Thorium · element 90
Named after the god of thunder, once in every gas lantern and camera lens, and touted as the nuclear fuel of tomorrow
Key facts
Thorium is a soft, silvery metal that has spent two centuries being almost useful. It lit streets and campsites, sharpened camera lenses and strengthened aircraft engines. Now it is best known for a job it has never quite landed: powering nuclear reactors that would be cleaner and safer than uranium ones.
It is radioactive, but only mildly. Thorium-232 has a half-life of 14 billion years, about the age of the universe, so most of the thorium that formed with the Earth is still here, quietly warming the planet from within.
Where it is found
Thorium is about three times as abundant in the Earth's crust as uranium, roughly as common as lead. Its main source is monazite, a heavy phosphate sand rich in rare earth elements that can contain several per cent thorium oxide, found on the beaches of Kerala in India, in Brazil and in Australia. It also forms its own minerals, thorite and thorianite, and is present in most granite.
Because thorium comes out of the ground alongside rare earth metals, more is produced as an unwanted by-product than anyone uses, and it is mostly stockpiled or treated as low level waste. India, with vast monazite sands and little uranium, has the most ambitious plans to burn it as fuel.
How it was discovered
In 1828 the Norwegian mineralogist and priest Morten Thrane Esmark found an unfamiliar black mineral on the island of Lovoya and sent it to the Swedish chemist Jöns Jakob Berzelius, who extracted a new oxide and named the element thorium after Thor, the Norse god of thunder. The mineral became thorite.
Berzelius could not know his element was radioactive; radioactivity itself was unknown until 1896. In 1898 Gerhard Schmidt and, independently, Marie Curie showed that thorium gave off radiation, making it the second radioactive element identified after uranium.
What we use it for
For a century thorium's biggest job was light. Gas mantles, fabric bags soaked in thorium oxide with a pinch of cerium, glow brilliant white in a gas flame, and from the 1890s they lit streets, homes and camping lanterns. Thorium oxide also went into high quality camera lenses from the 1940s to the 1970s, because it bends light strongly without splitting colours, and into tungsten welding electrodes, where a little thorium helps strike an arc.
Thorium's nuclear promise rests on a two step trick. Thorium-232 cannot sustain a chain reaction itself, but when it absorbs a neutron it turns, via protactinium, into uranium-233, which can. A reactor seeded with a little fissile fuel could in principle breed its own fuel from cheap thorium and produce less long lived waste. An experimental molten salt reactor at Oak Ridge ran on uranium-233 from 1965 to 1969, and China has recently built a small experimental thorium reactor. The chemistry remains difficult, and no commercial thorium power station yet exists.
Reading the numbers
Thorium melts at about 1750 °C, higher than iron, and boils near 4790 °C, one of the highest boiling points of any element. Its oxide melts above 3000 °C, which is why it was chosen for gas mantles that must glow rather than melt. Its density of 11.7 grams per cubic centimetre is almost exactly that of lead.
Thorium is given a precise atomic mass, 232.04, unlike its radioactive neighbours, because almost all natural thorium is the single isotope thorium-232, and its 14 billion year half-life means about four fifths of the Earth's original thorium remains. The electron configuration [Rn] 6d2 7s2 gives four outer electrons, and thorium nearly always uses all four, forming Th4+ ions, so its chemistry resembles zirconium and hafnium more than the neighbouring actinides. Like actinium it has no 5f electrons; the 5f shell only starts filling at protactinium. Ionisation energies of 587, 1110, 1930 and 2780 kJ/mol rise steadily, and its electronegativity of 1.3 marks a reactive metal that tarnishes black in air.
Safety and the environment
Thorium's alpha radiation is stopped by skin, so a lump of thorium or an old lens is safe to handle. The hazard is inhaling or swallowing thorium dust, which lodges in bone and lungs for decades, and its decay chain produces radon-220, a short lived form of radon. The most tragic exposure was Thorotrast, a thorium dioxide suspension injected as an X-ray contrast agent from the 1930s to the 1950s, which caused liver cancers decades later.
Environmentally, thorium is a headache for the rare earth industry, whose waste piles contain it, and old gas mantle factories have left contaminated ground. In a thorium fuel cycle the waste would be less long lived than from uranium, though still highly radioactive for centuries.
Did you know?
- Thorium-232 has a half-life of about 14 billion years, roughly the age of the universe, so around 80 per cent of the thorium present when the Earth formed is still here.
- The decay of thorium, uranium and potassium in the Earth's interior produces a large share of the heat that drives plate tectonics and volcanoes.
- Thorium and vanadium are the only two elements named after Norse deities; vanadium honours Vanadis, a name for the goddess Freyja.
- Old camera lenses containing thorium oxide slowly turn yellow as radiation damages the glass, and can be bleached back by leaving them in sunlight.
- An experimental molten salt reactor at Oak Ridge ran on uranium-233 bred from thorium between 1965 and 1969.
- Berzelius first used the name thorium in 1815 for a mineral that turned out to be yttrium phosphate, and reused it thirteen years later for the real thing.
More in the actinides
The nearest neighbours of thorium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Compare Thorium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is thorium dangerous?
Solid thorium and old thorium containing objects are low risk because its alpha particles cannot penetrate skin. The danger is inhaling or swallowing thorium dust, which stays in the body for years, so thorium should never be ground, sanded or burned. Its decay also produces a short lived form of radon.
Can thorium replace uranium in nuclear reactors?
In principle yes. Thorium-232 absorbs neutrons and becomes uranium-233, which can sustain a chain reaction. Thorium is more abundant and produces less long lived waste, but a reactor needs fissile material to start and the fuel chemistry is complex. Experimental reactors have worked; no commercial one operates yet.
Why are old camera lenses radioactive?
From the 1940s to the 1970s some lens makers added thorium oxide to glass because it gives a high refractive index with little colour distortion. The thorium is mildly radioactive and slowly yellows the glass. The lenses are safe to use but should not be broken or ground.
Is thorium more common than uranium?
Yes, about three times more common in the Earth's crust, and roughly as abundant as lead. Most of it is locked in monazite sands and granite, and much is dug up incidentally during rare earth mining.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Thorium reference article. Spotted an error? Tell us.