Sentence:  Of several methods employed for tagging chlorine with radiochlorine , the exchange of inactive <WSD>chlorine</WSD> with tagged aluminum chloride at room temperature was found to be the most satisfactory . 
Meaning of the word marked with <WSD> tags:  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)