/ ˌprəʊtækˈtɪniəm; NAmE ˌproʊ-/ noun
[ U ] ( symb Pa ) a chemical element. Protactinium is a radioactive metal found naturally when uranium decays.
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WORD ORIGIN
early 20th cent.: from proto- original, earlier + actinium , so named because one of its isotopes decays to form actinium.