thorium

thorium (thôr`ēəm) [from Thor], radioactive chemical element; symbol Th; at. no. 90; at. wt. 232.0381; m.p. about 1,750°C;; b.p. about 4,790°C;; sp. gr. 11.7 at 20°C;; valence +4.

Thorium is a soft, ductile, lustrous, silver-white, radioactive metal. At ordinary temperatures it has a face-centered cubic crystalline structure. It is a member of the actinide series in Group 3 of the periodic table and is sometimes classed as one of the rare-earth metals. When pure, the metal is stable and resists oxidation, but it is usually contaminated with small amounts of the oxide, which cause it to tarnish rapidly. It reacts slowly with water and is attacked only by hydrochloric acid among the common acids. The finely divided metal readily ignites when heated, burning with a brilliant white flame; the oxide formed has the highest melting point of all oxides. Thorium forms numerous compounds with other elements.

Thorium is widely distributed in small amounts in the earth's crust, being about half as abundant as lead and three times as abundant as uranium. The chief commercial source of thorium is monazite sands obtained from India and Brazil. It is also found in the minerals thorite (thorium silicate, ThSiO₄) and thorianite (mixed thorium and uranium oxides). Vast deposits of low-grade thorium ore in New Hampshire are a potential source. Thorium metal is isolated with difficulty; it is obtained from certain of its compounds by electrolysis or by chemical reduction. Thorium is used in magnesium alloys and in tungsten filaments for light bulbs and electronic tubes. The most important thorium compound is the oxide (thoria, ThO₂), which is the major incandescent component of the Welsbach mantle; it is also used in crucibles, in special highly refractive optical glass, and in catalysts for several industrially important chemical reactions. Important uses of the element result from its natural radioactivity.

There are 26 known radioactive isotopes, only 12 of which have half-lives greater than 1 sec. The most stable is thorium-232 (half-life 1.41 × 10¹⁰ years); it is the major component of naturally occurring thorium, which has atomic weight 232.038 atomic mass units. Thorium-232 undergoes natural disintegration and eventually is converted through a 10-step chain of isotopes to lead-208, a stable isotope; alpha and beta particles are emitted during this decay. One intermediate product is the gas radon-220, also called thorium emanation or thoron. Thorium and its decay products are sometimes used in radiotherapy. Although thorium-232 is not itself a nuclear reactor fuel since it will not sustain a chain reaction, it is expected to become increasingly important for conversion into the fissionable fuel uranium-233.

Thorium-232 can react with a thermal (slow) neutron to form thorium-233, emitting a gamma ray. Thorium-233 decays (half-life about 22 min) to protactinium-233, emitting a beta particle. The protactinium-233 decays (half-life about 27 days) with another beta particle emission to uranium-233. Fission of the uranium-233 can provide neutrons to start the cycle again. This cycle of reactions is known as the thorium cycle. Nuclear reactors that use a cycle like this to produce fuel are called breeder reactors. Thorium was discovered in 1828 by Jöns Jakob Berzelius but had few uses until the invention of the Welsbach mantle in 1885.

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thorium

Metallic chemical element, chemical symbol Th, atomic number 90. One of the actinide series of elements, natural thorium is a mixture of radioactive isotopes, predominantly thorium-232 (half-life of more than 10 billion years). It is a dense metal that is silver-white in pure form but turns gray or black on prolonged exposure to air. Although not a nuclear reactor fuel itself, thorium-232 can be used in breeder reactors because, on capturing slow-moving neutrons, it decays into fissionable uranium-233. Thorium is added to magnesium and its alloys to improve their high-temperature strength. Added to glass, it yields glasses with a high refractive index, useful for specialized optical applications. It was formerly in great demand as a component of mantles for gas and kerosene lamps and has been used in the manufacture of tungsten filaments for lightbulbs and vacuum tubes.

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thorium

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, ²³²Th: 1.41 × 10¹⁰ years; valency: 4; relative density: 11.72; melting pt.: 1755°C; boiling pt.: 4788°C

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thorium [′thȯr·ē·əm]

(chemistry)

An element of the actinium series, symbol Th, atomic number 90, atomic weight 232; soft, radioactive, insoluble in water and alkalies, soluble in acids, melts at 1750°C, boils at 4500°C.

(metallurgy)

A heavy malleable metal that changes from silvery-white to dark gray or black in air; potential source of nuclear energy; used in manufacture of sunlamps.