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

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Etymology

Origin of thorium

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

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Example Sentences

See Examples For:

thorium

A laser shines onto the crystal, interacting with the thorium nuclei inside.

From Science Daily ● Oct. 11, 2026

To make this possible, the researchers developed a system built around a special crystal containing thorium atoms, manufactured at TU Wien.

From Science Daily ● Oct. 11, 2026

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

From Barron's ● May 18, 2026

Over long periods of time, that uranium slowly transforms into thorium.

From Science Daily ● May 9, 2026

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

From "Meltdown" by Deirdre Langeland

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