Chlorine is a chemical element, having the chemical symbol Cl. Its atomic number (the number of protons) is 17. It has a standard atomic weight of 35.453 g•mol −1, and is typically found as a solid in its elemental form.
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.