thorium

American  

[thawr-ee-uhm, thohr-] / ˈθɔr i əm, ˈθoʊr- /

noun

Chemistry.

1. a grayish-white, lustrous, somewhat ductile and malleable, radioactive metallic element present in monazite: used as a source of nuclear energy, as a coating on sun-lamp and vacuum-tube filament coatings, and in alloys. Th; 232.038; 90; 11.7.

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thorium British  

/ ˈθɔːrɪəm /

noun

1. a soft ductile silvery-white metallic element. It is radioactive and occurs in thorite and monazite: used in gas mantles, magnesium alloys, electronic equipment, and as a nuclear power source. Symbol: Th; atomic no: 90; atomic wt: 232.0381; half-life of most stable isotope, ²³² Th: 1.41 × 10 ¹⁰ years; valency: 4; relative density: 11.72; melting pt: 1755°C; boiling pt: 4788°C

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thorium Scientific  

/ thôr′ē-əm /

1. A silvery-white, radioactive metallic element of the actinide series. It is used for fuel in some nuclear reactors and for improving the high-temperature strength of magnesium alloys. The only naturally occurring isotope of thorium, Th 232, is also its most stable, having a half-life of 14.1 billion years. Atomic number 90; atomic weight 232.038; approximate melting point 1,750°C; approximate boiling point 4,500°C; approximate specific gravity 11.7; valence 4.

2. See Periodic Table

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Other Word Forms

• thoric adjective

Etymology

Origin of thorium

< New Latin (1829); see Thor, -ium

Example Sentences

This includes valuable natural resources such as marble, gold, iridium, uranium and thorium and also possible deposits of oil and gas, with the information prepared by UK government officials, working for the Helmand reconstruction team.

From BBC • Feb. 11, 2026

Smye explained that earlier in Earth's history, the amount of heat produced from the radioactive elements that made up the crust -- uranium, thorium and potassium -- was about double what it is today.

From Science Daily • Oct. 16, 2025

Unlike uranium, thorium doesn't dissolve well in water, so it precipitates out on particles in the water column.

From Science Daily • May 30, 2024

Heavy elements may result from this "nucleosynthesis"; in fact, half of the heavy isotopes up to bismuth and all of thorium and uranium in the universe may have been created by the r process.

From Science Daily • Mar. 13, 2024

But a properly functioning coal-burning plant spreads radioactive lead, uranium, radon, polonium, and thorium every day.

From "Meltdown" by Deirdre Langeland