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17 sulfurchlorineargon


Name, symbol, number chlorine, Cl, 17
Chemical serieshalogens
Group, period, block 173, p
Appearanceyellowish green
Standard atomic weight 35.453(2) g·mol−1
Electron configuration [Ne] 3s2 3p5
Electrons per shell 2, 8, 7
Physical properties
Density(0 °C, 101.325 kPa)
3.2 g/L
Melting point171.6 K
(-101.5 °C, -150.7 °F)
Boiling point239.11 K
(-34.4 °C, -29.27 °F)
Critical point416.9 K, 7.991 MPa
Heat of fusion(Cl2) 6.406 kJ·mol−1
Heat of vaporization(Cl2) 20.41 kJ·mol−1
Heat capacity(25 °C) (Cl2)
33.949 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 128 139 153 170 197 239
Atomic properties
Crystal structureorthorhombic
Oxidation states±1, 3, 5, 7
(strongly acidic oxide)
Electronegativity3.16 (Pauling scale)
Ionization energies
1st: 1251.2 kJ·mol−1
2nd: 2298 kJ·mol−1
3rd: 3822 kJ·mol−1
Atomic radius100 pm
Atomic radius (calc.)79 pm
Covalent radius99 pm
Van der Waals radius175 pm
Magnetic orderingnonmagnetic
Electrical resistivity(20 °C) > 10 Ω·m
Thermal conductivity(300 K) 8.9x10-3  W·m−1·K−1
CAS registry number7782-50-5
Selected isotopes
Main article: Isotopes of chlorine
iso NA half-life DM DE (MeV) DP
35Cl 75.77% Cl is stable with 18 neutrons
36Cl syn 3.01×105 y β- 0.709 36Ar
ε - 36S
37Cl 24.23% Cl is stable with 20 neutrons
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Chlorine (IPA: /ˈklɔəriːn/, Greek: χλωρóς chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VIIa or VIIb). As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its common elemental form (Cl2 or "dichlorine") under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm[1] and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine based molecules have been implicated in the destruction of the ozone layer.



Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it dephlogisticated muriatic acid (see phlogiston theory) and mistakenly thought it contained oxygen. Scheele isolated chlorine by reacting MnO2 with HCl. Chlorine was given its current name in 1810 by Sir Humphry Davy, who insisted that it was in fact an element.

World War I


Iraq War

Chlorine gas has also been used by insurgents in the Iraq War as a chemical weapon to terrorize the local population and coalition forces. On March 17, 2007, for example, three chlorine filled trucks were detonated in the Anbar province killing 2 and sickening over 350.[3] Other chlorine bomb attacks resulted in higher death tolls, with more than 30 deaths on two separate occasions.[4] Most of the deaths were caused by the force of the explosions rather than the effects of chlorine, since the toxic gas is readily dispersed and diluted in the atmosphere by the blast. The Iraqi authorities have tightened up security for chlorine, which is essential for providing safe drinking water for the population.


Main article: Isotopes of chlorine

Chlorine has isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), giving chlorine atoms in bulk an apparent atomic weight of 35.5 g/mol.



Notable characteristics

  Chlorine gas is diatomic, with the formula Cl2. It combines readily with all elements except O2 and N2[5] and the noble gases. Compounds with oxygen, nitrogen, and xenon are known but do not form by direct reaction of the elements.[6] Chlorine is not as extremely reactive as fluorine. Pure chlorine gas does, however, support combustion of organic compounds such as hydrocarbons, although the carbon component tends to burn incompletely, with much of it remaining as soot.[7] At 10°C and atmospheric pressure, one liter of water dissolves 3.10 L of gaseous chlorine, and at 30°C, 1 L of water dissolves only 1.77 liters of chlorine.[5]

This element is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. As the chloride ion, Cl, it is also the most abundant dissolved ion in ocean water.


See also Halide minerals.

In nature, chlorine is found primarily as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans — about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits. Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Common chloride minerals include halite (sodium chloride), sylvite (potassium chloride), and carnallite (potassium magnesium chloride hexahydrate). Over 2000 naturally-occurring organic chlorine compounds are known.[8]


2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH


Chlorine gas extraction


Cathode: 2 H+ (aq) + 2 e → H2 (g)


Mercury cell electrolysis




Diaphragm cell electrolysis

In diaphragm cell electrolysis, an asbestos (or polymer-fiber) diaphragm separates cathode and anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathode.[13] This technology was also developed at the end of the nineteenth century. There are several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird cell (1901), the Gibbs cell (1908), and the Townsend cell (1904).[14][15] The cells vary in construction and placement of the diaphragm, with some having the diaphragm in direct contact with the cathode.


As a result, diaphragm methods produce alkali that is quite dilute (about 12%) and of lower purity than do mercury cell methods. But diaphragm cells are not burdened with the problem of preventing mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings over the mercury cell method[15], but large amounts of steam are required if the caustic has to be evaporated to the commercial concentration of 50%.

Membrane cell electrolysis

Development of this technology began in the 1970s. The electrolysis cell is divided into two "rooms" by a cation permeable membrane acting as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration.[16] Sodium (or potassium) hydroxide solution is circulated through the cathode compartment, exiting at a higher concentration. A portion of the concentrated sodium hydroxide solution leaving the cell is diverted as product, while the remainder is diluted with deionized water and passed through the electrolyzer again.


Other electrolytic processes

Although a much lower production scale is involved, electrolytic diaphragm and membrane technologies are also used industrially to recover chlorine from hydrochloric acids solutions, producing hydrogen (but no caustic alkali) as a co-product.


Other methods


4 HCl + O2 → 2 Cl2 + 2 H2O

This reaction is accomplished with the use of CuCl2 as a catalyst and is performed at high temperarature (about 400°C). The amount of extracted chlorine is approximately 80%. Due to the extremely corrosive reaction mixture, industrial use of this method is difficult and several pilot trials failed in the past. Nevertheless, recent developments are promising.





In the laboratory, small amounts of chlorine gas can also be created by adding concentrated hydrochloric acid (typically about 5M) to sodium hypochlorite or sodium chlorate solution.

Industrial production

Large-scale production of chlorine involves several steps and many pieces of equipment. The description below is typical of a membrane plant. The plant also produces simultaneously sodium hydroxide (referred to in the industry as caustic soda) and hydrogen gas. A typical plant consists of brine production/treatment, cell operations, chlorine cooling & drying, chlorine compression & liquefaction, liquid chlorine storage & loading, caustic handling, evaporation, storage & loading and hydrogen handling.




After the ion exchangers the brine is considered pure, and is transferred to storage tanks to be pumped into the cell room. Brine fed to the cell line is heated to the correct temperature to control exit brine temperatures according to the electrical load. Brine exiting the cell room must be treated to remove residual chlorine and control pH before being returned to the saturation stage. This can be accomplished via dechlorination towers with acid and sodium bisulfite addition. Failure to remove chlorine can result in damage to the cells. Brine should be monitored for accumulation of chlorate and sulfate and either have treatment systems in place or purging of the brine loop to maintain safe levels, since chlorate can diffuse through the membranes and contaminate the caustic, while sulfate can damage the anode surface coating.

Cell room


Direct electrical current is supplied via rectifiers. Plant load is controlled by varying the current to the cells. As the current is increased flow rates for brine, caustic and deionized water are increased while lowering the feed temperatures.

Cooling and drying

Chlorine gas exiting the cell line must be cooled and dried since the exit gas can be over 80º C and contains moisture that allows chlorine gas to be corrosive to iron piping. Cooling the gas allows for a large amount of moisture from the brine to condense out of the gas stream. Cooling also improves the efficiency of the compression and liquefaction stage that follows. Chlorine exiting is ideally between 18º C and 25º C. After cooling the gas stream passes through a series of towers with counter flowing sulfuric acid. These towers progressively remove any remaining moisture from the chlorine gas. After exiting the drying towers the chlorine is filtered to remove any sulfuric acid droplets.

Compression and liquefaction


Storage and loading


Caustic handling, evaporation, storage and loading


Hydrogen handling

Hydrogen produced may be vented unprocessed directly to the atmosphere or cooled, compressed and dried for use in other processes on site or sold to a customer via pipeline, cylinders or trucks. Some possible uses are hydrochloric acid or hydrogen peroxide production, desulfurization of petroleum oils and use as a fuel in boilers or fuel cells.

Energy consumption

Production of chlorine is extremely energy intensive.[18] Energy consumption per unit weight of product is not far below that for iron and steel manufacture[19] and greater than for the production of glass[20] or cement.[21]



See also Chlorine compounds

For general references to the chloride ion (Cl), including references to specific chlorides, see chloride. For other chlorine compounds see chlorate (ClO3), chlorite (ClO2), hypochlorite(ClO), and perchlorate(ClO4), and chloramine (NH2Cl).[22]

Other chlorine-containing compounds include:

Oxidation states

Name Formula Example compounds
−1 chlorides Cl ionic chlorides, organic chlorides, hydrochloric acid
0 chlorine Cl2 elemental chlorine
+1 hypochlorites ClO sodium hypochlorite, calcium hypochlorite
+3 chlorites ClO2 sodium chlorite
+5 chlorates ClO3 sodium chlorate, potassium chlorate, chloric acid
+7 perchlorates ClO4 potassium perchlorate, perchloric acid,magnesium perchlorate
organic perchlorates, ammonium perchlorate


Cl2 + 2OH → Cl + ClO + H2O

In hot concentrated alkali solution disproportionation continues:

2ClO → Cl + ClO2
ClO + ClO2 → Cl + ClO3


4ClO3 → Cl + 3ClO4

This same progression from chloride to perchlorate can be accomplished by electrolysis. The anode reaction progression is:[23]

Reaction Electrode
Cl + 2OH → ClO + H2O + 2e +0.89 volts
ClO + 2OH → ClO2 + H2O + 2e +0.67 volts
ClO2 + 2OH → ClO3 + H2O + 2e +0.33 volts
ClO3 + 2OH → ClO4 + H2O + 2e +0.35 volts

Each step is accompanied at the cathode by

2H2O + 2e → 2OH + H2          −0.83 volts

Applications and uses

Production of industrial and consumer products

Chlorine's principal applications are in the production of a wide range of industrial and consumer products.[24] [25] For example, it is used in making plastics, solvents for dry cleaning and metal degreasing, textiles, agrochemicals and pharmaceuticals, insecticides, dyestuffs, etc.

Purification and disinfection








Other uses






See also


  1. ^ Merck Index of Chemicals and Drugs, 9th ed., monograph 2065
  2. ^ Weapons of War: Poison Gas. First World Retrieved on 2007-08-12.
  3. ^ Mahdi, Basim. "Iraq gas attack makes hundreds ill", CNN, 2007-03-17. Retrieved on 2007-03-17. 
  4. ^ "'Chlorine bomb' hits Iraq village", BBC News, 2007-05-17. Retrieved on 2007-05-17. 
  5. ^ a b – Chlorine. Mark Winter [The University of Sheffield and WebElements Ltd, UK]. Retrieved on 2007-03-17.
  6. ^ Merck Index of Chemicals and Drugs, 9th ed.
  7. ^ Lange's Handbook of Chemistry, 10th ed
  8. ^ Risk assessment and the cycling of natural organochlorines. Euro Chlor. Retrieved on 2007-08-12.
  9. ^ Pauling, Linus, General Chemistry, 1970 ed., Dover publications
  10. ^ Electrolytic Processes for Chlorine and Caustic Soda. Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands. Retrieved on 2007-03-17.
  11. ^ Mercury cell. Euro Chlor. Retrieved on 2007-08-15.
  12. ^ Regional Awareness-raising Workshop on Mercury Pollution. UNEP. Retrieved on 2007-10-28.
  13. ^ Diaphragm cell. Euro Chlor. Retrieved on 2007-08-15.
  14. ^ The Electrolysis of Brine. Salt Manufacturers' Association. Retrieved on 2007-03-17.
  15. ^ a b Kiefer, David M.. When the Industry Charged Ahead. Chemistry Chronicles. Retrieved on 2007-03-17.
  16. ^ Membrane cell. Euro Chlor. Retrieved on 2007-08-15.
  17. ^ The Chlorine Industry. Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands. Retrieved on 2007-03-17.
  18. ^ Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Chlor-Alkali Manufacturing Industry. European Commission. Retrieved on 2007-09-02.
  19. ^ Integrated Pollution Prevention and Control (IPPC) - Best Available Techniques Reference Document on the Production of Iron and Steel. European Commission. Retrieved on 2007-09-02.
  20. ^ Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Glass Manufacturing Industry. European Commission. Retrieved on 2007-09-02.
  21. ^ Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Cement and Lime Manufacturing Industries. European Commission. Retrieved on 2007-09-02.
  22. ^ Chlorine compounds of the month. Euro Chlor. Retrieved on 2007-08-29.
  23. ^ Cotton, F. Albert and Wilkinson, Geoffrey, Advanced Inorganic Chemistry 2nd ed. John Wiley & sons, p568
  24. ^ Uses. Euro Chlor. Retrieved on 2007-08-20.
  25. ^ Chlorine Tree. Chlorine Tree. Retrieved on 2007-08-20.
  26. ^ Chlorine. Los Alamos National Laboratory. Retrieved on 2007-03-17.
  27. ^ a b "Chlorine." MSDS. Issued on October 23, 1997; Revised on November 1, 1999; Retrieved on September 8, 2007.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chlorine". A list of authors is available in Wikipedia.