Thorium Element Facts

The chemical element thorium is classed as an actinide metal. It was discovered in 1828 by Jöns Jacob Berzelius.

Thorium rods. Photo: Department of Energy
Thorium rods.

Data Zone

Classification: Thorium is an actinide metal
Color: silvery
Atomic weight: 232.0381, no stable isotopes
State: solid
Melting point: 1750 oC, 2023 K
Boiling point: 4790 oC, 5063 K
Electrons: 90
Protons: 90
Neutrons in most abundant isotope: 142
Electron shells: 2,8,18,32,18,10,2
Electron configuration: [Rn] 6d2 7s2
Density @ 20oC: 11.7 g/cm3
Atomic volume: 19.9 cm3/mol
Structure: face-centered cubic
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 19.9 cm3/mol
Structure: face-centered cubic
Specific heat capacity 0.113 J g-1 K-1
Heat of fusion 16.1 kJ mol-1
Heat of atomization 575 kJ mol-1
Heat of vaporization 514.4 kJ mol-1
1st ionization energy 587 kJ mol-1
2nd ionization energy 1110 kJ mol-1
3rd ionization energy 1930 kJ mol-1
Electron affinity
Minimum oxidation number 0
Min. common oxidation no. 0
Maximum oxidation number 4
Max. common oxidation no. 4
Electronegativity (Pauling Scale) 1.3
Polarizability volume 32.1 Å3
Reaction with air mild, with heat,ignites ⇒ Th2O3
Reaction with 15 M HNO3 passivated
Reaction with 6 M HCl mild
Reaction with 6 M NaOH none
Oxide(s) ThO2 (thoria)
Hydride(s) ThH2, Th4H15
Chloride(s) ThCl4
Atomic radius 179 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion)
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 54 W m-1 K-1
Electrical conductivity 7.1 x 106 S m-1
Freezing/Melting point: 1750 oC, 2023 K

Jöns Jacob Berzelius

Jöns Jacob Berzelius. A portrait from the Royal Swedish Academy of Sciences

Discovery of Thorium

Dr. Doug Stewart

Thorium was discovered by Jöns Jacob Berzelius in 1828, in Stockholm, Sweden after he received a sample of an unusual black mineral from Hans Esmark found on an island close to Brevik, Norway.

The mineral contained a large number of known elements including iron, manganese, lead, tin and uranium plus another substance Berzelius could not identify.

He concluded that the mineral contained a new element.

He called the black mineral thorite, in honor of the Scandinavian god Thor.

His analysis indicated that 57.91% of thorite was an oxide of the proposed new element, which he called thorium. (1)

To isolate thorium metal, Berzelius found the most effective method was to react thorium chloride with potassium, to yield potassium chloride and thorium. (Berzelius made thorium chloride by mixing thorium oxide with carbon and heating to red-heat in a stream of chlorine gas.) (2)

Berzelius’s isolation of thorium from its chloride using potassium was similar to the approach used by Wöhler and Bussy to isolate beryllium in 1828 and by Ørsted to isolate aluminum in 1825.

Thorium was discovered to be radioactive by Gerhard Schmidt in 1898 – the first element after uranium to be identified as such.

Marie Curie also found this, independently, later in the same year. (3)

In the early 1900s Ernest Rutherford and Frederick Soddy found that thorium decayed into other elements at a fixed rate – a key discovery in our understanding of the radioactive elements. (4), (5)

A method for producing high purity thorium metal was discovered in 1925 by Anton Eduard van Arkel and Jan Hendrik de Boer. Thorium iodide is decomposed on a white hot tungsten filament creating a crystal bar of pure thorium. (6)

Prior to his discovery of thorium, Berzelius had discovered two other elements, cerium in 1803 and selenium in 1817.

India’s experimental Thorium Fuel Cycle Nuclear Reactor. NDTV Report

Thorium-232 decay chain.

Thorium-232 decay chain. This is what thorium does naturally. If, however, we bombard it with neutrons we can make uranium-233, from which we can generate nuclear energy.(Photo: BatesIsBack)


Appearance and Characteristics

Harmful effects:

Thorium is radioactive. It collects in living animal bones, including human bone, where it can remain for a long period of time. (7)


Thorium is a radioactive, bright, soft, silvery-white metal, which tarnishes extremely slowly (over many months) to the black oxide. The most stable isotope is thorium-232, with a half-life of 14.05 billion years. Nearly 100% of thorium found on Earth is thorium-232, which is only slightly radioactive because it has such a long half-life. (Uranium-235’s half-life is 700 million years, shorter by a factor of 20.)

Thorium is chemically reactive and is attacked by oxygen, hydrogen, the halogens and sulfur. (6a) Thorium powder is pyrophoric (ignites spontaneously in air). (7)

Thorium is dimorphic, changing from face centered cubic to body centered cubic above 1360 oC. (6)

Thorium has the largest liquid range of any element, spanning over 3000 degrees between its melting point of 2023 K (1750 oC) and its boiling point of 5063 K (4790 oC).

Thorium dioxide (thoria) has the highest melting point of any known oxide.

Almost all naturally occurring thorium is thorium-232 which decays slowly to the Group 2 metal radium by emission of alpha particles.

Thorium-232 can be converted by thermal (slow) neutrons to fissionable uranium-233 via the following reaction sequence:

232Th+ n ⇒ 233Th

ß decay       ß decay

233Th     ⇒    233Pa     ⇒     233U

Fission of the uranium-233 can provide neutrons to start the cycle again. This cycle of reactions is known as the thorium cycle. (6b)

Uses of Thorium

An exciting possibility for the future is fueling nuclear reactors with thorium. Not only is thorium more abundant on Earth than uranium, but 1 ton of mined thorium can produce as much energy as 200 tons of mined uranium. (8)

The difference in the energy output of the two elements arises because most uranium mined is uranium-238, which is not fissile. (Naturally occurring uranium is over 99% uranium-238 with only about 0.7% of the fissile uranium-235.) Nearly all mined thorium, however, can easily be made into the fissile uranium isotope uranium-233 through neutron bombardment (as shown above).

Waste from a thorium reactor is expected to lose its dangerous radioactivity after about 400-500 years, compared with many thousands of years for nuclear waste produced today. (8)

Thorium fuel research is continuing in several countries including the USA and India. (9)

Most non-nuclear uses of thorium are driven by the unique properties of its oxide.

Thorium dioxide was used in Welsbach gas mantles in the 19th century and today these mantles may still be found in camping lanterns. (Thorium dioxide’s very high melting point ensures it stays solid, glowing with an intense, bright white light at the temperature of the lantern’s burning gas.)

Thorium dioxide is used for heat resistant ceramics.

Glass that contains thorium dioxide has a high refractive index and low dispersion, so thorium dioxide is added to glass for use in high quality lenses and scientific equipment.

Thorium-magnesium alloys are used in the aerospace industry for aircraft engines. These alloys are lightweight and have excellent strength and creep resistance at high temperatures.

Thorium is used to coat tungsten filaments in light bulbs.

The demand for thorium in non-nuclear applications is decreasing because of environmental and health concerns due to its radioactivity.

Abundance and Isotopes

Abundance earth’s crust: 6 parts per million by weight, 0.5 parts per million by moles

Abundance solar system: 0.3 parts per billion by weight, 2 parts per trillion by moles

Cost, pure: $ per 100g

Cost, bulk: $ per 100g

Source: Thorium is not found free in nature but is found in a number of minerals: mainly monazite and bastnasite. Commercially thorium is extracted from monazite sand (phosphate mineral). The chemical inertness of monazite makes extraction a complex and multi-stage process. (6c)

Thorium metal can be isolated by electrolysis of the anhydrous thorium chloride with calcium.

Isotopes: Thorium has 28 isotopes whose half-lives are known, with mass numbers 210 to 237. All are radioactive. The most stable isotope is 232Th, with a half-life of 14.05 billion years and an abundance of virtually 100%.


1. The Quarterly Journal of Science, Literature and Art., The Royal Institure of Great Britain., July to December 1829 p412.
2. Jöns Jacob Berzelius, The Quarterly Journal of Science, Literature and Art., The Royal Institure of Great Britain., January to June 1830, p88.
3. Lawrence Badash, The Discovery of Thorium’s Radioactivity., Journal of Chemical Education, (April 1966) p219.
4. Ernest Rutherford, The Cause and Nature of Radioactivity., The Collected Papers of Lord Rutherford of Nelson, Vol. 1, pp. 472-94.
5. Jean Pierre Adloff, Robert Guillaumont, Fundamentals of Radiochemistry., CRC Press, 1993, p2.
6. M. S. Wickleder, B. Fourest,P. K. Dorhout, The Chemistry of the Actinide and Transactinide Elements., Springer., Vol 1.3, p61.
6a. M. S. Wickleder, B. Fourest,P. K. Dorhout, The Chemistry of the Actinide and Transactinide Elements., Springer., Vol 1.3, p63.
6b. M. S. Wickleder, B. Fourest,P. K. Dorhout, The Chemistry of the Actinide and Transactinide Elements., Springer., Vol 1.3, p53.
6c. M. S. Wickleder, B. Fourest,P. K. Dorhout, The Chemistry of the Actinide and Transactinide Elements., Springer., Vol 1.3, p56- 59.
7. Argonne National Laboratory, Thorium Human Health Fact Sheet
8. Carlo Rubbia, Using Thorium Could Reduce Risk of Nuclear Power., 2011.
9. World Nuclear Association, Thorium

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