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.

Chlorine, 17Cl
Pronunciation/ˈklɔːrn, -n/ (KLOR-een, -yne)
Appearancepale yellow-green gas
Standard atomic weight Ar, std(Cl)[35.446, 35.457] conventional: 35.45
Chlorine in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Atomic number (Z)17
Groupgroup 17 (halogens)
Periodperiod 3
Block  p-block
Electron configuration[Ne] 3s2 3p5
Electrons per shell2, 8, 7
Physical properties
Phase at STPgas
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[1]
Critical point416.9 K, 7.991 MPa
Heat of fusion(Cl2) 6.406 kJ/mol
Heat of vaporisation(Cl2) 20.41 kJ/mol
Molar heat capacity(Cl2)
33.949 J/(mol·K)
Vapour pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 128 139 153 170 197 239
Atomic properties
Oxidation states−1, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)
ElectronegativityPauling scale: 3.16
Ionisation energies
  • 1st: 1251.2 kJ/mol
  • 2nd: 2298 kJ/mol
  • 3rd: 3822 kJ/mol
  • (more)
Covalent radius102±4 pm
Van der Waals radius175 pm
Color lines in a spectral range
Spectral lines of chlorine
Other properties
Natural occurrenceprimordial
Crystal structure orthorhombic
Speed of sound206 m/s (gas, at 0 °C)
Thermal conductivity8.9×103 W/(m⋅K)
Electrical resistivity>10 Ω⋅m (at 20 °C)
Magnetic orderingdiamagnetic[2]
Molar magnetic susceptibility−40.5×10−6 cm3/mol[3]
CAS NumberCl2: 7782-50-5
Discovery and first isolationCarl Wilhelm Scheele (1774)
Recognized as an element byHumphry Davy (1808)
Main isotopes of chlorine
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
35Cl 76% stable
36Cl trace 3.01×105 y β 36Ar
ε 36S
37Cl 24% stable
 Category: Chlorine
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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.