Sentence:  Because of the simplicity of the molecules , isotopic exchange reactions between elemental <WSD>halogens</WSD> and the corresponding carbon tetrahalides would appear to offer particularly fruitful possibilities for obtaining unambiguous basic kinetic data . 
Meaning of the word marked with <WSD> tags:  any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions