thorium

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Definition of thorium in English English dictionary: a chemical element (symbol Th) with atomic number 90; Symbol Th; A metallic element found in certain rare minerals, as thorite, pyrochlore, monazite, etc; a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands; Atomic weight 232; {i} radioactive element (Chemistry); and isolated as an infusible gray metallic powder which burns in the air and forms thoria; formerly called also thorinum
thorium-228: A radioactive isotope of thorium, 22890Th, having a half-life of 1.4 x 1010 years; it occurs naturally, and is a source of nuclear energy
thorium dioxide: A heavy white powder, ThO, obtained from monazite and used mainly in ceramics, glass, and gas mantles and as a catalyst. Also called thoria
thorium-228: radioactive isotope of thorium with mass number 228
uranium-thorium-lead dating: or common-lead dating Method of dating very old rocks by means of the amount of common lead they contain. Common lead is any lead from a rock or mineral that contains a large amount of lead and a small amount of the radioactive precursors of lead (i.e., the isotopes uranium-235, uranium-238, and thorium-232). By this method, the age of the Earth has been estimated to be 4.6 billion years. This figure is in good agreement with the age of meteorites and the age of the Moon as determined independently