(Science: chemical) chlorine. A greenish-yellow, diatomic gas that is a member of the halogen family of elements. It has the atomic symbol cl, atomic number 17, and atomic weight 70.906. It is a powerful irritant that can cause fatal pulmonary oedema. Chlorine is used in manufacturing, as a reagent in synthetic chemistry, for water purification, and in the production of chlorinated lime, which is used in fabric bleaching.