Chlorine

Chlorine is a chemical element with the symbol Cl and atomic number 17. It is a halogen in group 17 of the periodic table and is the third most abundant in the halogen series. At room temperature it forms a diatomic molecule, Cl2, which is a pale greenish-yellow gas with a strong, pungent odor. It is highly reactive and one of the most electronegative and oxidizing elements, readily forming compounds with most other elements. In nature it is not found free in appreciable quantities but occurs primarily as chloride ions in minerals and in seawater.

Chlorine possesses notable physical and chemical properties: it has a high tendency to gain electrons and form bonds with a wide range of elements, and its compounds are widespread in industry and the environment. Its melting point is −101.5 °C and its boiling point is −34.04 °C. The element is typically produced and handled as a gas or in solutions, often in combination with other elements in various chlorinated compounds.

Industrial production and reactions: today chlorine is produced mainly by electrolysis of brine in the chlor-alkali process, yielding chlorine gas, hydrogen, and sodium hydroxide (2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH). It forms common salts such as sodium chloride and a broad range of chlorinated organics used in solvents, refrigerants, and agrochemicals.

Uses and safety: chlorine is essential for disinfection in drinking water and swimming pools, and it is a key feedstock in the production of polyvinyl chloride (PVC) and other materials. It is toxic and corrosive; exposure can irritate the eyes, skin, and lungs, and appropriate safety measures are required in its handling and use.

History: chlorine was identified in the 18th century, with Scheele recognizing its presence in 1774, and Humphry Davy isolating it as an element in 1807, naming it chlorine from the Greek chloros, meaning greenish-yellow.