Chlorine is a member of the halogen family, also known as group 17. This group contains chlorine, bromine, fluorine, iodine and astatine. The halogen family is characterized in part by its tendency to form acids when bonded to hydrogen.
Chlorine was discovered by Carl Wilhelm Scheele in 1774. In standard conditions, chlorine is a green gas with a distinctive odor characteristic of household bleach. The gas can be turned to liquid under pressurized conditions. It is used industrially in plastic production, cosmetic production, water sanitization and in medicine. Chlorine was also used as a chemical weapon in World War I, as it is highly toxic when concentrated.