Dictionary![]() ![]() prot•ac•tin•i•umPronunciation: (prō"tak-tin'ē-um), [key] —n. Chem. a radioactive, metallic element. Symbol: Pa; at. no.: 91. Also,protoactinium. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.
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