Thorium
Atomic Data
| Atomic Number | 90 |
| Symbol | Th |
| Atomic Weight | 232.04 u |
| Density (STP) | 11.72 g/cm³ |
| Melting Point | 1841.85 °C (2115 K) |
| Boiling Point | 4787.85 °C (5061 K) |
| Electronegativity | 1.3 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d2 7s2 |
| Oxidation States | +4 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-29-1 |
Electron Configuration
[Rn] 6d2 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| P | 6 | 6s | 2 | 80 |
| P | 6 | 6p | 6 | 86 |
| P | 6 | 6d | 2 | 88 |
| Q | 7 | 7s | 2 | 90 |
| Total | 90 | 90 | ||
Isotopes of Thorium
Thorium is monoisotopic: ²³²Th is its only naturally occurring stable isotope, accounting for 100% of all natural Thorium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Thorium-232 | ²³²Th | 90 | 142 | 99.98 | Stable |
Abundance & Occurrence
Thorium is present in Earth's crust at approximately 9.6 ppm by mass and at approximately 0.04 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Radioactivity: Thorium-232 (t½ = 14 × 109 years) is a weak alpha emitter; its decay chain through radium to radon means that thorium ore dust and thorium oxide aerosols carry a complex mixed radiation hazard.
- Thorium dioxide (ThO2) dust: Historically used in gas mantles and high-index glass; inhalation of ThO2 dust deposits alpha-emitting material in the lungs; thorotrast (colloidal ThO2), used as a medical contrast agent until the 1950s, caused liver cancers decades later in exposed patients.
- Chemical toxicity: In addition to radiological hazard, thorium compounds have moderate chemical toxicity; thorium nitrate and chloride solutions are irritants and potential teratogens.
- Regulatory controls: Thorium use is regulated as both a radioactive material and a toxic substance; work with thorium-containing materials requires radiation protection training, dosimetry, and a licence in most jurisdictions.
Thorium in the Real World
Real-World Uses
- Thorium nuclear reactor fuel (research): Thorium-232 is fertile; it captures neutrons to breed fissile uranium-233, which sustains a chain reaction in the thorium fuel cycle. Thorium molten salt and solid-fuel reactors are under active development in India, China, and several research programmes as a potentially safer and more abundant alternative to uranium.
- Gas lantern mantles: Thorium dioxide (ThO₂)-impregnated fabric mantles emit an intense white light when incandescent, used in camping lanterns and gas lamps; modern mantles increasingly substitute yttrium or cerium to avoid thorium's radioactivity.
- High-temperature ceramics and crucibles: ThO₂ has one of the highest melting points of any oxide (3390 °C) and was used in refractory crucibles for melting reactive metals and in high-temperature furnace components; now largely replaced by less radioactive alternatives.
- High-refractive-index optical glass (historical): Thorium oxide was a key component of high-refractive-index, low-dispersion optical glass used in precision camera and microscope lenses from the 1940s to 1970s; production was discontinued due to radioactivity and replaced by lanthanum-based glasses.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is thorium used for?
Thorium's main current use is in high-temperature gas-cooled nuclear reactors (as thorium fuel cycles) and in thorium-magnesium alloys for aerospace applications. Thorium oxide is used in high-refractive-index optical glass and in tungsten welding electrodes (thoriated tungsten TIG electrodes glow more consistently). Thorium was historically used in gas mantles for camping lanterns and in aircraft engine parts, and there is renewed interest in thorium as a nuclear fuel.
Can thorium be used as nuclear fuel?
Yes. Thorium-232 is a fertile material: it does not itself fission easily, but when bombarded by neutrons it converts to uranium-233, which is an excellent nuclear fuel. A thorium fuel cycle could potentially produce less long-lived radioactive waste than a uranium cycle and offers different non-proliferation characteristics. India, which has large thorium reserves, has the most advanced thorium reactor programme. Several other countries are pursuing thorium reactor designs.
How was thorium discovered?
Thorium was discovered in 1828 by Swedish chemist Jöns Jacob Berzelius, who isolated it from a Norwegian mineral sent to him by mineralogist Morten Thrane Esmark. Berzelius named it thorium after Thor, the Norse god of thunder: continuing the pattern of naming newly discovered heavy elements after powerful deities. It was later found to be radioactive by Gerhard Carl Schmidt and Marie Curie in 1898.
Why do gas lantern mantles contain thorium?
The original Welsbach gas mantle, invented in 1885, was made of a fabric impregnated with a mixture of 99% thorium oxide and 1% cerium oxide. When heated in a gas flame, this mixture produces exceptionally bright, white incandescent light: far brighter than the bare flame alone. Thorium oxide has the right combination of thermal stability and light emission at flame temperatures. Due to thorium's radioactivity, modern mantles mostly use yttrium oxide or other rare-earth oxides as replacements.