pro·tac·tin·i·um
noun/ˌprəʊtækˈtɪniəm/
/ˌprəʊtækˈtɪniəm/
[uncountable] (symbol Pa)
- a chemical element. Protactinium is a radioactive metal found naturally when uranium decays .
镤（放射性化学元素） Word Originearly 20th cent.: from proto- ‘original, earlier’ + actinium, so named because one of its isotopes decays to form actinium.