A gaseous → chemical element of the halogen group, which is greenish yellow and poisonous; symbol Cl. → Atomic number 17; → atomic weight 35.453; → melting point -100.98°C; → boiling point -34.6°C. Chlorine is about two and one-half times as dense as air. It is used for water purification, in the making of bleaching powder. Its compounds occur as common → salt (sodium chloride), NaCl, in sea water and as rock salt. Chlorine is the first poison gas to be used in warfare (by German army, the Second Battle of Ypres, 1915). It has several → radioactive isotopes, in particular 36Cl with a half-life of 3 × 105 years. Chlorine was discovered by the Swedish pharmacist and chemist Carl-Wilhelm Scheele (1742-1786) in 1774. In 1810, the English chemist Humphry Davy (1778-1829) proved it was an element and gave it the name chlorine.