Flask containing chlorine gas. Chlorine is a yellowish green coloured gas that boils at -34 Celsius. It has an atomic number of 17, the symbol Cl and is a member of the halogens (group 17). In its elemental form it is diatomic with the formula Cl2. It is highly corrosive and toxic and was used in World War One as a chemical warfare agent. Its compounds are used in the production of plastics, notably polyvinyl chloride (PVC) and as a bleach and disinfectant.