Name: Thorium
Symbol: Th
Atomic Number: 90
Atomic Weight: 232.038100
Family: Rare Earth Elements
CAS RN: 7440-29-1
Description: A silvery white, soft, malleable, ductile radioactive  metal.
State (25 C): Solid
Oxidation states: +4

Molar Volume: 19.8 cm3/mole
Valence Electrons: 6d27s2

Boiling Point: 5063.15oK, 4790oC, 8654oF
Melting Point: 2028K, 1755C, 3191F

Electrons Energy Level: 2, 8, 18, 32, 18, 10, 2
Isotopes: 30 + None Stable
Heat of Vaporization: 514.4 kJ/mol
Heat of Fusion: 16.1 kJ/mol
Density: 11.724 g/cm3 @ 300K
Specific Heat: 0.12 J/gK
Atomic Radius: 179.8 pm
Ionic Radius: 0.972
Electronegativity: 1.3 (Pauling), 1.11 (Allrod Rochow)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10 6s2p6d2 7s2


The Reverend Has Morten Thrane Esmark found a black mineral on Lvy 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 Jons Jakob Berzelius for examination in 1828.  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.  Esmark's mineral is now known as thorite (ThSiO4).

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.


When pure, thorium is a silvery white metal that retains its luster 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 the highest melting point of any oxide (3300C).  When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10
6s2 6p6 6d2


Thorium makes up about 0.0007% of the earth's crust and is primarily obtained from thorite, thorianite (ThO2) and monazite [(Ce, La, Th, Nd, Y)PO4].   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.


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

Country Th Reserves (tons) Th Reserve Base (tons)
Australia 300,000 340,000
India 290,000 300,000
Norway 170,000 180,000
United States 160,000 300,000
Canada 100,000 100,000
South Africa 35,000 39,000
Brazil 16,000 18,000
Malaysia 4,500 4,500
Other Countries 95,000 100,000
World Total 1,200,000 1,400,000
Country RAR Th (tons) EAR Th (tons)
Brazil 606,000 700,000
Turkey 380,000 500,000
India 319,000 -
United States 137,000 295,000
Norway 132,000 132,000
Greenland 54,000 32,000
Canada 45,000 128,000
Australia 19,000 -
South Africa 18,000 -
Egypt 15,000 309,000
Other Countries 505,000 -
World Total 2,230,000 2,130,000

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


Large deposits of thorium minerals have been reported in New England and elsewhere, but these have not yet been exploited. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum.  Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals, 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.


Thorium Dioxide (ThO2):

Thorium dioxide (ThO2), one of thorium's compounds, has many uses.  It is primarily used in a type of lantern mantel known as a Welsbach mantle.   This mantle, which also contains about 1% cerium oxide, glows with a bright white light when it is heated in a gas flame.  Thorium dioxide has a very high melting point, about 3300C, and is used to make high temperature crucibles. Thorium dioxide is also used to make glass with a high index of refraction that is used to make high quality camera lenses.

Thorium dioxide is used as a catalyst in the production of sulfuric acid (H2SO4), in the cracking of petroleum products and in the conversion of ammonia (NH3) to nitric acid (HNO3).

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 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 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 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 fast neutron reactors, holds considerable potential long-term.  Thorium is significantly more abundant than uranium, and is a key factor in sustainable nuclear energy.

India, having about 25% of the world's reserves, 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 reactor are efforts in this direction.


Thorium dioxide is a material for heat-resistant ceramics, e.g., for high temperature laboratory crucibles.  When added to glass, it helps increase refractive index and decrease dispersion.  Such glass finds application in high-quality lenses for cameras and scientific instruments.

Thorium dioxide (ThO2) d thorium nitrate (Th(NO3)4) were used in mantles of portable gas lights, including natural gas lamps, oil lamps and camping lights.  These mantles glow with an intense white light (unrelated to radioactivity) when heated in a gas flame, and its color could be shifted to yellow by addition of cerium.

Thorium dioxide was used to control the grain size of tungsten metal used for spirals of electric lamps.  Thoriated tungsten elements were also found in the active ingredient of microwave frequencies and were applied in microwave ovens and radars.

Thorium dioxide has been used as a catalyst in the conversion of ammonia (NH3) to nitric acid (HNO3), in petroleum cracking and in producing sulfuric acid.  It 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 Thortrast.

Despite its radioactivity, thorium fluoride (ThF4) is used as an antireflection material in multilayered optical coatings.  It has excellent optical transparency and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.  Thorium fluoride was also used in manufacturing carbon arc lamps, which provided high-intensity illumination for movie projectors and search lights.

Thorium Ore
Thorite, Thorianite, ThSiO4 Thoria, Thorium Dioxide, ThO2
Monazite, (Ce, La, Th, Nd, Y)PO4


Naturally occurring thorium is composed of one isotope: 232Th.  Thirty radioisotopes have been characterized, with the most abundant and/or stable being 232Th, an alpha emitter, 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.

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

232Th goes through six alpha and four beta decay steps before becoming the stable isotope 208Pb. 232Th is sufficiently radioactive to expose a photographic plate in a few hours. Thorium disintegrates with the production of "thoron" (220Rn), which is an alpha emitter and presents a radiation hazard.  Good ventilation of areas where thorium is stored or handled is therefore essential.

When bombarded with neutrons, 232Th becomes 233Th, which eventually decays into 233U through a series of beta decays.   233U is a fissionable material and can be used as a nuclear fuel.

Thorium's most stable isotope, thorium-232, decays into radium-228 through alpha decay or decays through spontaneous fission.

atom.gif (700 bytes)

Atomic Mass
209Th 209.01772 7 ms
210Th 210.015075 17 ms
211Th 211.01493 48 ms
212Th 212.01298 36 ms
213Th 213.01301 140 ms
214Th 214.011500 100 ms
215Th 215.011730 1.2 seconds
216Th 216.011062 26.8 ms
217Th 217.013114 240 s
218Th 218.013284 109 ns
219Th 219.01554 1.05 s
220Th 220.015748 9.7 s
221Th 221.018184 1.73 ms
222Th 222.018468 2.237 ms
223Th 223.020811 0.60 seconds
224Th 224.021467 1.05 seconds
225Th 225.023951 8.72 minutes
226Th 226.024903 30.57 minutes
227Th 227.0277041 18.68 days
228Th 228.0287411 1.9116 years
229Th 229.031762 7.34 x 103 years
230Th 230.0331338 7.538 x 104 years
231Th 231.0363043 25.52 hours
232Th 232.0380553 1.405 x 1010 years
233Th 233.0415818 21.83 minutes
234Th 234.043601 24.10 days
235Th 235.04751 7.2 minutes
236Th 236.04987 37.5 minutes
237Th 237.05389 4.8 minutes
238Th 238.0565 9.4 minutes


80px-Flammable.jpg (2186 bytes) Powdered thorium metal is often pyrophoric and should be handled carefully.  When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.

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.

atom.gif (700 bytes)

Thorium Data

Atomic Structure

  • Atomic Radius: 179.8 pm
  • Atomic Volume: 19.9cm3/mol
  • Covalent Radius: 1.65
  • Cross Section (Thermal Neutron Capture) Barns: 7.37
  • Crystal Structure: Cubic face centered
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10 6s2p6d2 7s2
  • Electrons per Energy Level: 2, 8, 18, 32, 18, 10, 2
  • Ionic Radius: 0.972
  • Filling Orbital: 6d2
  • Number of Electrons (with no charge): 90
  • Number of Neutrons (most common/stable nuclide): 142
  • Number of Protons: 90
  • Oxidation States: 4
  • Valence Electrons: 6d2 7s2

Chemical Properties

  • Electrochemical Equivalent: 2.1643 g/amp-hr
  • Electron Work Function: 3.41eV
  • Electronegativity: 1.3 (Pauling); 1.11 (Allrod Rochow)
  • Heat of Fusion: 16.1 kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 6.08
    • Second: 11.504
    • Third: 20.003
  • Valence Electron Potential (-eV): 59.3

Physical Properties

  • Atomic Mass Average: 232.0381
  • Boiling Point: 5061K, 4788C, 8650F
  • Coefficient of Lineal Thermal Expansion/K-1: 12.5E-6
  • Conductivity
    Electrical: 0.0653 106/cm
    Thermal: 0.54 W/cmK
  • Density: 11.724 g/cm3 @ 300K
  • Description:
    Soft silvery radioactive metal that is ductile and soft.
  • Elastic Modulus:
    • Bulk: 54/GPa
    • Rigidity: 30.8/GPa
    • Youngs: 78.3/GPa
  • Enthalpy of Atomization: 565 kJ/mole @ 25C
  • Enthalpy of Fusion: 15.65 kJ/mole
  • Enthalpy of Vaporization: 543.9 kJ/mole
  • Flammablity Class:
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 400 MN m-2
    • Mohs: 3
    • Vickers: 350 MN m-2
  • Heat of Vaporization: 514.4 kJ/mol
  • Melting Point: 2028K, 1755C, 3191F
  • Molar Volume: 19.8 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.12 J/gK

Regulatory / Health

  • CAS Number
    • 7440-29-1
  • NFPA 704
    • Health:
    • Fire:
    • Reactivity:
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • No limits set by OSHA
    • OSHA PEL Vacated 1989
      • No limits set by OSHA
    • NIOSH Recommended Exposure Limit (REL)
      • No limits set by NIOSH
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: 0.00016
      • Bone/p.p.m: 0.02-0.012
      • Liver/p.p.m: n/a
      • Muscle/p.p.m: n/a
      • Daily Dietary Intake: 0.00005-0.003 mg
      • Total Mass In Avg. 70kg human: 0.1 mg

    Who / Where / When / How

    • Discoverer: Jns Jakob Berzelius
    • Discovery Location: Stockholm Sweden
    • Discovery Year: 1829
    • Name Origin:
      Named for Thor the Scandinavian god of war.
    • Abundance:
      • Estimated Crustal: 9.6 milligrams per kilogram
      • Estimated Oceanic: 110-6 milligrams per liter
      • Atmosphere: N/A
      • Sun (Relative to H=1E12): 2
    • Sources:
      Found in various minerals like monazite and thorite and dispersed on pegmatite rocks. Annual world wide production is around 31,000 tons. Primary mining areas are USA, Brazil, India Sri Lanka, Madagascar, Russia and Australia.
    • Uses:
      Used in making strong alloys. Also in ultraviolet photoelectric cells. Bombarded with neutrons make uranium-233, a nuclear fuel.


Debye Temperature: 100.00K
Lattice Constant: 5.080
Van der Waals Radius: 0.182 nm
First Ionizing Energy: 670.4 kJ/mol
Energy of First Ionization: 1107.6 kJ/mol
Energy of Second Ionization: 1962.4 kJ/mol
Energy of Third Ionization: 2774 kJ/mol

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.