Thorium
[Rn] 6d² 7s² · 2 · 8 · 18 · 32 · 18 · 10 · 2
A weakly radioactive metal three times more abundant in the crust than uranium. Thorium does not fission itself, but absorbing a neutron turns it into fissile uranium-233 — the basis of the thorium fuel cycle.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 90
- Atomic mass
- 232.038 а.е.м.
- Electron configuration
- [Rn] 6d² 7s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 10 · 2
- Block
- d-block
- Group
- no data
- Period
- 7
- Electronegativity (Pauling)
- 1.3
- Electronegativity (Allen)
- 1.11
- Atomic radius
- 237 pm
- Covalent radius
- 206 pm
- Van der Waals radius
- 237 pm
- Ionisation energy 1
- 587 kJ/mol
- Ionisation energy 2
- 1 110 kJ/mol
- Ionisation energy 3
- 1 930 kJ/mol
- Ionisation energy 4
- 2 780 kJ/mol
- Electron affinity
- 112.7 kJ/mol
- Common oxidation states
- +4
- Oxidation states
- +1, +2, +3, +4
Physical properties
- State at 25 °C
- solid
- Density
- 11.72 g/cm³
- Melting point
- 2 115 K · 1 841.9 °C
- Boiling point
- 5 061 K · 4 787.9 °C
- Speed of sound
- 2 490 m/s
Thermal properties
- Heat of fusion
- 13.81 kJ/mol
- Heat of vaporisation
- 514 kJ/mol
- Specific heat capacity
- 0.113 J/(g·K)
- Thermal conductivity
- 54 W/(m·K)
Mechanical properties
- Young's modulus
- 79 GPa
- Shear modulus
- 31 GPa
- Bulk modulus
- 54 GPa
- Poisson ratio
- 0.27
- Mohs hardness
- 3
- Brinell hardness
- 400 MPa
Electrical and magnetic properties
- Electrical resistivity
- 157 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 1.4 K
Crystal structure
- Crystal structure
- face-centred cubic (fcc)
- Lattice constants
- a = 508.42 pm
Abundance
- In the crust
- 9.6 mg/kg
- In the ocean
- 1·10⁻⁶ mg/L
- In the universe
- 0.0004 mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 230Th | — | 75 419.2 yr | α |
| 232Th | 100 % | 14.04 Gyr | α |
Thorium-232 is essentially all of natural thorium and has a 14-billion-year half-life, longer than the age of the universe. Thorium-229 is the source of actinium-225 for alpha therapy.
Discovery
- Year of discovery
- 1829
- Discovered by
- Jöns Jacob Berzelius
- Where
- Sweden
- Origin of the name
- after Thor, the Norse god of thunder
History
Berzelius isolated thorium in 1829 from a Norwegian mineral and named it after Thor. Marie Curie and Gerhard Schmidt independently established in 1898 that it is radioactive — the second radioactive element known, after uranium.
Where it occurs
9.6 mg/kg of the crust — commoner than lead. The main mineral is monazite, mined alongside the rare earths.
How it is produced
From monazite sands during rare-earth processing. Significant quantities accumulate as an unwanted by-product.
Role in living things
No biological role.
Safety
Weakly radioactive, but inhaled dust lodges in the lungs and liver. Thorotrast, a thorium dioxide contrast agent used from the 1930s to the 1950s, caused liver cancer decades after the examination.
Uses
- Welsbach gas mantles — a historic mass application that gave a brilliant white light
- Thoriated tungsten in TIG welding electrodes
- High-refractive-index optical glass (largely historical)
- The thorium nuclear fuel cycle, under development in India and China
Curiosities
- The crust holds three to four times more thorium than uranium, and it is all one isotope — no enrichment needed.
- The Welsbach mantle with thorium dioxide gave the brightest light available before electricity.
- A thorium reactor cannot go into an uncontrolled chain reaction without an external neutron source.
- Thorium is named after Thor, the Norse god of thunder.