chlorine

Greenish-yellow, gaseous, non-metallic element with a pungent odour, atomic number 17, relative atomic mass 35.453. It is a member of the halogen group and is widely distributed, in combination with the alkali metals, as chlorides.

Chlorine was discovered in 1774 by the German chemist Karl Scheele, but English chemist Humphry Davy first proved it to be an element in 1810 and named it after its colour. In nature it is always found in the combined form, as in hydrochloric acid, produced in the mammalian stomach for digestion. Chlorine is obtained commercially by the electrolysis of concentrated brine and is an important bleaching agent and germicide, used for sterilizing both drinking water and swimming pools. As an oxidizing agent it finds many applications in organic chemistry. The pure gas (Cl₂) is a poison and was used in gas warfare in World War I, where its release seared the membranes of the nose, throat, and lungs, causing pneumonia. Chlorine is a component of chlorofluorocarbons (CFCs) and is partially responsible for the depletion of the ozone layer; it is released from the CFC molecule by the action of ultraviolet radiation in the upper atmosphere, making it available to react with and destroy the ozone. The concentration of chlorine in the atmosphere in 1997 reached just over 3 parts per billion. It peaked in the late 1990s and subsequently declined, due to international action to curb ozone-destroying chemicals.