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any of the five chemical elements comprising Group VIa of the periodic classification-namely, oxygen, sulfur, selenium, tellurium, and polonium. (See .) A relationship among the first three members of the group was recognized as early as 1829; tellurium was assigned its place by 1865, and the discovery of polonium in 1898 completed the group. By the early 1970s, the possibility of finding element 116, the next member of Group VIa, in nature appeared to be negligible.