thorium

noun

tho·​ri·​um ˈthȯr-ē-əm 

: 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-4 is naturally produced in the mantle through the decay of uranium and thorium, so there’s a lot of it. —Robin George Andrews, Scientific American, 14 May 2026

Scientists also struggled to explain strange chemical deposits rich in thorium and iron found southwest of the basin. —Rupendra Brahambhatt, Interesting Engineering, 9 May 2026

Byproducts include acidic wastewater, heavy metals, and radioactive elements like thorium and uranium. —Charlie Campbell, Time, 21 Apr. 2026

First, in most materials, the thorium nuclei release their energy not as light, but through a process called internal conversion, where the energy is transferred to an electron in the material instead. —Andrei Derevianko, The Conversation, 14 Apr. 2026

Word History

Etymology

New Latin, from Old Norse Thōrr Thor

First Known Use

1832, in the meaning defined above

Rhymes for thorium

• fusarium
• terrarium

Kids Definition

thorium

noun

tho·​ri·​um ˈthōr-ē-əm 

ˈthȯr-

: a radioactive metallic element see element

Medical Definition

thorium

noun

tho·​ri·​um ˈthōr-ē-əm, ˈthȯr- 

: a radioactive metallic element that occurs combined in minerals and is usually associated with rare earths

—symbol Th see Chemical Elements Table