Thorium
Element 90 · Actinide
Thorium is a naturally occurring actinide that is only weakly radioactive, with thorium-232 having a half-life of about 14 billion years. It is a bright silvery metal that tarnishes slowly in air. Thorium is fertile rather than fissile: in a reactor it absorbs neutrons and converts to uranium-233, which can sustain fission. It was once widely used in gas lantern mantles and camera lenses, and remains of interest as an alternative nuclear fuel.
Key facts
| Atomic number | 90 |
|---|---|
| Atomic weight | 232.04 |
| Category | Actinide |
| Group / Period | — / 7 |
| Block | f |
| Electron configuration | [Rn] 6d2 7s2 |
| Common oxidation states | +4, +3 |
| State at STP | Solid |
| Electronegativity | 1.3 |
| Density | 11.72 g/cm3 |
| Melting point | 1750 °C |
| Boiling point | 4788 °C |
Occurrence
Thorium is relatively abundant, roughly as common as lead in the Earth's crust. It is mined from monazite sands in India, Brazil, Australia, and the United States.
Applications
Potential breeder fuel for thorium-based nuclear reactors
Historical: incandescent gas mantles and high-index camera lenses
Magnesium-thorium alloys for aerospace components (historical)
Common compounds
- Thorium dioxide (ThO2)
- Thorium nitrate (Th(NO3)4)
- Thorium fluoride (ThF4)
Thorium was discovered in 1828 by Jons Jacob Berzelius and named after Thor, the Norse god of thunder.
Data source: IUPAC standard atomic weights 2021