Chlorine
Chlorine is a chemical element, having the chemical symbol Cl.
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Its atomic number (the number of protons) is 17. It has a Standard Atomic Weight of 35.453(2), and is typically found as a gas in its elemental form.
Chlorine is considered to be a member of the "Halogen" class of elements, has a boiling point of -34.04 °C , and a melting point of -101.5 °C .
Chemical warfare
Chlorine was the first agent used as a large-scale chemical weapon, by German forces at the Second Battle of Ypres in 1915. 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 treatment and as an industrial chemical precursor, could be the source of a chemical terrorism incident.