Cl

Cl is the chemical symbol for chlorine, a chemical element with atomic number 17. It is a halogen and a highly reactive, nonmetal. In its elemental form (Cl2), it is a yellow-green gas with a sharp, suffocating odor and is a powerful oxidizer. Chlorine is extremely reactive and forms a wide range of compounds, especially with hydrogen, carbon, and metals. It exists under standard conditions as a diatomic molecule and is denser than air.

Chlorine occurs primarily as chloride ions (Cl−) in seawater, rocks, and minerals; free chlorine gas is not common in nature. Industrial production is dominated by the chlor-alkali process, which uses electrolysis of brine to yield chlorine gas along with sodium hydroxide and hydrogen. The element is also released in various industrial processes and can be present in trace amounts in the atmosphere near production sites.

Chlorine is used to manufacture a broad array of products. It is essential in water disinfection (e.g., sodium hypochlorite and chlorinated compounds), bleaching processes, and the production of polyvinyl chloride (PVC) and other polymers. It also serves as a reagent in organic synthesis and in the manufacture of solvents, paints, and pharmaceuticals.

Safety and biology: chlorine gas is toxic and corrosive to the respiratory tract. In water, chlorine reacts to form hypochlorous and hydrochloric acids, which contribute to disinfection but can be harmful in high concentrations. Chlorine has two stable isotopes, 35Cl and 37Cl, with natural abundances of about 75.8% and 24.2%, respectively.

History: chlorine was discovered in 1774 by Carl Wilhelm Scheele and identified as an element in the early 19th century by Humphry Davy, who named it from the Greek chloros, meaning greenish-yellow.