alchemists, which commonly involved the heating of chloride salts like ammonium chloride (sal ammoniac) and sodium chloride (common salt), producing various 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 pure element, and this was confirmed by Sir Humphry Davy in 1810, who named it from Ancient Greek: χλωρός, romanized: khlōrós, 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 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 electrolysis, predominantly in the chlor-alkali process. The high oxidising potential of elemental chlorine led to the development of commercial bleaches and disinfectants, and a reagent 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 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-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 ions, 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 upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Small quantities of elemental chlorine are generated by oxidation of chloride to hypochlorite in neutrophils as part of an immune system response against bacteria.