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

See Examples For:

thorium

Monazite tends to contain higher levels of radioactive thorium, which can be a processing issue.

From Barron's ● May 18, 2026

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

Scientists had believed that thorium needed to be embedded in a transparent material so laser light could reach and excite the nucleus.

From Science Daily ● Jan. 8, 2026

The team discovered how to reproduce their earlier results while using only a tiny fraction of the thorium previously required.

From Science Daily ● Jan. 8, 2026

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

From "Meltdown" by Deirdre Langeland