Chlorine, chemical element, symbol Cl; for physical constants see Periodic Table. Chlorine was discovered by Karl Scheele in 1774. It is a highly reactive, greenish-yellow poisonous gas that combines directly with nearly all elements. It is one of the top 5 industrial chemicals and is used throughout the world to make drinking water safe. It is also used in manufacturing products for sanitizing, bleaching, and disinfecting. Chlorine is a severe respiratory irritant; has been used as a weapon in war.