thorium

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thorium (symbol Th) Radioactive, metallic element of the actinide series, first discovered in 1828 by Swedish chemist J. J. Berzelius who named it after the Norse god Thor. The chief ore is monazite. The metal is used in photoelectric and thermionic emitters. One decay product is radon-220. Thorium is sometimes used in radiotherapy, and is increasingly used for conversion into uranium-233 for nuclear fission. Chemically reactive, it burns in air but reacts slowly in water. Properties: at.no. 90; r.a.m. 232.0381; r.d. 11.72; m.p. 1750°C (3182°F); b.p. 4790°C (8654°F); most stable isotope Th232 (1.41 × 1010 yrs).

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