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About Chlorine

Chlorine Bohr

Chlorine in the form of hydrochloric acid has a long history of use; as a component of aqua regia, it was used by alchemists in their experiments as early as the fourteenth century. Pure hydrochloric acid, however, was not produced until centuries later, and its chemical constituents were not known. In 1774, Carl Wilhelm Scheele was the first to document the production of a yellow-green corrosive gas, now known to be elemental chlorine, from a reaction of hydrochloric acid with magnesium oxide. However, it was a common view in chemistry at the time that all acids must contain oxygen, which initially led to the conclusion that this acid-like gas must be a compound. Despite the poor understanding its chemistry, chlorine gas quickly found practical use: in 1785, Claude Berthollet began using it to bleach textiles, shortly thereafter more convenient bleaching agents, calcium and sodium hypochlorites, came into widespread use as textile bleaches and disinfectants. Science finally came to understand the elemental nature of chlorine gas in 1810, thanks to the experimental work of Sir Humphry Davy, who named the element from the Greek chloros in reference to the distinctive color of its gaseous phase.

Chlorine continued to find additional applications throughout the eighteenth century. The photosensitivity of silver halides, including silver chloride, was widely exploited for the production of photographic images starting in 1839, and the organochlorine compound chloroform was first used as an anesthetic in 1847. In 1892, the introduction of the chloralkali process allowed the first production of chlorine on an industrial scale, an advancement which allowed for even more widespread use of chlorine in bleaches, antiseptics, and photography--all applications which continue today--as well as for expanded use of chlorine in industry.

Millions of tons of chlorine are produced and used each year, a fact that reflects the enormous importance of chlorine in modern industry. A large percentage of this chlorine is used directly in the production of polyvinyl chloride (PVC), a versatile and ubiquitous plastic used in everything from water pipes to clothing. Additionally, a significant amount of chlorine is processed to hydrochloric acid, a workhorse of an industrial chemical that finds direct use in steel production, desulfurizing petroleum products, modifying pH in oil wells, coagulating latex, and in many forms of food processing, including the refining of sugar.

Hydrochloric acid is also used to produce many other important chlorine chemicals, including metal chlorides and chlorosilanes. Metal chlorides have many uses: nickel chloride is used for nickel electroplating, zinc chlorides are used for galvanizing and as an electrolyte material in certain types of batteries, iron chloride is used in water treatment, hydrated aluminum chlorides are used in deodorants, iron and aluminum chlorides are important catalysts in organic synthesis, and several metal pentachlorides are used for chemical vapor deposition of their constituent metals in the form of thin films. Chlorosilanes are essential for the production of high purity silicon used in the semiconductor industry in the production of silicones.

Organochlorine compounds may be produced using a variety of chlorine-containing reagents and have a vast range of functions. Low molecular weight chlorinated hydrocarbons such as chloromethanes and tetrachloroethylene are vital non-polar solvents, used in applications such as degreasing and dry cleaning. A number of important herbicides and detergents are also organochlorine compounds. Organochlorine intermediates are used in the production of many types of polymers, including polycarbonates, polyurethanes, silicones, and polytetrafluoroethylene. Additionally, a significant percentage of pharmaceutical ingredients are organic compounds containing chlorine.

Chlorofluorocarbons (CFCs) and polychlorinated biphenyls (PCB) are classes of organochlorine compounds that were once found in a broad range of products, but which have since been largely phased out of use due to the discovery of their potential for damage to health and the environment. CFCs were used as refrigeration fluids, propellants in aerosols, and as solvents, but unfortunately were found to cause considerable damage to the earth’s ozone layer. They have since been replaced in most applications by the much less damaging hydrofluorocarbons (HFCs). PCBs are known to cause both acute symptoms from large exposures and cancer when they accumulate in the body over time. Once used as plasticizers, fire retardants, and coolants, and found in adhesives and paints, PCBs have now been completely banned in some countries, while others have significantly limited their use.

Chlorine’s relationship to biological organisms is somewhat paradoxical. Chlorine ions, usually obtained in the form of sodium chloride--table salt--are absolutely necessary for life: the human body uses them to maintain pH and electrical charge balances in body fluids, makes hydrochloric acid to breakdown food in the stomach, and even produces hypochlorite (chlorine bleach) compounds to help destroy infectious agents. However, chlorine in the form of gas, concentrated hydrochloric acid, or many other chlorine compounds is quite toxic. Chlorine gas has been used as a chemical weapon, and many other chemical warfare agents are chlorine compounds.

Additionally, many organochlorine compounds are both toxic and known to persist for long periods in the environment. For this reason, many such compounds have been either withdrawn from use entirely or tightly regulated. The United Nations Environmental Program produced a list of twelve “dirty dozen” persistent organic pollutants (POPs) of particular concern in 2001; all of which are chlorinated organic compounds. However, it must be noted that the problems with many chlorine compounds--their toxicity and tendency to persist in biological systems--are intimately tied to the advantages of using chlorine in many applications. In pharmaceutical manufacturing, carbon-chlorine bonds enable a drug to remain intact and active in the body for longer periods, while chlorine disinfectants directly exploit the reactivity and resultant toxicity of some chlorine products. It is therefore vital that use of chlorine products be managed responsibly, as their associated risks can never be completely eliminated.

The most common natural chlorine compound, sodium chloride, maybe be mined as rock salt or produced from evaporated saltwater. Much rock salt is used directly for deicing roads and sidewalks, and additionally some salt is simply processed to suitable forms for use in food. The rest is processed using the chloralkali process, producing both chlorine gas and sodium hydroxide through the electrolysis of brine. Very pure hydrochloric acid is produced by reacting chlorine and hydrogen gases. Additionally, hydrochloric acid is produced as a byproduct of many organic synthesis reactions, and therefore a significant quantity of technical and industrial grade hydrochloric acid are recovered from industrial chemical processes.

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Chlorine Products

For additional products, see the Chemicals and Compounds Information Center.

Chlorine Properties

Chlorine Element SymbolChlorine is a Block P, Group 17, Period 3 element.. Its electron configuration is [Ne]3s23p5. The chlorine atom has a covalent radius of 102±4 pm and its Van der Waals radius is 175 pm. In its elemental form, CAS 7440-44-0, chlorine is a yellow-green gas. Chlorine is the second lightest halogen after fluorine. it has the third highest electronegativity and the highest electron affinity of all the elements making it a strong oxidizing agent. It is rarely found by itself in nature. Chlorine was discovered and first isolated by Carl Wilhelm Scheele in 1774. It was first recognized as an element by Humphry Davy in 1808.

Symbol: Cl
Atomic Number: 17
Atomic Weight: 35.45
Element Category: halogen
Group, Period, Block: 17 (halogens), 3, p
Color: greenish-yellow/  yellowish green
Other Names: Le chlore, Chlor, Cloro, Klor
Melting Point: -101.5 °C, -150.7 °F, 171.65 K
Boiling Point: -34.04 °C, -29.272 °F, 239.11 K
Density: 2030 kg·m3
Liquid Density @ Melting Point: 1.5625 g/cm3
Density @ 20°C: 0.003214 g/cm3
Density of Solid: 2030 kg·m3
Specific Heat: N/A
Superconductivity Temperature: N/A
Triple Point: N/A
Critical Point: 416.9 K, 7.991 MPa
Heat of Fusion (kJ·mol-1): 6.41
Heat of Vaporization (kJ·mol-1): 20.4033
Heat of Atomization (kJ·mol-1): 120
Thermal Conductivity: 8.9 x 10?3  W·m-1·K-1
Thermal Expansion: N/A
Electrical Resistivity: (20 °C) 10 nΩ ·m
Tensile Strength: N/A
Molar Heat Capacity: 33.949 J·mol-1·K-1
Young's Modulus: N/A
Shear Modulus: N/A
Bulk Modulus: N/A
Poisson Ratio: N/A
Mohs Hardness: N/A
Vickers Hardness: N/A
Brinell Hardness: N/A
Speed of Sound: (gas, 0 °C) 206 m·s-1
Pauling Electronegativity: 3.16
Sanderson Electronegativity: 3.48
Allred Rochow Electronegativity: 2.83
Mulliken-Jaffe Electronegativity: 3.10 (14.3% s orbital)
Allen Electronegativity: 2.869
Pauling Electropositivity: 0.84
Reflectivity (%): N/A
Refractive Index: 1.000773
Electrons: 17
Protons: 17
Neutrons: 18
Electron Configuration: [Ne]3s23p5
Atomic Radius: N/A
Atomic Radius,
non-bonded (Å):
1.75
Covalent Radius: 102±4 pm
Covalent Radius (Å): 1
Van der Waals Radius: 175 pm
Oxidation States: 7, 6, 5, 4, 3, 2, 1, -1 (strongly acidic oxide)
Phase: Gas
Crystal Structure: orthorhombic
Magnetic Ordering: diamagnetic
Electron Affinity (kJ·mol-1) 348.602
1st Ionization Energy: 1251.2 kJ·mol-1
2nd Ionization Energy: 2298 kJ·mol-1
3rd Ionization Energy: 3822 kJ·mol-1
CAS Number: 7782-50-5
EC Number: 231-959-5
MDL Number: MFCD00010934
Beilstein Number: 3902968
SMILES Identifier: [Cl]
InChI Identifier: InChI=1S/Cl
InChI Key: ZAMOUSCENKQFHK-UHFFFAOYSA-N
PubChem CID: 24526
ChemSpider ID: 4514529
Earth - Total: 19.9 ppm
Mercury - Total: 0.23 ppm
Venus - Total: 20.9 ppm
Earth - Seawater (Oceans), ppb by weight: 19870000
Earth - Seawater (Oceans), ppb by atoms: 3470000
Earth -  Crust (Crustal Rocks), ppb by weight: 170000
Earth -  Crust (Crustal Rocks), ppb by atoms: 100000
Sun - Total, ppb by weight: 8000
Sun - Total, ppb by atoms: 300
Stream, ppb by weight: 8000
Stream, ppb by atoms: 230
Meterorite (Carbonaceous), ppb by weight: 380000
Meterorite (Carbonaceous), ppb by atoms: 160000
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: N/A
Universe, ppb by atom: N/A
Discovered By: Carl Wilhelm Scheele
Discovery Date: 1774
First Isolation: N/A

Health, Safety & Transportation Information for Chlorine

Safety data for Chlorine and its compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific material or compound referenced in the Products tab. The below information applies to elemental Chlorine.

Safety Data
Material Safety Data Sheet MSDS
Signal Word Danger
Hazard Statements H270-H280-H315-H319-H331-H335-H400
Hazard Codes T,N
Risk Codes 23-36/37/38-50
Safety Precautions 9-45-61
RTECS Number FO2100000
Transport Information UN 1017 2.3
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)
Environment-Hazardous to the aquatic environment Skull and Crossbones-Acute Toxicity  Oxidizing Liquid - Oxidizing Gas Gas Cylinder - Gases Under Pressure

Chlorine Isotopes

Chlorine has two stable isotopes: 35Cl and 37Cl.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
28Cl 28.02851(54)# N/A p to 27S (1+)# N/A 181.04 -
29Cl 29.01411(21)# <20 ns p to 28S (3/2+)# N/A 202.16 -
30Cl 30.00477(21)# <30 ns p to 29S (3+)# N/A 219.56 -
31Cl 30.99241(5) 150(25) ms ß+ to 31S; ß+ + p to 30P 3/2+ N/A 238.82 -
32Cl 31.985690(7) 298(1) ms ß+ to 32S; ß+ + a to 28Si; ß+ + p to 31P 1+ N/A 253.42 -
33Cl 32.9774519(5) 2.511(3) s ß+ to 33S 3/2+ N/A 268.95 -
34Cl 33.97376282(19) 1.5264(14) s ß+ to 34S 0+ N/A 280.76 -
35Cl 34.96885268(4) STABLE - 3/2+ 0.8218736 293.49 75.78
36Cl 35.96830698(8) 3.01(2)E+5 y ß- to 36Ar 2+ 1.28547 301.57 -
37Cl 36.96590259(5) STABLE - 3/2+ 0.684123 312.44 24.22
38Cl 37.96801043(10) 37.24(5) min ß- to 38Ar 2- 2.05 317.73 -
39Cl 38.9680082(19) 55.6(2) min ß- to 39Ar 3/2+ N/A 325.81 -
40Cl 39.97042(3) 1.35(2) min ß- to 40Ar 2- N/A 332.02 -
41Cl 40.97068(7) 38.4(8) s ß- to 41Ar (1/2+,3/2+) N/A 340.1 -
42Cl 41.97325(15) 6.8(3) s ß- to 42Ar N/A N/A 345.38 -
43Cl 42.97405(17) 3.07(7) s ß- to 43Ar; ß- + n to 42Ar 3/2+# N/A 352.53 -
44Cl 43.97828(12) 0.56(11) s ß- to 44Ar; ß- + n to 43Ar N/A N/A 356.88 -
45Cl 44.98029(13) 400(40) ms ß- to 45Ar; ß- + n to 44Ar 3/2+# N/A 363.1 -
46Cl 45.98421(77) 232(2) ms ß- + n to 45Ar; ß- to 46Ar N/A N/A 367.45 -
47Cl 46.98871(64)# 101(6) ms ß- to 47Ar; ß- + n to 46Ar 3/2+# N/A 371.8 -
48Cl 47.99495(75)# 100# ms [>200 ns] ß- to 48Ar N/A N/A 374.29 -
49Cl 49.00032(86)# 50# ms [>200 ns] ß- to 49Ar 3/2+# N/A 376.78 -
50Cl 50.00784(97)# 20# ms ß- to 50Ar N/A N/A 378.34 -
51Cl 51.01449(107)# 2# ms [>200 ns] ß- to 51Ar 3/2+# N/A 379.9 -
Chlorine Elemental Symbol

Recent Research & Development for Chlorine

  • Development of Combined Dry Heat and Chlorine Dioxide Gas Treatment with Mechanical Mixing for Inactivation of Salmonella enterica Serovar Montevideo on Mung Bean Seeds.. Annous BA, Burke A.. J Food Prot. 2015 May
  • Kinetic Study of the Gas-Phase Reactions of Chlorine Atoms with 2-Chlorophenol, 2-Nitrophenol, and Four Methyl-2-nitrophenol Isomers.. Bejan I, Duncianu M, Olariu R, Barnes I, Seakins PW, Wiesen P.. J Phys Chem A. 2015 May 21
  • Decay of free residual chlorine in drinking water at the point of use.. Sheikhi R, Alimohammadi M, Askari M, Moghaddasian MS.. Iran J Public Health. 2014 Apr
  • Emerging nitrogenous disinfection byproducts: Transformation of the antidiabetic drug metformin during chlorine disinfection of water.. Armbruster D, Happel O, Scheurer M, Harms K, Schmidt TC, Brauch HJ.. Water Res. 2015 Apr 27
  • [Determination of chlorine in gasoline by inductively coupled plasma atomic emission spectrometry].. Zhao Y, Chen XY, Xu DY, Zhang SY, Chen ZY.. Guang Pu Xue Yu Guang Pu Fen Xi. 2014 Dec
  • Surrogate testing suggests that chlorine dioxide gas exposure would not inactivate Ebola virus contained in environmental blood contamination.. Lowe JJ, Hewlett AL, Iwen PC, Smith PW, Gibbs SG.. J Occup Environ Hyg. 2015 May 8:0.
  • A novel microfluidic mixer-based approach for determining inactivation kinetics of Escherichia coli O157:H7 in chlorine solutions.. Zhang B, Luo Y, Zhou B, Wang Q, Millner PD.. Food Microbiol. 2015 Aug
  • The effect of chlorine and fluorine substitutions on tuning the ionization potential of benzoate-bridged paddlewheel diruthenium(ii, ii) complexes.. Kosaka W, Itoh M, Miyasaka H.. Dalton Trans. 2015 Apr 21
  • Potential biodefense model applications for portable chlorine dioxide gas production.. Stubblefield JM, Newsome AL.. Health Secur. 2015 Jan-Feb
  • Chlorine Functionalization of a Model Phenolic C8-Guanine Adduct Increases Conformational Rigidity and Blocks Extension by a Y-Family DNA Polymerase.. Witham A, Verwey A, Sproviero M, Manderville RA, Sharma P, Wetmore SD.. Chem Res Toxicol. 2015 May 24.
  • Combination treatment of chlorine dioxide gas and aerosolized sanitizer for inactivating foodborne pathogens on spinach leaves and tomatoes.. Park SH, Kang DH.. Int J Food Microbiol. 2015 May 2