tho·​ri·​um ˈthȯr-ē-əm How to pronounce thorium (audio)
: a radioactive metallic element that is obtained especially from monazite and is usually associated with rare earths see Chemical Elements Table

Examples of thorium in a Sentence

Recent Examples on the Web Helium is formed by the radioactive decay of uranium and thorium deep inside the earth’s crust. Michael Braga, The Arizona Republic, 14 May 2024 In 2003 physicists first suggested that a synthetic isotope called thorium 229 could be the key to nuclear timekeeping. Kenna Hughes-Castleberry, Scientific American, 20 Oct. 2023 But one stood out: shooting protons at targets made of thorium 232. Sarah Scoles, Scientific American, 13 Feb. 2024 Although uranium and thorium have been widely-detected on Mars, Zubrin predicts that the technology to support the large-scale settlement of Mars is fusion power. Bruce Dorminey, Forbes, 18 Feb. 2024 There’s a resistance to processing, particularly heavy rare earths that tend to come coupled together with uranium and thorium. IEEE Spectrum, 7 Dec. 2023 In the study, a team of physicists at CERN's nuclear physics facility, ISOLDE, spotted and measured thorium 229's nuclear transition for the first time. Kenna Hughes-Castleberry, Scientific American, 20 Oct. 2023 Over time, minerals accumulate energy as radioactive elements like uranium, thorium, and potassium decay and release radiation. Paul Bierman, Fortune, 24 July 2023 Phoenix finds mine sites where the tailings waste is free from radioactive elements such as thorium and uranium. Yusuf Khan, WSJ, 20 July 2023

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'thorium.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


New Latin, from Old Norse Thōrr Thor

First Known Use

1832, in the meaning defined above

Time Traveler
The first known use of thorium was in 1832

Dictionary Entries Near thorium

Cite this Entry

“Thorium.” Dictionary, Merriam-Webster, Accessed 24 Jun. 2024.

Kids Definition


tho·​ri·​um ˈthōr-ē-əm How to pronounce thorium (audio)
: a radioactive metallic element see element

Medical Definition


tho·​ri·​um ˈthōr-ē-əm, ˈthȯr- How to pronounce thorium (audio)
: a radioactive metallic element that occurs combined in minerals and is usually associated with rare earths
symbol Th
see Chemical Elements Table

More from Merriam-Webster on thorium

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