chlorines

Chlorine, chemical element with symbol Cl and atomic number 17, is a halogen. At room temperature it exists as a diatomic gas (Cl2) that is pale green and highly reactive. It is a strong oxidizing agent and forms a wide range of compounds. Its two stable isotopes are 35Cl and 37Cl, with natural abundances about 75.8% and 24.2%.

Occurrence and production: Although chlorine is abundant in the Earth's crust, free chlorine is rare due to its reactivity. It is found mainly as chloride minerals and in seawater. Industrial production is dominated by electrolysis of sodium chloride brine (the chlor-alkali process), which yields chlorine gas on one electrode and produces caustic soda (sodium hydroxide) and hydrogen gas as byproducts.

Properties and reactions: Chlorine reacts with almost all elements to form chlorides. It reacts with water to form hydrochloric acid and hypochlorous acid, a reaction exploited in disinfection. Chlorine gas can react with organics to form various chlorinated compounds, some of which are persistent environmental pollutants. In pure form it is toxic and corrosive to skin and mucous membranes.

Applications: Major uses include production of polyvinyl chloride (PVC) plastics, bleaching of paper and textiles, water treatment and pool sanitation, and synthesis of various organic chemicals and solvents.

History and naming: Chlorine was first prepared by Carl Wilhelm Scheele in 1774; the element was identified as such by Humphry Davy in 1807–1810; the name chlorine derives from Greek chloros meaning greenish-yellow.