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WordReference Random House Unabridged Dictionary of American English © 2024prot•ac•tin•i•um (prō′tak tin′ē əm),USA pronunciation n. [Chem.]- Chemistrya radioactive, metallic element. Symbol: Pa;
at. no.: 91.
Also, protoactinium. Collins Concise English Dictionary © HarperCollins Publishers:: protactinium /ˌprəʊtækˈtɪnɪəm/ n - a toxic radioactive metallic element that occurs in uranium ores and is produced by neutron irradiation of thorium. Symbol: Pa; atomic no: 91; half-life of the most stable isotope, 231Pa: 32 500 years; valency: 4 or 5; relative density: 15.37 (calc.); melting pt: 1572°C
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