Chlorine is one of the chemical elements in the halogen group of gases. This yellow-green gas is active, reacting with water (bleach is a chlorine solut- ion), organic compounds and many metals. The gas will not burn in air, but will support combustion itself; for example, a paraffin candle will burn in a pure chlorine atmosphere. Chlorine has an irritating odour and is dangerous in high concentrations; it was the first poison gas to be used during World War I. Chlorine is widely used in the chemical industry to make many compounds. Chlorine has the chemical symbol Cl, an atomic number of 17 and an atomic weight of 35.5.