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(thôr`ēəm) [from ThorThor
, Germanic Donar
, Norse god of thunder. An ancient and highly revered divinity, Thor was the patron and protector of peasants and warriors. As a god of might and war he was represented as extremely powerful and fearless, occasionally slow-witted, armed with a
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], radioactive chemical element; symbol Th; at. no. 90; mass number of most stable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 232; 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 seriesactinide series,
a series of radioactive metallic elements in Group 3 of the periodic table. Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
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 in Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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 and is sometimes classed as one of the rare-earth metalsrare-earth metals,
in chemistry, group of metals including those of the lanthanide series and actinide series and usually yttrium, sometimes scandium and thorium, and rarely zirconium. Promethium, which is not found in nature, is not usually considered a rare-earth metal.
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. 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 monazitemonazite
, yellow to reddish-brown natural phosphate of the rare earths, mainly the cerium and lanthanum metals, usually with some thorium. Yttrium, calcium, iron, and silica are frequently present.
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 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 mantleWelsbach mantle
or Welsbach burner
[for C. A. von Welsbach], cylindrical framework of gauze impregnated with oxides of thorium and cerium. When heated in a gas flame, it produces a very bright light because of the incandescence of the oxides.
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; 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-liveshalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 greater than 1 sec. The most stable is thorium-232 (half-life 1.40 × 1010 years); it is the major component of naturally occurring thorium. 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 radonradon
, gaseous radioactive chemical element; symbol Rn; at. no. 86; mass no. of most stable isotope 222; m.p. about −71°C;; b.p. −61.8°C;; density 9.73 grams per liter at STP; valence usually 0. Radon is colorless and the most dense gas known.
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-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 may be converted into the fissionable fuel uranium-233, but uranium-233 has not proven to be a practical alternative to natural uranium.

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 thermal breeder reactors. Thorium was discovered in 1828 by Jöns Jakob BerzeliusBerzelius, Jöns Jakob, Baron
, 1779–1848, Swedish chemist, M.D. Univ. of Uppsala, 1802. He was noted for his work as teacher at the medical school and other institutions in Stockholm and for his discoveries in diverse fields of chemistry.
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 but had few uses until the invention of the Welsbach mantle in 1885.



Th, a radioactive chemical element; the first member of the actinide series in Group III of Mendeleev’s periodic system. Atomic number, 90; atomic weight, 232.038. A silvery white ductile metal. Natural thorium consists almost entirely of the long-lived isotope 232Th and four short-lived isotopes. The isotope 232Th, the parent of one of the radioactive series, has a half-life of 1.39 × 1010 years (the content of isotope 228Th, present in equilibrium with 232Th, being insignificant [1.37 × 10–8percent ]). Of the four short-lived isotopes, two belong to the uranium-radium series, namely 234Th (half-life, 24.1 days) and 230Th (half-life, 8.0 × 104 years), and two belong to the actinium series, namely 231Th (half-life, 25.6 hours) and 227Th (half-life, 18.17 days). Of the artificially obtained isotopes, 229Th (half-life, 7,340 years) is the most stable.

Thorium was discovered by J. J. Berzelius in 1828 in a syenite in Norway. The element was named for Thor, the god of thunder in Scandinavian mythology. The mineral thorium silicate received the name thorite.

Occurrence. Thorium typically occurs in the upper part of the earth’s crust in the granite layer and the sedimentary layer, where its average content is, respectively, 1.8 × 10–3 and 1.3 × 10–3percent by weight. Thorium, which migrates to only a slight extent, participates mainly in magmatic processes, accumulating in granites, alkali rocks, and pegmatites. It has only a slight tendency to form concentrations. There are 12 thorium minerals as such, and the element is also contained in, among other minerals, monazite, uraninite, zircon, apatite, and orthite. Monazite placer deposits, both marine and continental, constitute the major industrial source of thorium. The content of thorium in natural water is especially low: 2 × 10–9 percent in fresh water and 1 × 10–9 percent in seawater. The element migrates only slightly in the biosphere and in hydrothermal solutions.

Physical and chemical properties. Thorium exists in two modifications: the α form, which has a face-centered cubic lattice and exists at temperatures up to 1400°C (a = 5.086 angstroms [Å]), and the β form, which has a body-centered cubic lattice and exists at temperatures above 1400°C (a = 4.11 Å). The density (X-ray) of thorium is 11.72 g/cm3 at 25°C; the atomic diameter is 3.59 Å for the α form and 3.56 Å for the β form. The ionic radius of Th3+ is 1.08 Å; that of Th4+ is 0.99 Å. The element has a melting point of 1750°C and a boiling point in the range 3500°–4200°C.

The molar heat capacity of thorium is 27.32 kilojoules/kilomole·°K (6.53 calories/gram atom·°C) at 25°C; the thermal conductivity at 20°C is 40.19 watts/m·°K (0.096 calorie/cm·sec·°C). Thorium has a temperature coefficient of linear expansion of 12.5 × 10–6/°C (25°–100°C), an electrical resistivity of 13–18 × 10–6 ohm·cm (25°C), and a temperature coefficient of resistivity of 3.6–4.0 × 10–3/°C. The element is paramagnetic, with a magnetic susceptibility at 20°C of 0.54 × 10–6. At 1.4°K it becomes a superconductor.

Thorium is readily deformed when cold-worked. Since the mechanical properties are extremely sensitive to the metal’s purity, the tensile strength varies from 150 to 290 meganewtons/m2 (15–29 kilograms-force/mm2), and the Brinell number from 450 to 700 meganewtons/m2 (45–70 kilograms-force/mm2). The electron configuration of the outer subshells is 6d27s2.

Although thorium belongs to the actinide series, some of its properties are similar to those of elements in the second subgroup of Group IV of Mendeleev’s periodic system, namely Ti, Zr, and Hf. In most of its compounds, thorium exhibits an oxidation state of + 4.

Thorium oxidizes only slightly in the air at room temperature, forming a protective black film. It oxidizes rapidly above 400°C to form ThO2, the only oxide of thorium, which melts at 3200°C and is highly stable. Th02 is obtained by the thermal decomposition of thorium nitrate, thorium oxalate, or thorium hydroxide. Above 200°C, thorium reacts with hydrogen to form powdery hydrides having such compositions as ThH2 and ThH3. At 700°–800°C in a vacuum, all bound hydrogen may be removed from thorium. When thorium is heated in nitrogen above 800°C, the nitrides ThN and Th2N3 are formed, which are decomposed by water with the release of ammonia. With carbon, thorium forms two carbides, namely ThC and ThC2, which are decomposed by water with the release of methane and acetylene. The sulfides ThS, Th2S3, Th7S12, and ThS2 may be obtained by heating thorium metal to 600°–800°C in the presence of sulfur vapor.

Thorium reacts with fluorine at room temperature and with the other halogens upon heating to yield halides of the type ThX4 (where X is a halogen). Of the halides, the fluoride (ThF4) and chloride (ThCl4) have the greatest industrial importance. The fluoride is obtained by the action of HF on ThO2 at elevated temperatures, while the chloride is obtained by the chlorination of a mixture of ThO2 and carbon at elevated temperatures. Thorium fluoride is sparingly soluble in water and mineral acids, while the chloride, bromide, and iodide of thorium are hygroscopic and readily soluble in water. Crystal hydrates, which separate out from aqueous solutions, are known for all thorium halides.

Massive thorium metal slowly corrodes in water at temperatures up to 100°C and is covered by a protective oxide film. Above 200°C, the metal reacts vigorously with water, forming ThO2 and liberating hydrogen. In the cold, thorium metal reacts slowly with nitric, sulfuric, and hydrofluoric acids and readily dissolves in hydrochloric acid and aqua regia. Salts of thorium are formed as crystal hydrates. Solubilities of the salts in water vary: the nitrates [Th(NO3)4·n H2O] are readily soluble, the sulfates [Th(SO4)2·n H2O], the basic carbonate (ThOCO3·8H2O), and the phosphates Th3(PO4)4·4H2O and ThP2O7·2H2O are difficultly soluble, and the oxalate Th(C2O4)2·6H2O is practically insoluble. Solutions of alkalies react only slightly with thorium. Thorium hydroxide [Th(OH)4] settles out from thorium salts in the pH range 3.5–3.6 as an amorphous precipitate. Th4+ ions show a marked tendency in aqueous solutions to form complexes and binary salts.

Preparation. Thorium is extracted mainly from monazite concentrates, in which it is present as a phosphate. The two methods of industrial importance for breaking down these concentrates are treatment with concentrated sulfuric acid at 200°C and treatment by alkaline solutions at 140°C. Thorium, phosphoric acid, and all the rare-earth elements enter the solution containing the products of the sulfatizing process in sulfuric acid. When the pH of this solution is brought to 1, thorium phosphate precipitates; the precipitate is removed and dissolved in nitric acid. The thorium nitrate thus formed is then extracted by an organic solvent, from which thorium is easily washed out in the form of complexes. With the second method, that is, the alkaline breakdown of the concentrates, the hydroxides of all the metals remain in the precipitate, while trisodium phosphate enters the solution. The precipitate is removed and dissolved in hydrochloric acid; when the pH of this solution is increased to 3.6–5.0, thorium is precipitated in the form of its hydroxide. These isolated and purified thorium compounds are used to obtain ThO2, ThCl4, and ThF4—the principal starting materials for the production of metallic thorium through methods involving thermal reduction or the electrolysis of molten salts.

Among the methods employing thermal reduction are the reduction of ThO2 by calcium in the presence of CaCl2 in an argon atmosphere at 1100°–1200°C, the reduction of ThCl4 by magnesium at 825°–925°C, and the reduction of ThF4 by calcium in the presence of ZnCl2. The last method yields an alloy of thorium, from which the zinc is removed by holding the alloy in a vacuum at 1100°C. In all these methods, thorium is obtained as a powder or sponge. Electrolysis of molten salts is carried out with electrolytes containing ThCl4 and NaCI or with baths consisting of a mixture of ThF4, NaCI, and KC1. Thorium is deposited at the cathode as a powder, which is separated from the electrolyte by treatment with water or dilute alkaline solution. Techniques of powder metallurgy, here the sintering of thorium powder in a vacuum at 1100°–1350°C, and melting of thorium compacts in induction furnaces under a vacuum in crucibles made of ZrO2 or BeO are used to obtain massive thorium metal. Thermal dissociation of thorium iodide is used to obtain thorium of very high purity.

Use. Thoriated cathodes are used in electronic tubes, while electrodes treated with thorium oxide are used in magnetrons and powerful oscillator tubes. The addition of 0.8–1.0 percent ThO2 to tungsten stabilizes the structure of incandescent lamp filaments. ThO2 is used as a refractory material and as a resistance element in high-temperature furnaces. Thorium and its compounds are widely used as constituents of catalysts in organic synthesis; they are also used in alloying magnesium and other alloys that have acquired great importance in jet aviation and rocketry. Metallic thorium is used in thorium reactors.

Procedures of radiation safety must be observed when working with thorium.


In organisms. Thorium is always present in the tissues of plants and animals. The element’s coefficient of accumulation, that is, the ratio of its concentration in the organism to the concentration in the environment, is 1,250 in marine plankton, 10 in benthic algae, 50–300 in the soft tissues of invertebrates, and 100 in fish. In freshwater mollusks (Unio mancus), the concentration ranges from 3 × 10–7 to 1 × 10–5 percent, while in marine animals it varies from 3 × 10–7 to 3 × 10–6 percent. Thorium is absorbed mostly by the liver and spleen, as well as by bone marrow, lymph glands, and adrenal glands. Thorium is poorly assimilated from the gastrointestinal tract. In humans, the daily intake of thorium with food and water is 3 micrograms; the element is eliminated from the body in the urine and feces (0.1 and 2.9 micrograms, respectively). Thorium has low toxicity, although as a natural radioactive element it contributes to background radiation.



Torii, ego syr’evye resursy, khimiia i tekhnologiia. Moscow, 1960.
Zelikman, A. N. Metallurgiia redkozemel’nykh metallov, toriia i urana. Moscow, 1961.
Emel’ianov, V. S., and A.I. Evstiukhin. Metallurgiia iadernogo goriuchego, 2nd ed. Moscow, 1968.
Seaborg, G. T., and J. Katz. Khimiia aktinidnykh elementov. Moscow, 1960. (Translated from English.)
Bowen, H. J. M. Trace Elements in Biochemistry. London–New York, 1966.


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.
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.


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