A silvery-white radioactive metallic element , one of the actinides. Its chief ore is monazite. Thorium is tetravalent, resembling zirconium and hafnium. It is used in magnesium alloys and to produce uranium-233 for atomic fuel.
Thorium has 13 radioactive isotopes only one of which, thorium-232, occurs naturally.
The refractory thorium (IV) oxide was used to make incandescent gas mantles; it is added in small amounts to the tungsten filaments in electric lights.
Related categories ATOMIC AND NUCLEAR PHYSICS
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