Thorium
Thorium is a chemical element with symbol Th and atomic number 90. A radioactive actinide metal, thorium is one of only two significantly radioactive elements that still occur naturally in large quantities as a primordial element (the other being uranium). It was discovered in 1828 by the Norwegian Reverend and amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder.
Identity
- Atomic mass
- 232.04 u
- Category
- Actinides
- Group
- 3
- Period
- 7
- Block
- f
- Oxidation States
- +4
- Phase (25 °C)
- Solid
- Appearance
- silvery, often with black tarnish
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d2
- Short configuration
- [Rn] 6d2 7s2
- Electrons per shell
- 2, 8, 18, 32, 18, 10, 2
- Electronegativity (Pauling)
- 1.30
- Electron affinity
- 113 kJ/mol
- 1st ionization energy
- 587 kJ/mol
- Ionization energies
- 587 · 1110 · 1930 · 2780 kJ/mol
Physical properties
- Melting point
- 1749.8 °C (2023 K)
- Boiling point
- 4787.9 °C (5061 K)
- Density
- 11.724 g/cm³
- Molar heat capacity
- 26.2 J/(mol·K)
Chemical behavior of thorium
Thorium has four outer electrons ([Rn] 6d² 7s²) and almost always acts as +4, with low electronegativity (1.30). It is a fertile actinide, dense and refractory: it melts at 1,750 °C and forms extremely stable oxides. Less radioactive than uranium, but toxic and inhalable as dust. Its dioxide is the ceramic with the highest known melting point.
Reactivity
Th + O₂ → ThO₂
In air it coats itself with a protective oxide; when heated it burns to the white dioxide, which melts at 3,390 °C and serves as a crucible for melting other metals.
Th + 2 H₂O → ThO₂ + 2 H₂
Water barely attacks it in the cold because of its oxide layer, but hot water and steam react with it, releasing hydrogen.
Th + 4 HCl → ThCl₄ + 2 H₂
It dissolves slowly in concentrated acids to give thorium(IV) salts. The chloride is the route for purifying it by crystallization.
Th + 2 F₂ → ThF₄
With fluorine it forms the tetrafluoride, a molten salt key to thorium molten-salt reactors and to metal production by reduction.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Thorium dioxide | ThO₂ | Crucibles, optics, and fertile fuel |
| Thorium tetrafluoride | ThF₄ | Molten salts and metal production |
| Thorium nitrate | Th(NO₃)₄ | Purification and incandescent mantles |
| Thorium carbide | ThC₂ | Advanced nuclear fuels |
| WT20 electrode | W-ThO₂ | Precision TIG welding |
Electron shell distribution
View in the atom builderIsotopes
All natural thorium is thorium-232 (²³²Th), with 142 neutrons and a 14-billion-year half-life: it is fertile and three times more abundant than uranium. On capturing a neutron it breeds fissile uranium-233. Thorium-230 (²³⁰Th, 75,000 years) dates sediments alongside protactinium-231.
Occurrence and production
It is relatively abundant, at about 10 ppm, in monazite (up to 12% ThO₂), thorite, and thorianite. The main deposits are in India, Brazil, Australia, and the United States. It is obtained as a rare-earth byproduct through acid dissolution and solvent extraction, and the metal is produced by reduction with calcium.
History and origin of the name
Jöns Jacob Berzelius isolated it in 1829 from Norwegian thorite and named it thorium, after Thor, the Norse god of thunder: a warrior's baptism for a heavy metal. Marie Curie and Gerhard Schmidt discovered its radioactivity. Auer gas mantles carried it to every streetlamp until radiation protection retired it.
Applications and relevance of thorium
Uses of the element
As fertile Th-232 it feeds advanced cycles that breed U-233 in molten-salt and high-temperature reactors. As a metal it alloys aerospace magnesium and tungsten electrodes that strike the TIG arc.
Uses of its compounds
The dioxide forms crucibles that melt platinum, high-index lenses, and historic incandescent mantles. The fluoride dissolved in LiF-BeF₂ salt is the liquid fuel of thorium MSRs.
Health, safety and environment
Biological role
It has no biological function and is toxic and radioactive: inhaled dust irradiates the lung for years and its daughters emit radon-220. Historic Thorotrast, an X-ray contrast agent, caused liver cancers.
Safety
It is handled with dust control, ventilation, and alpha monitors in mining and welding. Thoriated electrodes require grinding with extraction; waste is managed as NORM with light shielding.
Environment
Monazite mining mobilizes thorium and uranium that must be contained in tailings ponds. In return, a well-managed thorium cycle breeds fewer minor actinides. India treats it as its strategic energy reserve.
Frequently asked questions about Thorium
Can thorium be used as nuclear fuel?
Yes, but not directly in most current reactors. Thorium-232 is a fertile material: when it absorbs a neutron it becomes uranium-233, a fissile isotope capable of sustaining a chain reaction. Thorium can therefore be used in alternative fuel cycles, although these require specific reactors and reprocessing technology. Its development remains experimental or limited compared with conventional uranium technology.
What has thorium been used for besides nuclear energy?
Thorium has been used as an alloying agent in magnesium to improve high-temperature strength, in TIG welding electrodes, and in industrial catalysts. It was also used in incandescent mantles, optics, and scientific instrumentation, although many of these uses have declined for radiological and regulatory reasons. Today its main interest lies in advanced nuclear fuel research.
Where does thorium come from?
Thorium is obtained mainly from the mineral monazite, where it occurs as thorium oxide alongside rare earths. It can also be found in thorite and other minerals. After extraction it is purified through chemical processes to obtain metallic thorium, thorium dioxide, or compounds for research. It is roughly three times more abundant than uranium in Earth's crust, although its energy use remains limited.
What is the difference between thorium and uranium?
Uranium-235 can sustain a chain reaction directly, whereas thorium-232 must first convert into uranium-233 to become fissile. Thorium is more abundant and may offer advantages in some nuclear cycles, but it requires less mature reactor and reprocessing technologies. Uranium currently dominates commercial nuclear power because its supply chain, reactors, and regulation are far more developed.
Is thorium safer than uranium?
Thorium has potential advantages, such as greater abundance, better thermal properties, and possibly less long-lived waste. However, it is not "safe" in an absolute sense: it is radioactive, toxic, and its cycle can produce dangerous isotopes, including uranium-233. Safety depends on the reactor design, fuel management, reprocessing, and waste control, not solely on the element used.
Is thorium dangerous to people?
Thorium is radioactive and toxic, although its radioactivity is lower than that of many other actinides. The main risk comes from inhaling or ingesting dust, as well as from prolonged exposure to radioactive materials. It has no known biological function. In everyday life significant exposure is uncommon, but industrial, mining, or waste handling requires strict radiation protection controls.
Why is thorium associated with molten-salt reactors?
Molten-salt reactors are one of the most studied concepts for using thorium, because they dissolve the fuel in a molten salt and operate at high temperatures at low pressure. In these designs thorium-232 can convert into uranium-233 and contribute to fission. Their possible advantages include greater passive safety and an alternative waste management route, although the technology still has to overcome materials, regulatory, and economic challenges.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.