any number of entities (members) considered as a unit

a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium

a nonmetallic heavy volatile corrosive dark brown liquid element belonging to the halogens; found in sea water

a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water)

a nonmetallic univalent element belonging to the halogens; usually a yellow irritating toxic flammable gas; a powerful oxidizing agent; recovered from fluorite or cryolite or fluorapatite

a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)