Chlorine (Cl) is a chemical element and the second lightest member of the halogen elements. In 1774, Carl Wilhelm Scheele produced chlorine gas, but he didn't realize it was an element. In 1810, Humphry Davy named it chlorine, from the Greek chloros , meaning yellowish green. Chlorine is a toxic, corrosive, greenish-yellow gas that irritates the eyes and respiratory system. It has a choking smell, and inhalation can cause suffocation and fluid in the lungs. Exposure to one part per thousand in the air can be fatal within minutes. Chlorine is used in many chemical processes, including the production of carbon tetrachloride , glycols , and polyvinyl chloride . It is also used to sterilize water and as a bleaching agent for paper and textiles.

Where can chlorine be found in nature?

Chlorine, with the symbol Cl, is found in nature primarily in the form of chemical compounds, not as a free element, due to its high reactivity. It makes up about 0.017 percent of the Earth's crust. The most common chlorine compound is sodium chloride (NaCl), known as crystalline rock salt, often with impurities. Sodium chloride is also abundant in seawater, at an average concentration of roughly 2 percent. Higher concentrations of dissolved salt, up to 33 percent, can be found in landlocked seas such as the Caspian Sea , Dead Sea , and Great Salt Lake in Utah. Other chlorine-containing minerals include sylvite (potassium chloride), bischofite, carnallite, and kainite. Small amounts of sodium chloride are also present in blood and milk.

What are some common uses of chlorine?

Chlorine is a widely used element with several applications. It plays a significant role in the manufacture of chemical compounds , including sulfur chloride, thionyl chloride, and phosgene. It is also used to create organic chemicals like methyl chloride and ethylene chloride. Another key application is in water treatment , where chlorine and its compounds disinfect public water supplies, killing harmful microorganisms. Additionally, chlorine is used as a bleaching agent for paper and textiles. It is also used in the production of plastics, dyes, and synthetic rubber.

What are the physical properties of chlorine?

Chlorine (Cl) is a chemical element and a member of the halogen group. At room temperature, chlorine is a greenish-yellow gas, about two and a half times heavier than air. It becomes a liquid at -34 °C (-29 °F). It has a choking smell, and inhalation can cause suffocation, chest constriction, and fluid buildup in the lungs after severe exposure. Here's a list of its other physical properties:

Why is chlorine important for clean water?

Chlorine , a greenish-yellow gas, is crucial for clean water because it disinfects and destroys harmful bacteria, preventing waterborne diseases. This process, called chlorination , is typically the final step in treating drinking water. Chlorine is economical for disinfecting large volumes of water, and proper dosages minimize taste and odor issues. While chlorine can react with organic compounds to form potentially harmful by-products, the risk is small when applied correctly. An alternative is chloramine, which lasts longer and reduces harmful by-products.

What are the safety concerns when handling chlorine?

Chlorine gas poses significant safety concerns due to its irritating and potentially toxic nature. It can severely irritate the eyes and the respiratory tract. Exposure occurs primarily through inhalation, absorption through the skin, or accidental ingestion in the workplace. To prevent chlorine poisoning, ensure adequate ventilation and regular air quality testing. If exposed, move the affected individual to fresh air and administer artificial respiration and oxygen. Because some gases have insidious, delayed effects, medical observation is essential.

chlorine

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Images

A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used.

Diagram showing the formation of an ionic bond between sodium (Na) and chlorine (Cl) atoms. Sodium loses an electron to become Na+, chlorine gains an electron to become Cl-, resulting in sodium chloride (NaCl), or table salt.

Explore an interactive periodic table of the elements

A scientific graph plots average nuclear binding energy per nucleon (in MeV) against atomic mass number, showing a sharp rise at low mass numbers, peaking near iron (Fe), then gradually declining for heavier elements. Key isotopes are labeled along the curve.

Figure 13: Transformations of chlorine compounds in the atmosphere.

Diagram showing the chemical reaction where methane (CH4) and chlorine (Cl2) combine to produce chloromethane (CH3Cl) and hydrogen chloride (HCl), with molecular structures and labels.

At a Glance

chlorine summary

chlorine

chemical element

Also known as: Cl

Written and fact-checked by

Britannica Editors

Last updated

Oct. 22, 2026 •History

Top Questions

What is chlorine?
Chlorine (Cl) is a chemical element and the second lightest member of the halogen elements. In 1774, Carl Wilhelm Scheele produced chlorine gas, but he didn't realize it was an element. In 1810, Humphry Davy named it chlorine, from the Greek chloros, meaning yellowish green.

Chlorine is a toxic, corrosive, greenish-yellow gas that irritates the eyes and respiratory system. It has a choking smell, and inhalation can cause suffocation and fluid in the lungs. Exposure to one part per thousand in the air can be fatal within minutes.

Chlorine is used in many chemical processes, including the production of carbon tetrachloride, glycols, and polyvinyl chloride. It is also used to sterilize water and as a bleaching agent for paper and textiles.
2 Britannica Sources
1. chlorine
2. organohalogen compound: Natural occurrence
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where can chlorine be found in nature?
Chlorine, with the symbol Cl, is found in nature primarily in the form of chemical compounds, not as a free element, due to its high reactivity. It makes up about 0.017 percent of the Earth's crust.

The most common chlorine compound is sodium chloride (NaCl), known as crystalline rock salt, often with impurities. Sodium chloride is also abundant in seawater, at an average concentration of roughly 2 percent. Higher concentrations of dissolved salt, up to 33 percent, can be found in landlocked seas such as the Caspian Sea, Dead Sea, and Great Salt Lake in Utah.

Other chlorine-containing minerals include sylvite (potassium chloride), bischofite, carnallite, and kainite. Small amounts of sodium chloride are also present in blood and milk.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some common uses of chlorine?
Chlorine is a widely used element with several applications. It plays a significant role in the manufacture of chemical compounds, including sulfur chloride, thionyl chloride, and phosgene. It is also used to create organic chemicals like methyl chloride and ethylene chloride.

Another key application is in water treatment, where chlorine and its compounds disinfect public water supplies, killing harmful microorganisms. Additionally, chlorine is used as a bleaching agent for paper and textiles. It is also used in the production of plastics, dyes, and synthetic rubber.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the physical properties of chlorine?
Chlorine (Cl) is a chemical element and a member of the halogen group. At room temperature, chlorine is a greenish-yellow gas, about two and a half times heavier than air. It becomes a liquid at -34 °C (-29 °F). It has a choking smell, and inhalation can cause suffocation, chest constriction, and fluid buildup in the lungs after severe exposure.

Here's a list of its other physical properties:
- Atomic number: 17
- Atomic weight: 35.446 to 35.457
- Melting point: -103 °C (-153 °F)
- Boiling point: -34 °C (-29 °F)
- Density: 3.214 g/litre (0.429 ounce/gallon)
- Oxidation states: −1, +1, +3, +5, +7
- Electron configuration 1s22s22p63s23p5
1 Britannica Source
1. chlorine
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is chlorine important for clean water?
Chlorine, a greenish-yellow gas, is crucial for clean water because it disinfects and destroys harmful bacteria, preventing waterborne diseases. This process, called chlorination, is typically the final step in treating drinking water.

Chlorine is economical for disinfecting large volumes of water, and proper dosages minimize taste and odor issues. While chlorine can react with organic compounds to form potentially harmful by-products, the risk is small when applied correctly. An alternative is chloramine, which lasts longer and reduces harmful by-products.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the safety concerns when handling chlorine?
Chlorine gas poses significant safety concerns due to its irritating and potentially toxic nature. It can severely irritate the eyes and the respiratory tract. Exposure occurs primarily through inhalation, absorption through the skin, or accidental ingestion in the workplace.

To prevent chlorine poisoning, ensure adequate ventilation and regular air quality testing. If exposed, move the affected individual to fresh air and administer artificial respiration and oxygen. Because some gases have insidious, delayed effects, medical observation is essential.
2 Britannica Sources
1. occupational disease: Gases
2. chlorine deficiency
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

chlorine (Cl), chemical element, the second lightest member of the halogen elements, or Group 17 (Group VIIa) of the periodic table. Chlorine is a toxic, corrosive, greenish yellow gas that is irritating to the eyes and to the respiratory system.

Element Properties
atomic number	17
atomic weight	35.446 to 35.457
melting point	−103 °C (−153 °F)
boiling point	−34 °C (−29 °F)
density (1 atm, 0 °C or 32 °F)	3.214 g/litre (0.429 ounce/gallon)
oxidation states	−1, +1, +3, +5, +7
electron configuration	1s²2s²2p⁶3s²3p⁵

History

Rock salt (common salt, or sodium chloride) has been known for several thousand years. It is the main constituent of the salts dissolved in seawater, from which it was obtained in ancient Egypt by evaporation. In Roman times, soldiers were partially paid in salt (salarium, the root of the modern word salary). In 1648 the German chemist Johann Rudolf Glauber obtained a strong acid, which he called spirit of salt, by heating moist salt in a charcoal furnace and condensing the fumes in a receiver. Later he obtained the same product, now known to be hydrochloric acid, by heating salt with sulfuric acid.

In 1774 the Swedish chemist Carl Wilhelm Scheele treated powdered black oxide of manganese with hydrochloric acid and obtained a greenish-yellowish gas, which he failed to recognize as an element. The true nature of the gas as an element was recognized in 1810 by English chemist Humphry Davy, who later named it chlorine (from the Greek chloros, meaning “yellowish green”) and provided an explanation for its bleaching action.

Occurrence and distribution

Dead SeaSalt deposits on the southwestern shore of the Dead Sea near Masada, Israel.

Apart from very small amounts of free chlorine (Cl) in volcanic gases, chlorine is usually found only in the form of chemical compounds. It constitutes 0.017 percent of Earth’s crust. Natural chlorine is a mixture of two stable isotopes: chlorine-35 (75.53 percent) and chlorine-37 (24.47 percent). The most common compound of chlorine is sodium chloride, which is found in nature as crystalline rock salt, often discoloured by impurities. Sodium chloride is also present in seawater, which has an average concentration of about 2 percent of that salt. Certain landlocked seas, such as the Caspian Sea, the Dead Sea, and the Great Salt Lake of Utah, contain up to 33 percent dissolved salt. Small quantities of sodium chloride are present in blood and in milk. Other chlorine-containing minerals are sylvite (potassium chloride [KCl]), bischofite (MgCl₂∙6H₂O), carnallite (KCl∙MgCl₂∙6H₂O), and kainite (KCl∙MgSO₄∙3H₂O). It is found in evaporite minerals such as chlorapatite and sodalite. Free hydrochloric acid is present in the stomach.

Present-day salt deposits must have been formed by evaporation of prehistoric seas, the salts with the least solubility in water crystallizing first, followed by those with greater solubility. Because potassium chloride is more soluble in water than sodium chloride, certain rock salt deposits—such as those at Stassfurt, Germany—were covered by a layer of potassium chloride. In order to gain access to the sodium chloride, the potassium salt, important as a fertilizer, is removed first.

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Occurrence and distribution