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Meaning of radioactinium

ra•di•o•ac•tin•i•um

Pronunciation: (rā"dē-ō-ak-tin'ē-um), [key]
— n. Chem.
  1. the radioactive isotope of thorium having a mass number 227 and a half-life of 18.8 days. Symbol: RdAc, Th 227
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.