Chlorine is a chemical element, typically found as a gas in its elemental form. It has the chemical symbol Cl, atomic number (number of protons) Z = 17, and a standard atomic weight of 35.453 g/mol.
Chlorine is considered a member of the "Halogen" class of elements. At a pressure of 101.325 kPa, it has a boiling point −34.04 °C, and a melting point of −101.5 °C.
Chlorine was the first agent used as a large-scale chemical weapon, by German forces at the Second Battle of Ypres in 1915; see World War I, poison gas. It was released from cylinders carried by trains, but was replaced by agents that were sufficiently toxic, for weight, to be used in artillery shells (e.g., phosgene).
Concern remains that chlorine, widely used in water purification and as an industrial chemical precursor, could be the source of a chemical terrorism incident. In industrialized countries, greater tracking and security is being applied to the large amounts shipped by rail.