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radioactinium

American  
[rey-dee-oh-ak-tin-ee-uhm] / ˌreɪ di oʊ ækˈtɪn i əm /

noun

Chemistry.

  1. the radioactive isotope of thorium having a mass number 227 and a half-life of 18.8 days. RdAc, Th 227

Etymology

Origin of radioactinium

First recorded in 1905–10; radio- + actinium

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.