Thorium (Th)
Thorium (symbol Th) is the chemical element with atomic number 90: an actinide, solid at 0 °C, radioactive, with an atomic mass of 232.04 u. Discovered in 1829 (Jöns Jacob Berzelius).
Thorium is a weakly radioactive metal, about three times more abundant than uranium in the Earth's crust. Its main isotope, thorium-232, has a half-life of 14 billion years, longer than the age of the universe. It cannot sustain a nuclear chain reaction on its own, but it can be converted into fissile uranium-233, making it a serious candidate for future reactors. Together with uranium and potassium, it helps keep the Earth's interior hot.
Key facts
- Atomic number
- 90
- Symbol
- Th
- French name
- Thorium
- Atomic mass
- 232.04 u
- Molar mass
- 232.04 g/mol
- Family
- Actinides
- Position
- period 7, f-block
- State at 0 °C
- Solid
- Melting point
- 1,749.9 °C
- Boiling point
- 4,787.9 °C
- Density
- 11.72 g/cm³
- Electronegativity (Pauling)
- 1.3
- First ionization energy
- 587 kJ/mol
- Atomic radius
- 180 pm
- Electron configuration
- [Rn] 6d² 7s²
- Abundance in Earth's crust
- 9.6 ppm (38th most abundant element)
- Indicative price
- 290 $/kg (order of magnitude)
- Radioactive
- yes
- Discovery
- 1829, Jöns Jacob Berzelius
Origin of the name
From Thor, the Norse god of thunder.
What does thorium look like?
A shiny, silvery metal, fairly soft and ductile, that slowly tarnishes in air, turning gray and then black. As a fine powder, it can ignite spontaneously.
Where is thorium found?
About 6 to 10 ppm in the Earth's crust, mostly in granites and in the heavy mineral sands of beaches. It is concentrated in monazite, a rare earth phosphate found notably along the coasts of India (Kerala) and Brazil, where some beaches are naturally radioactive.
Natural sources
- Monazite
- Thorite
- Thorianite
How is thorium produced?
It is recovered as a by-product of processing monazite for rare earths. The ore is broken down with acid, then the thorium is separated by chemical extraction and obtained as an oxide. With few buyers, much of it is simply stockpiled.
Top producers
- India
- Brazil
- Australia
- United States
What is thorium used for?
- Potential nuclear fuel (thorium–uranium-233 cycle), under development in India and China
- Thoriated tungsten welding electrodes
- Formerly: gas lamp mantles (incandescent thorium oxide)
- Formerly: high-index optical glass for camera lenses
- Magnesium alloys for aerospace
In everyday life
- Mantles of old camping lanterns
- Old yellowed camera lenses (1940s–1970s)
- Red-tipped TIG welding electrodes
Role in living things
No biological role. It is barely absorbed through the gut, but when inhaled as dust, it lodges in the lungs, liver and bones.
Hazards of thorium
Weakly radioactive as a metal, it becomes dangerous as inhaled dust. Its decay products, including radon-220 (“thoron”), are far more radioactive. Thorotrast, a thorium-based X-ray contrast agent used until the 1950s, caused liver cancers decades later.
Cosmic origin
- Neutron stars: Created when two neutron stars collide — the “forge” of gold.
History of thorium
Swedish chemist Jöns Jacob Berzelius identified it in 1828 in a Norwegian mineral and named it after Thor, the Norse god of thunder. In 1898, Gerhard Schmidt and Marie Curie independently discovered that it is radioactive. Rutherford and Soddy then studied its decay chain and realized that radioactive elements transmute into one another.
Did you know?
- Berzelius had already used the name “thorium” in 1815 for a mineral that turned out to be… yttrium phosphate. He recycled the name.
- The gas mantles invented by Carl Auer von Welsbach lit up Europe's streets in the late 19th century thanks to thorium oxide.
- India, poor in uranium but rich in thorium, has built its entire long-term nuclear program around this element.
Isotopes of thorium
No stable isotopes, but thorium-232 lives so long (14 billion years) that it is almost as abundant as lead in rocks. Thorium-229 has an excited state of such low energy that it could serve as a “nuclear clock.”
- thorium-228: radioactive, half-life 1.91 years
- thorium-229: radioactive, half-life 7,900 years. Its excited state, reachable with a laser, opens the way to ultra-precise nuclear clocks.
- thorium-230: 0.02% of natural thorium, radioactive, half-life 75,400 years. Used to date corals and marine sediments up to several hundred thousand years old.
- thorium-232: 99.98% of natural thorium, radioactive, half-life 1.4 × 10¹⁰ years. Can be turned into fissile uranium-233; seen as a possible nuclear fuel of the future.
- thorium-234: radioactive, half-life 24.1 days
Related elements
Same family (actinides): Actinium (Ac), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).