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Thorium (revised)


Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.


Thorium is a member of the actinide family. The actinide elements are located in Row 7 of the periodic table. They have atomic numbers between 90 and 103. The periodic table is a chart that shows how chemical elements are related to one another. The actinide series is named for element 89, actinium, which is sometimes included in the actinide family.

Thorium was discovered in 1828 by Swedish chemist Jons Jakob Berzelius (1779-1848). At the time, Berzelius did not realize that thorium was radioactive. That was discovered 70 years later, in 1898, by Polish-French physicist Marie Curie (1867-1934) and English chemist Gerhard C. Schmidt (1864-1949).

Thorium is a relatively common element with few commercial applications. There is some hope that it can someday be used in nuclear power plants, in which nuclear reactions are used to generate electricity.






Discovery and naming

In 1815, Berzelius was studying a new mineral found in the Falun district of Sweden. From his analysis, he concluded that he had found a new element. He named the element thorium, in honor of the Scandinavian god Thor.

Ten years later, Berzelius announced that he had made an error. The substance he had found was not a new element, but the compound yttrium phosphate (YPO4).

Shortly thereafter, Berzelius again reported that he had found a new element. This time he was correct. He chose to retain thorium as the name for this element.

At the time Berzelius made his discovery, the concept of radioactivity was unknown. Radioactivity refers to the process by which an element spontaneously breaks down and gives off radiation. In that process, the element often changes into a new element. One of the first scientists to study radioactivity was Curie. She and Schmidt announced at almost the same time in 1898 that Berzelius' thorium was radioactive.

Physical properties

Thorium is a silvery white, soft, metal, somewhat similar to lead . It can be hammered, rolled, bent, cut, shaped, and welded rather easily. Its general physical properties are somewhat similar to those of lead. It has a melting point of about 1,800°C (3,300°F) and a boiling point of about 4,500°C (8,100°F). The density of thorium is about 11.7 grams per cubic centimeter.

Chemical properties

Thorium is soluble in acids and reacts slowly with oxygen at room temperature. At higher temperatures, it reacts with oxygen more rapidly, forming thorium dioxide (ThO2).

Occurrence in nature

Thorium is a relatively abundant element in the Earth's crust. Scientists estimate that the crust contains about 15 parts per million of the element. That fact is important from a commercial standpoint. It means that thorium is much more abundant than another important radioactive element, uranium . Uranium is used in nuclear reactors to generate electricity and in making nuclear weapons (atomic bombs). Scientists believe thorium can replace uranium for these purposes. With more thorium than uranium available, it would be cheaper to make electricity with thorium than uranium.

The most common ores of thorium are thorite and monazite. Monazite is a relatively common form of beach sand. It can be found, among other places, on the beaches of Florida. This sand may contain up to 10 percent thorium.

Thorium in place of uranium?

U ranium is one of the most important elements in the world today. Why? One of its isotopes undergoes nuclear fission. Nuclear fission occurs when neutrons collide with the nucleus of a uranium atom. When that happens, the uranium nucleus splits apart. Enormous amounts of energy are released. That energy can be used for mass destruction in the form of atomic bombs, or used for peaceful energy production in nuclear power plants.

But there are two problems with using uranium for nuclear fission. First, of uranium's three isotopes (uranium-234, uranium-235, and uranium-238), only oneuranium-235undergoes fission. The second problem is that this isotope of uranium is quite rare. Out of every 1,000 atoms of uranium, only seven are uranium-235. Tons of uranium ore must be processed and enriched to make tiny amounts of this critical isotope. It is difficult and extremely expensive.

Scientists know that another isotope of uranium, uranium-233, will also undergo fission. The problem is that uranium-233 does not occur in nature. So how can it be used to make atomic weapons or nuclear power?

The trick is to start with an isotope of thorium, thorium-232. Thorium-232 has a very long half life of 14 billion years. If thorium-232 is bombarded with neutrons, it goes through a series of nuclear changes, first to thorium-233, then to protactinium-233, and finally to uranium-233. The whole process only takes about a month. At the end of the month, a supply of uranium-233 has been produced. This isotope of uranium has a fairly long half life, about 163,000 years. So once it has been made, it stays around for a long time. It can then be used for nuclear fission.

Scientists would like to find a way to use this process to make uranium-233 economically. Thorium is much more abundant than uranium. It would be far cheaper to make nuclear bombs and nuclear power plants with thorium than with uranium.

Unfortunately, no one has figured how to make the process work on a large scale. One nuclear reactor using thorium was built near Platteville, Colorado, in 1979. However, a number of economic and technical problems developed. After only ten years of operation, the plant was shut down. The promise of thorium fission plants has yet to become reality.

There is some hope that thorium can someday be used in nuclear power plants, where nuclear reactions are used to generate electricity.


More than two dozen isotopes of thorium are known. All are radioactive. The isotope with the longest half life is thorium-232. Its half life is about 14 billion years. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

The half life of a radioactive element is the time it takes for half of a sample of the element to break down. After one half life (14 billion years), only 5 grams of a ten-gram sample of thorium-232 would be left. The remaining 5 grams would have broken down to form a new isotope.


The thorium in monazite, thorite, or other minerals is first converted to thorium dioxide (ThO2). This thorium dioxide is then heated with calcium to get the free element:

Uses and compounds

Thorium and its compounds have relatively few uses. The most important thorium compound commercially is thorium dioxide. This compound has the highest melting point of any oxide, about 3,300°C (6,000°F). It is used in high-temperature ceramics. A ceramic is a material made from earthy materials, such as sand or clay. Bricks, tiles, cement, and porcelain are examples of ceramics. Thorium dioxide is also used in the manufacture of specialty glass and as a catalyst. A catalyst is a substance used to speed up or slow down a chemical reaction without undergoing any change itself.

The one device in which most people are likely to have seen thorium dioxide is in portable gas lanterns. These lanterns contain a gauzy material called a mantle. Gas passing through the mantle is ignited to produce a very hot, bright white flame. That flame provides the light in the lantern. The mantle in most lanterns was once made of thorium dioxide because it can get very hot without melting.

The thorium dioxide in a gas mantle is radioactive. But it is of no danger to people because the amount used is so small. Still, gas mantles in the United States are no longer made with thorium. Safer substitutes have been found.

Another thorium compound, thorium fluoride (ThF4), is used in carbon arc lamps for movie projectors and searchlights. A carbon arc lamp contains a piece of carbon (charcoal) to which other substances (such as ThF4) have been added. When an electric current is passed through the carbon, it gives off a bright white light. The presence of thorium fluoride makes this light even brighter.

Health effects

As with all radioactive materials, thorium is dangerous to the health of humans and other animals. It must be handled with great caution. Living cells that absorb radiation are damaged or killed. Inhaling a radioactive element is especially dangerous because it exposes fragile internal tissues.

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thorium (thôr´ēəm) [from Thor], radioactive chemical element; symbol Th; at. no. 90; mass number of most stable isotope 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 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 1.40 × 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 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 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 Berzelius but had few uses until the invention of the Welsbach mantle in 1885.

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melting point: 1,750°C
boiling point: 4,820°C
density: 11.724 g/cm3
most common ions: Th4+

Thorium is a radioactive chemical element that belongs to the actinide series. Its ground state electronic configuration is [Rn]5f06d27s2. Thorium was discovered by Jöns Jacob Berzelius in 1828. Its name is derived from "Thor," the god of war in the Scandinavian mythology. Thorium chemistry is dominated by the tetravalent thorium ion (Th4+). Its ionic radius is very similar to that of the trivalent cerium ion (Ce3+). For that reason it is no surprise that thorium occurs in nature together with the rare earth elements . Thorium is recovered commercially from the rare earth ore monazite (mainly CePO4), which contains up to 9 percent ThO2. Other thorium-containing minerals are thorite and thorianite. The most abundant isotope is 232Th, with a half-life of 1.4 × 1010 years. None of the twenty-five known isotopes of thorium (with atomic masses ranging between 212 and 236) is stable.

Pure thorium is a silvery-white metal (melting point 1,750°C) that tarnishes upon exposure to air. Its density is 11.724 g/cm3 at 25°C (77°F), similar to that of lead. The best-known application of thorium is its use in incandescent mantles for gas lamps. These mantles consist of a metal oxide skeleton (99% ThO2 and 1% CeO2). Thorium(IV) oxide is used by chemists

as a catalyst in different organic reactions, and in the conversion of ammonia to nitric acid. Thorium is about three times as abundant as uranium, and therefore it may become an important nuclear fuel in the future. Because one of the disintegration products of thorium is the radioactive noble gas radon (220Rn), good ventilation of areas and places where thorium is stored or handled is necessary.

see also Actinium; Berkelium; Einsteinium; Fermium; Lawrencium; Mendelevium; Neptunium; Nobelium; Plutonium; Protactinium; Rutherfordium; Uranium.

Koen Binnemans


Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press.

Katz, Joseph J.; Seaborg, Glenn T.; and Morss, Lester R. (1986). The Chemistry of the Actinide Elements, 2nd edition. New York: Chapman and Hall.

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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: 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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tho·ri·um / ˈ[unvoicedth]ôrēəm/ • n. the chemical element of atomic number 90, a white radioactive metal of the actinide series. (Symbol: Th)

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thorium (chem.) dark-grey metallic element. XIX. f. Thor Norse god + -IUM.

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