Thorium is a radioactive chemical element with the symbol Th and atomic number 90. A thorium atom has 90 protons in its nucleus. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. The radioactivity of thorium was discovered in 1896 by French chemist Henri Becquerel. In 1932, American chemists John D. Cockcroft and Ernest T.S. Walton used it to successfully split an atom for the first time; this was an important step toward nuclear power generation from fission reactors (in which uranium-235 or plutonium-239 are used as fuel).
Thorium metal has been used as an alloying agent in ferrous metals and as a getter in certain vacuum tubes; however its radioactivity limits these applications. It also forms alloys with nonferrous metals such as magnesium, lead, and tin; these alloys have found use in torches, aircraft engines, welding rods, and castings for holding molten metals during casting processes such as investment casting because they absorb impurities more readily than other materials.
Because of their high melting points (above 1700 °C or 3090 °F), refractory thorium compounds are among the most useful ceramic materials available; they find use in high-temperature crucibles for growing single crystals of metals, electrical insulators at elevated temperatures (including furnace insulation bricks), catalysts for Claus process plants that convert hydrogen sulfide waste gases into elemental sulfur (used extensively in the petroleum industry), pigments for glasses and enamels that are highly resistant to color changes upon exposure to ultraviolet light or other radiation sources (such as those used in museum display cases), etc..
The major commercial source of thorium is monazite sand ((ThPO_4) which contains up to 12% thorium oxide but no more than 0.5% uranium oxide; another source is euxenite ((Y(Nb0_3)_3(U0_4TiO_3))_2O_3–xSiO_2–yHfO 2 –zNd 2 O 3 , which typically contains about 1% thorium oxide along with significant amounts of neodymium oxide). Most natural samples also contain small amounts of uranium oxides ((UO2)2+(Th)4+(Nd)2+) because the two elements often occur together in nature due to their similar geochemical properties (elements belonging to groups 3 through 6 on the periodic table tend to form minerals composed primarily of either Th 4+ or U 4+ cations).
Approximately three quarters of the world’s supply of Thorianite ore comes from India.(1) Other large deposits exist in Australia near Broken Hill((PbZn)(Fe)(As)(S)), China’s Bayan Obo deposit((FeREE)(NbCu))))and Mountain Pass California((CaREE)). Smaller quantities can be found associated with placer deposits ((AuAg))of rare earth elements around the world including Brazil’s Catalao deposit((Y La Ce Nd Pr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu)), Canada’s Bikita district Zimbabwe((LiCs Na)); United States: Bear Lodge Wyoming((W Cu Fe As Sb Ag Pb Zn REE U Th)), Black Hills South Dakota & Wyoming ((Pt Au Ag Ir Os Ru Rh Re Ga Ge As Se Te S Sb Bi Po At Hg Tl Pb Zn Cu Ni Co Fe Mn Cr Ti V Zr Nb Mo W Sn Ta Re Os Ir Pt Au Hg Tl Pb Bi Po At No Rf Ds Md Lr))and Colorado Rare Earths North Park District ((La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Lu Y La Ce Pr Nd Sm Eu Gd Dy Ho Er Y)).
thor·i·um ˈthȯr-ē-əm noun Symbol Th Atomic number: 90 Element category: actinide A radioactive metallic element that occurs naturally only combined with other elements such as silica