Chlorine - Cl
Chlorine (Cl) is a reactive halogen, usually encountered as compounds such as sodium chloride. Elemental chlorine gas, chloride ions and the disinfecting forms of chlorine in water are distinct chemical species.
Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment
Chemical properties of chlorine
| Symbol / atomic number | Cl / 17 |
|---|---|
| Group / period / block | 17 / 3 / p |
| Standard atomic weight interval | [35.446, 35.457] |
| Electron configuration | [Ne] 3s2 3p5 |
| Usual elemental form | Cl2, a yellow-green gas |
| Gas density (20 °C, 1 bar; calculated) | 2.91 g L−1 |
| Melting point | −101.5 °C |
| Boiling point | −34.04 °C |
| Electronegativity (Pauling) | 3.16 |
| Non-bonded atomic radius | 0.175 nm |
| First ionisation energy | 1251.2 kJ mol−1 |
| Stable isotopes | 35Cl and 37Cl |
| First isolated | Carl Wilhelm Scheele, 1774 |
| CAS registry number (Cl2) | 7782-50-5 |
Atomic weight follows CIAAW. Intervals reflect natural isotopic variation; parentheses indicate uncertainty in the final digits. Other properties follow RSC unless noted. Atomic radii depend on their definition. Gas density is an ideal-gas estimate at 20 °C and 1 bar, calculated from ρ = pM/RT using conventional atomic weights; it is not a liquid density.
Properties, uses and occurrence
Elemental chlorine is a yellow-green, reactive gas. It is manufactured mainly from brine and used to make many compounds and materials, including disinfectants and plastics. In nature, chlorine is found in compounds such as sodium chloride, not as free gas.
Chlorine is a member of the halogen group with fluorine, bromine and iodine. The name “chloride” refers to the Cl− ion and its salts.
Health effects and safe handling
Chlorine gas is corrosive and can seriously injure the lungs, eyes and skin. Concentrated hypochlorite products are also hazardous and must be handled according to their labels. Never mix bleach with acids or ammonia-containing products; dangerous gases can be released.
Chloride ion in water is chemically different from chlorine gas. Health and environmental assessments must identify the actual compound and exposure rather than treating every chlorine-containing substance alike.
Chlorine in the environment
When chlorine gas or hypochlorous acid enters water, it reacts with other substances and can be transformed into chloride and other products. Chloride is persistent in many natural waters, and high concentrations can affect taste, salinity and freshwater organisms. Chlorinated organic compounds may form when disinfectants react with natural organic matter; formation depends on water quality and treatment conditions.
Chlorine in water treatment
In water, added chlorine gas or hypochlorite forms free chlorine, mainly hypochlorous acid (HOCl) and hypochlorite ion (OCl−). Their relative amounts depend on pH. Free chlorine is used to inactivate many disease-causing microorganisms and can provide a residual through distribution systems.
Chloramine is a different disinfectant formed from chlorine and ammonia. Disinfectant choice and operation depend on source-water quality, demand, contact conditions, distribution needs and disinfection-by-product requirements. Operators monitor the disinfectant residual and relevant by-products in line with applicable standards; no universal dose or target applies to every system.
After disinfection, some processes use activated carbon or a reducing agent to remove residual chlorine when needed. Chloride ions are not removed by disinfection. See Lenntech's water disinfection information.
Chlorine page last reviewed: by Lenntech.
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