2007 Schools Wikipedia Selection. Related subjects: Chemical elements

90 actiniumthoriumprotactinium


Periodic Table - Extended Periodic Table
Name, Symbol, Number thorium, Th, 90
Chemical series Actinides
Group, Period, Block n/a, 7, f
Appearance silvery white
Atomic mass 232.03806 (2) g/mol
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
( more)
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 ( IPA: /ˈθɔːriəm/) is a chemical element in the periodic table that has 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 lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium dioxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

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


Applications of thorium:

  • As an alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures.
  • Thorium is used to coat tungsten wire used in electronic equipment, improving the electron emission of heated cathodes.
  • Thorium has been used in gas tungsten arc welding electrodes and heat-resistant ceramics.
  • Uranium-thorium age dating has been used to date hominid fossils.
  • As a fertile material for producing nuclear fuel. In particular, the proposed energy amplifier reactor design would employ thorium. Since thorium is more abundant than uranium, some designs of nuclear reactor incorporate thorium in their nuclear fuel cycle.
  • Thorium is a very effective radiation shield, although it has not been used for this purpose as much as have lead or depleted uranium.
  • Thorium may be used in subcritical reactors instead of uranium as fuel. This produces less waste and cannot melt down.

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.


Thorium was discovered in 1828 by the Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder. The metal had virtually no uses until the invention of the lantern mantle in 1885.

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.

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.


Monazite, a rare-earth-and-thorium-phosphate mineral is the primary source of the world's thorium
Monazite, a rare-earth-and-thorium-phosphate mineral is the primary source of the world's thorium

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.

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 decays to protactinium-233 (233Pa) and then 233U. 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 the need for fast neutron reactors, holds considerable potential long-term. Thorium is significantly more abundant than uranium, so it is a key factor in the sustainability of nuclear energy.

Australia and India have particularly large reserves of thorium. India has planned its nuclear power program to eventually use thorium exclusively, phasing out uranium as an input material. This ambitious plan uses both fast and thermal breeder reactors. The Advanced Heavy Water Reactor and KAMINI reactor are efforts in this direction.

The current thorium mineral reserve estimates (in tons)

  • 300,000 Australia
  • 290,000 India
  • 170,000 Norway
  • 160,000 United States
  • 100,000 Canada
  • 35,000 South Africa
  • 16,000 Brazil
  • 95,000 Others


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-lifes that are less than thirty days and the majority of these have half lifes that are less than ten minutes. This element also has one meta state.

The known isotopes of thorium range in atomic weight from 210 amu (210Th) to 236 amu (236Th).


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

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.

In popular culture

David Hahn, the so-called "radioactive boy scout," bombarded thorium from lantern mantles with neutrons to produce small quantities of fissionable material in his backyard. He had to abandon his project when he began to detect elevated radiation levels several houses away from his own.

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