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90 actiniumthoriumprotactinium


Name, Symbol, Number thorium, Th, 90
Chemical series Actinides
Group, Period, Block n/a, 7, f
Appearance silvery white
Standard atomic weight 232.0381(2)  g·mol−1
Electron configuration [Rn] 6d2 7s2
Electrons per shell 2, 8, 18, 32, 18, 10, 2
Physical properties
Phase solid
Density (near r.t.) 11.7  g·cm−3
Melting point 2115 K
(1842 °C, 3348 °F)
Boiling point 5061 K
(4788 °C, 8650 °F)
Heat of fusion 13.81  kJ·mol−1
Heat of vaporization 514  kJ·mol−1
Heat capacity (25 °C) 26.230  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2633 2907 3248 3683 4259 5055
Atomic properties
Crystal structure cubic face centered
Oxidation states 4
(weakly basic oxide)
Electronegativity 1.3 (Pauling scale)
Ionization energies
1st:  587  kJ·mol−1
2nd:  1110  kJ·mol−1
3rd:  1930  kJ·mol−1
Atomic radius 180  pm
Magnetic ordering no data
Electrical resistivity (0 °C) 147 nΩ·m
Thermal conductivity (300 K) 54.0  W·m−1·K−1
Thermal expansion (25 °C) 11.0  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 2490 m/s
Young's modulus 79  GPa
Shear modulus 31  GPa
Bulk modulus 54  GPa
Poisson ratio 0.27
Mohs hardness 3.0
Vickers hardness 350  MPa
Brinell hardness 400  MPa
CAS registry number 7440-29-1
Selected isotopes
Main article: Isotopes of thorium
iso NA half-life DM DE (MeV) DP
228Th syn 1.9116 years α 5.520 224Ra
229Th syn 7340 years α 5.168 225Ra
230Th syn 75380 years α 4.770 226Ra
231Th trace 25.5 hours β 0.39 231Pa
232Th 100% 1.405×1010 years α 4.083 228Ra
234Th trace 24.1 days β 0.27 234Pa

Thorium (pronounced /ˈθɔriəm/) is a chemical element with the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium.


Notable characteristics

When pure, thorium is a silvery white metal that retains its luster for several months. However, when it is exposed to oxygen, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium dioxide (ThO2), also called thoria, has the highest melting point of any oxide (3300°C).[1] When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

Thorium has the largest liquid range of any element: 2946 K between the melting point and boiling point.

See Actinides in the environment for details of the environmental aspects of thorium.


Applications of thorium:

Applications of thorium dioxide (ThO2):

  • Mantles in portable gas lights. These mantles glow with a dazzling light (unrelated to radioactivity) when heated in a gas flame.
  • Used to control the grain size of tungsten used for electric lamps.
  • Used for high-temperature laboratory crucibles.
  • Added to glass, it helps create glasses of a high refractive index and with low dispersion. Consequently, they find application in high-quality lenses for cameras and scientific instruments.
  • Has been used as a catalyst:
  • Thorium dioxide is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics. This use has been abandoned due to the carcinogenic nature of Thorotrast.


M. T. Esmark found a black mineral on Løvøy Island, Norway and gave a sample to Professor Jens Esmark, a noted mineralogist who was not able to identify it so he sent a sample to the Swedish chemist Jöns Jakob Berzelius for examination in 1828.[2] Berzelius analysed it and named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the gas mantle in 1885.

Between 1900 and 1903 Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity.[3]

The crystal bar process (or Iodide process) was discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925 to produce high-purity metallic thorium.[4]

The name ionium was given early in the study of radioactive elements to the 230Th isotope produced in the decay chain of 238U before it was realized that ionium and thorium were chemically identical. The symbol Io was used for this supposed element.


  Thorium is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 12 parts per million (ppm) of thorium. Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide. There are substantial deposits in several countries. 232Th decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than 232Th, though on a mass basis they are negligible. India is believed to have 25% of the world's Thorium reserves.[5]

See also thorium minerals.


Present knowledge of the distribution of Thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.[6] Under the prevailing estimate, Australia and India have particularly large reserves of thorium.

  • The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1997-2006):[7][8]
Country Th Reserves (tonnes) Th Reserve Base (tonnes)
United States160,000300,000
South Africa35,00039,000
Other Countries95,000100,000
World Total1,200,0001,400,000
  • Another estimate of Reasonably Assured Reserves (RAR) and Estimated Additional Reserves (EAR) of thorium comes from OECD/NEA, Nuclear Energy, "Trends in Nuclear Fuel Cycle", Paris, France (2001).[9]
Country RAR Th (tonnes) EAR Th (tonnes)
United States137,000295,000
South Africa18,000-
Other Countries505,000-
World Total2,230,0002,130,000

The two sources vary wildly for countries such as Brazil, Turkey, and Australia.

Thorium as a nuclear fuel

  Thorium, as well as uranium and plutonium, can be used as fuel in a nuclear reactor. Although not fissile itself, 232Th will absorb slow neutrons to produce uranium-233 (233U), which is fissile. Hence, like 238U, it is fertile. In one significant respect 233U is better than the other two fissile isotopes used for nuclear fuel, 235U and plutonium-239 (239Pu), because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (235U or 239Pu), a breeding cycle similar to, but more efficient than that currently possible with the 238U-to-239Pu cycle (in slow-neutron reactors), can be set up. The 232Th absorbs a neutron to become 233Th which normally emits an electron and an anti-neutrino (\bar{\nu}_e) by β decay to become protactinium-233 (233Pa) and then emits another electron and anti-neutrino by a second β decay to become 233U:

\mathrm\hbox{n}+{{}^2{}^{32}_{90}Th}\rightarrow\mathrm{{}^2{}^{33}_{90}Th}\rightarrow\mathrm{{}^2{}^{33}_{91}Pa}+ e^- + \bar{\nu}_e
\mathrm{{}^2{}^{33}_{91}Pa}\rightarrow\mathrm{{}^2{}^{33}_{92}U}+ e^- + \bar{\nu}_e

The irradiated fuel can then be unloaded from the reactor, the 233U separated from the thorium (a relatively simple process since it involves chemical instead of isotopic separation), and fed back into another reactor as part of a closed nuclear fuel cycle.

Problems include the high cost of fuel fabrication due partly to the high radioactivity of 233U which is a result of its contamination with traces of the short-lived 232U; the similar problems in recycling thorium due to highly radioactive 228Th; some weapons proliferation risk of 233U; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available.

Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without fast neutron reactors, holds considerable potential long-term benefits. Thorium is significantly more abundant than uranium, and is a key factor in sustainable nuclear energy. An example of this is the Liquid fluoride reactor.

One of the earliest efforts to use thorium fuel cycle took place at Oak Ridge National Laboratory in the 1960s. An experimental reactor was built based on Molten Salt Reactor technology to study the feasibility of such an approach. This effort culminated in a Molten Salt Breeder Reactor (MSBR) design that used 232Th as the fertile material and 233U as the fissile fuel. Due to a lack of funding, the MSBR program was discontinued in 1976.

India, having about 25% of the world's reserves [5], has planned its nuclear power program to eventually use thorium exclusively, phasing out uranium as a feed stock. This ambitious plan uses both fast and thermal breeder reactors. The Advanced Heavy Water Reactor and KAMINI reactor are efforts in this direction.

In 2007, Norway was debating whether or not to focus on thorium plants, due to the existence of large deposits of thorium ores in the country, particularly at Fensfeltet, near Ulefoss in Telemark county.

The primary fuel of the HT3R Project in Odessa, Texas, USA will be ceramic-coated thorium beads.


Main article: isotopes of thorium

Naturally occurring thorium is composed of one isotope: 232Th. Twenty-seven radioisotopes have been characterized, with the most abundant and/or stable being 232Th with a half-life of 14.05 billion years, 230Th with a half-life of 75,380 years, 229Th with a half-life of 7340 years, and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy of 3.5 eV.[10]

The known isotopes of thorium range in atomic weight from 210 u (210Th) to 236 u (236Th).[11]


Powdered thorium metal is often pyrophoric and should be handled carefully.

Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted cannot penetrate human skin. Owning and handling small amounts of thorium, such as a gas mantle, is considered safe if care is taken not to ingest the thorium -- lungs and other internal organs can be penetrated by alpha radiation. Exposure to aerosolized thorium can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases. This element has no known biological role. See also Thorotrast.

Thorium Extraction

Thorium has been extracted chiefly from monazite through a multi-stage process. In the first stage, the monazite sand is dissolved in an inorganic acid such as sulfuric acid (H2SO4). In the second, the Thorium is extracted into an organic phase containing an amine. Next it is separated or "stripped" using an anion such as nitrate, chloride, hydroxide, or carbonate, returning the thorium to an aqueous phase. Finally, the thorium is precipitated and collected.[12]

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thorium". A list of authors is available in Wikipedia.
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