#initial-load-header-loader { visibility: hidden!important; } #initial-load-content { visibility: visible!important; } .expandable .expandable-control { display: none!important; } .expandable .expandable-content { display: block!important; visibility: visible!important; max-height: none!important; height: auto!important; }

radioactinium

[rey-dee-oh-ak-tin-ee-uh m]

noun Chemistry.

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

Origin of radioactinium

First recorded in 1905–10; radio- + actinium

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019