Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where thorium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 180 pm — that is 0.18 nm, so about 2778 million of them side by side would span a millimetre.
Thermal range
Thorium is liquid over a 3038 K window, from 2023 K to 5061 K.
The story
Three times more abundant than uranium and a potential reactor fuel that cannot easily be weaponised.
Thorium was discovered in 1828 by Swedish chemist Jöns Jacob Berzelius, one of the founding fathers of modern chemistry. Berzelius identified the new element while analyzing a black mineral sample sent to him by Norwegian mineralogist Morten Thrane Esmark, who had found it near Løvøya island in Norway.
The original sample was discovered by Morten Thrane Esmark in 1828 on the island of Løvøya in the Langesundsfjord region of Norway. Esmark, unable to identify the unusual black mineral, sent it to Berzelius in Stockholm for analysis. This collaboration between Norwegian field work and Swedish analytical chemistry led to the element's discovery.
Berzelius used his advanced analytical techniques to isolate and characterize the new element. He initially thought he had discovered two new elements, which he named "thorium" and "thorina." Later analysis revealed these were the same element in different oxidation states. The name "thorium" was chosen after Thor, the Norse god of thunder.
The discovery occurred during the golden age of element discovery, when chemists were systematically analyzing new minerals using improved analytical methods. Berzelius's work with thorium contributed to the development of atomic weight determination and helped establish principles of chemical analysis that remain important today.
Thorium's radioactive properties weren't discovered until 1898 by Gerhard Carl Schmidt and Marie Curie independently. This discovery came shortly after Becquerel's discovery of radioactivity and established thorium as only the second known radioactive element after uranium, launching the field of nuclear chemistry.
The discovery of thorium exemplified 19th-century collaborative chemistry and established the foundation for understanding actinide elements. Berzelius's methodical approach to isolating and characterizing thorium became a model for discovering and studying other heavy elements.
Applications
Thorium is increasingly recognized as a next-generation nuclear fuel with significant advantages over uranium-based fuels. Thorium-232, when bombarded with neutrons, converts to uranium-233, which is an excellent fissile material for nuclear reactors. This thorium fuel cycle produces less long-lived radioactive waste and cannot easily be weaponized, making it an attractive option for peaceful nuclear energy.
Thorium is the preferred fuel for molten salt reactor (MSR) technology, where thorium fluoride is dissolved in molten salt that serves as both fuel and coolant. These reactors can operate at atmospheric pressure, have inherent safety features, and can consume existing nuclear waste. Countries like China and India are investing heavily in thorium MSR development.
Historically, thorium's most common use was in gas lantern mantles, where thorium dioxide provided intense, bright light when heated. Although largely phased out due to radioactivity concerns, thorium compounds are still used in specialized high-temperature applications, including refractory materials for crucibles and furnace linings that must withstand extreme heat.
Thorium is used to create specialized magnesium-thorium alloys for aerospace applications, where high strength-to-weight ratios are critical. These alloys maintain their properties at elevated temperatures, making them valuable for jet engine components and missile applications. The addition of small amounts of thorium significantly improves the creep resistance of magnesium alloys.
Research institutions use thorium in various applications, including neutron sources, radiation detection equipment calibration, and fundamental nuclear physics research. Thorium compounds serve as reference standards for radiation measurements and are used in studying heavy element chemistry and nuclear decay processes.
Advanced nuclear technologies are exploring thorium's potential in accelerator-driven systems (ADS) and small modular reactors (SMRs). These systems could provide clean, safe nuclear power with reduced waste production and enhanced proliferation resistance, making thorium a key element in future sustainable energy strategies.
Thorium's most promising modern application is in advanced nuclear reactor designs. Research programs worldwide are developing thorium molten salt reactors, which could provide safer, cleaner nuclear energy. India's three-stage nuclear program specifically targets thorium utilization, given the country's large thorium reserves.
Thorium dioxide (thoria) is used in specialized refractory applications where extreme temperature resistance is required. Its melting point of 3,300°C makes it valuable for crucibles, furnace linings, and high-temperature laboratory equipment used in materials research and metal processing.
Small amounts of thorium are added to magnesium alloys to create materials with excellent high-temperature properties for aerospace applications. These alloys maintain strength and resistance to creep at elevated temperatures, making them suitable for aircraft engines and structural components.
Thorium compounds serve as calibration standards for radiation detection equipment and as sources for neutron activation analysis. Research facilities use thorium isotopes for studying nuclear decay processes and developing new radiochemical techniques.
Thorium is being investigated for use in small modular reactors and other advanced nuclear technologies that could provide decentralized, clean energy. Its abundance and favorable nuclear properties make it an attractive alternative to uranium for future energy systems.
Where it comes from
9.6 mg/kg of Earth's crust · more abundant than 68% of elements
Thorium is relatively abundant in the Earth's crust, with an average concentration of 9.6 parts per million, making it about three times more abundant than uranium. It occurs primarily in igneous rocks and is concentrated in heavy mineral sands formed by weathering and erosion processes.
The most important thorium-bearing mineral is monazite (a rare earth phosphate containing 4-12% thorium dioxide), found in beach sands and river deposits. Other significant minerals include thorite (thorium silicate), thorianite (thorium dioxide), and bastnasite, which contains both thorium and rare earth elements.
Major thorium deposits are located in India, Australia, Brazil, and the United States. India possesses the world's largest thorium reserves, primarily in monazite beach sands along its eastern and southwestern coasts. Australia's reserves are mainly in mineral sands, while Brazil's deposits are in monazite-rich beach sands and weathered rock formations.
Thorium is typically obtained as a byproduct of rare earth element mining rather than being mined specifically for thorium. The extraction process involves acid leaching of monazite concentrates, followed by chemical separation techniques to isolate thorium from rare earth elements and other components.
Thorium occurs naturally in granite rocks, soil, and water at low concentrations. It's present in coal at levels of 1-20 parts per million, and burning coal releases thorium into the atmosphere. Natural background radiation includes a small contribution from thorium and its decay products.
Handling
Thorium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
WARNING: Thorium is naturally radioactive with a half-life of 14 billion years. While less intensely radioactive than many artificial isotopes, it still poses health risks through alpha radiation and the production of radioactive decay products, including radon gas.
The primary health concern is inhalation of thorium dust or particles, which can lodge in lung tissue and cause long-term radiation exposure. Workers in thorium processing facilities or research laboratories must use appropriate respiratory protection and work in well-ventilated areas.
Thorium compounds should be handled using standard radiological safety protocols, including personal protective equipment, radiation monitoring, and proper storage in shielded containers. Areas where thorium is used must be regularly monitored for radiation levels and contamination.
Past use of thorium in gas mantles and consumer products created potential exposure pathways. Old gas mantles should be handled carefully and disposed of as radioactive waste. Some vintage ceramics and welding rods may contain thorium and require proper handling.
Quick answers
Thorium (symbol Th) is element 90 on the periodic table, a actinide in period 7. Three times more abundant than uranium and a potential reactor fuel that cannot easily be weaponised. At room temperature it is a solid, and it is radioactive.
[Rn] 6d² 7s², giving 7 occupied shells holding 2, 8, 18, 32, 18, 10, 2 electrons respectively.Thorium melts at 2023 K (1749.9 °C) and boils at 5061 K (4787.9 °C). That puts it among the most refractory elements — it stays solid at temperatures that vaporise most metals.
The standard atomic weight of Thorium is 232.038 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
Thorium has a density of 11.72 g/cm³. Water is 1.0 g/cm³, so a block of thorium is about 11.7× heavier.
Thorium has a Pauling electronegativity of 1.3. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Thorium was discovered in 1828 by Jons Jacob Berzelius. It is named after thor, Norse god of thunder.
Thorium makes up about 9.6 mg/kg of the Earth's crust — uncommon, but not rare.
Yes. Thorium has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.