Thorium Th

Discovery

In 1815 the Swedish chemist Jöns Jacob Berzelius announced a new element he called thoria, but it turned out to be yttrium phosphate and he withdrew the claim.

In 1828 a black mineral found in Norway by the pastor and mineral collector Hans Morten Thrane Esmark was sent to Berzelius, who identified a genuine new element in it and gave it the name he had earlier been unable to use. The mineral is now called thorite. In 1898 Gerhard Carl Schmidt in Germany and Marie Curie in France independently discovered that thorium is radioactive.

Origin of the name

The name honours Thor, the Norse god of thunder.

Main uses

• Nuclear fuel research: thorium-232 cannot sustain a chain reaction on its own, but when it absorbs a neutron it is transformed into fissile uranium-233. India has long pursued a thorium fuel cycle, and several countries are researching thorium together with molten-salt reactors.
• Gas mantles: mantles made of thorium oxide with a little cerium oxide glow brilliantly white when heated and lit gas lamps from the late nineteenth century. Because of the radioactivity, most have been replaced by other materials.
• Thoriated tungsten electrodes: thorium oxide in tungsten electrodes gives a stable arc for TIG welding, though alternatives are increasingly used.
• Optical glass: its high refractive index led to its use in some mid-twentieth-century camera lenses.

Isotopes

Natural thorium is almost entirely thorium-232, with a half-life of about 14 billion years, similar to the age of the universe. Thorium-230 (half-life about 75,000 years) is used in uranium-thorium dating of corals and cave formations over hundreds of thousands of years. Thorium-229 has an exceptionally low-energy nuclear excited state; in 2024 researchers excited it with a laser, making thorium-229 the leading candidate for a "nuclear clock".

In everyday life

Thorium occurs in minerals such as monazite and is a by-product of rare-earth mining. Some mid-century camera lenses containing thorium glass have yellowed with age and register on radiation detectors. Thorotrast, a colloidal thorium dioxide used as an X-ray contrast agent in the 1930s and 1940s, remained in patients' bodies and was later found to cause liver cancer and other diseases decades afterwards. Thorium's radioactivity is weak, but it should not be inhaled or swallowed.