Chlorine
Definition and meaning of Chlorine in chemistry.
![Chlorine element card: symbol Cl, atomic number 17, atomic mass 35.45, electron configuration [Ne] 3s² 3p⁵, oxidation states +7 +5 +3 +1 -1, Halogen, group 17, period 3, gas at room temperature.](/assets/images/terms/chlorine-chemical-element.webp)
Chlorine, Cl2, is a diatomic gas made of two chlorine atoms joined by a single covalent bond. At room temperature it appears as a dense, yellow-green gas with a sharp, choking odor. It is a highly reactive nonmetal and one of the strongest common oxidizing agents among the halogens.
In more detail
Industrially, chlorine gas is produced by the chlor-alkali process, which uses electricity to split apart concentrated saltwater, or brine. This electrolysis produces chlorine gas at one electrode, along with hydrogen gas and sodium hydroxide as useful byproducts. Chlorine's strong oxidizing power makes it effective at killing bacteria and viruses, which is why it is used to disinfect drinking water and swimming pools.
It is also used to bleach paper and textiles and to manufacture PVC plastic and many other chlorine-containing organic compounds. Chlorine's name comes from the Greek word chloros, meaning pale green, a reference to its color. Despite its usefulness, chlorine gas is toxic and corrosive even in small amounts.
It irritates and damages the lungs, which is why it was infamously used as a chemical weapon during World War I. Modern water treatment plants handle chlorine carefully, using controlled doses and safety equipment to gain its disinfecting benefits without dangerous exposure. Chlorine also exists in nature as chloride ions dissolved in seawater and as the mineral halite, the source of most mined table salt.
Because of its reactivity, elemental chlorine rarely occurs freely in nature and is almost always found combined with other elements.
Key facts
| Formula | Cl2 |
|---|---|
| Field | Inorganic Chemistry |
| Molar mass | 70.90 g/mol |
| Appearance at 25°C | Yellow-green gas, pungent odor |
| Main industrial source | Chlor-alkali electrolysis of brine |
| Common uses | Water disinfection, bleaching, PVC production |
Chlorine reacts directly with hydrogen gas to form hydrogen chloride, following the equation Cl2 + H2 → 2 HCl. This reaction can proceed explosively if it is triggered by a spark or by bright light. Hydrogen chloride gas dissolves in water to form hydrochloric acid, a strong acid used widely in industry and found in dilute form in the stomach.
Frequently asked questions
Why is chlorine gas dangerous to breathe?
Cl2 is a strong oxidizer that reacts with moisture in the lungs to form hydrochloric and hypochlorous acids. These acids severely irritate and damage lung tissue even at low concentrations.
How is chlorine gas manufactured?
Most industrial Cl2 comes from the chlor-alkali process, in which electrolysis of aqueous NaCl brine produces chlorine gas at the anode along with hydrogen gas and sodium hydroxide.
Why is chlorine added to drinking water?
Chlorine is a strong oxidizer that kills bacteria and viruses in water, making it safe to drink at the low concentrations used in treatment.
Related terms
Sources & references
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