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radioactinium

[rey-dee-oh-ak-tin-ee-uh m]
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noun Chemistry.
  1. the radioactive isotope of thorium having a mass number 227 and a half-life of 18.8 days. Symbol: RdAc, Th 227
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Origin of radioactinium

First recorded in 1905–10; radio- + actinium
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018