One of a group of elements classified as the halogens. Chlorine, Cl2, the most common halogen, is a greenish yellow gas with an irritating odor. Chlorine is very reactive; it forms salts with metals, forms acids when dissolved in water, and combines readily with hydrocarbons. Various forms of chlorine are used to disinfect water. Chlorine is produced by the electrolysis of brine (a concentrated salt solution). Atomic number 17; atomic weight 35.45; freezing point –100.98EC; boiling point –34.6EC; specific gravity 1.56 (–33.6EC).