tho·​ri·​um | \ ˈthȯr-ē-əm How to pronounce thorium (audio) \

Definition of thorium

: 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 The white-colored material, which looks like the clean gypsum used in wallboard, contains measurable quantities of radium, uranium, thorium, radon, and a variety of toxic heavy metals. Mark Schleifstein,, "Groups challenge EPA rule allowing roads built with gypsum waste, prevalent in Louisiana," 18 Dec. 2020 The team also used measurements collected by NASA’s Mars Odyssey spacecraft of thorium, uranium, and potassium in Martian soils to model how heat might have traveled up from the interior. Kate Baggaley, Popular Science, "The most life-friendly places on ancient Mars were deep below its surface," 3 Dec. 2020 In practice, thorium and uranium are difficult to measure in this way, so in the new study, the researchers propose to instead look for europium—another element produced by neutron star collisions that sports a clearer spectroscopic signature. Marcus Woo, Scientific American, "Stellar Smashups May Fuel Planetary Habitability, Study Suggests," 17 Nov. 2020 These elements include uranium, potassium and thorium. Ashley Strickland, CNN, "Potential life on ancient Mars likely lived below the surface, study says," 2 Dec. 2020 And Stevenson noted that past in-situ copper mining has led to elevated levels of radioactive elements such as uranium and thorium in groundwater. Zak Podmore, The Salt Lake Tribune, "Ranchers, landowners worry copper mine’s plan will taint groundwater in San Juan County," 21 Nov. 2020 Wyoming can continue to be a leader in energy by investing in wind energy, and become a global mining capital of thorium and rare-earth elements. Bill Mckibben, The New Yorker, "What Have We Learned in Thirty Years of Covering Climate Change?," 7 Oct. 2020 Could a new blended thorium fuel improve U.S. nuclear power’s outlook? Caroline Delbert, Popular Mechanics, "The U.S. Government Made a Powerful New Kind of Nuclear Fuel," 24 Sep. 2020 Compared to terrestrial rare earth sources, the mud also has low levels of radioactive elements like uranium and thorium. Jennifer Frazer, Scientific American, "Mining Rare-Earth Elements from...Fossilized Fish???," 21 Sep. 2020

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'thorium.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of thorium

1832, in the meaning defined above

History and Etymology for thorium

New Latin, from Old Norse Thōrr Thor

Keep scrolling for more

Learn More about thorium

Time Traveler for thorium

Time Traveler

The first known use of thorium was in 1832

See more words from the same year

Statistics for thorium

Last Updated

31 Dec 2020

Cite this Entry

“Thorium.” Dictionary, Merriam-Webster, Accessed 20 Jan. 2021.

Keep scrolling for more

More Definitions for thorium


tho·​ri·​um | \ ˈthōr-ē-əm, ˈthȯr- How to pronounce thorium (audio) \

Medical Definition of thorium

: 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 Encyclopedia article about thorium

Comments on thorium

What made you want to look up thorium? Please tell us where you read or heard it (including the quote, if possible).


Test Your Vocabulary

Words of Snow and Ice Quiz

  • image1037863653
  • Which of the following refers to thin, bending ice, or to the act of running over such ice?
True or False

Test your knowledge - and maybe learn something along the way.


Anagram puzzles meet word search.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!