Thorium (Th)
Thorium (Th) is an actinide with atomic number 90 and a standard atomic weight of 232.04. At room temperature it is a solid that melts at 1,750 °C and boils at 4,785 °C.
Thorium key facts
Thorium at a glance
Identity
| Symbol | Th |
|---|---|
| Atomic number (Z) | 90 |
| Atomic weight | 232.04 ± 0.01 (CIAAW 2024) |
| Category | Actinide |
| Group | — (f-block: actinide series) |
| Period | 7 |
| Block | f-block |
| CAS number | 7440-29-1 |
Atomic properties
| Electron configuration | [Rn] 6d2 7s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d2 7s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 32, 18, 10, 2 |
| Oxidation states | +4 Less common: -1, +1, +2, +3 |
| Electronegativity (Pauling) | 1.3 |
| First ionisation energy | 6.307 eV (608.5 kJ/mol) 2nd: 12.1 eV · 3rd: 18.32 eV |
| Atomic radius (empirical) | 180 pm |
| Covalent radius | 175 pm |
| Van der Waals radius | 245 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 1,750 °C · 3,182 °F · 2,023.15 K |
| Boiling point | 4,785 °C · 8,645 °F · 5,058.15 K |
| Density | 11.7 g/cm³ |
| Specific heat capacity | 0.118 J/(g·K) |
| Crystal structure | Face-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | Yes |
| Natural isotopes (abundance) | 232Th 99.98% (radioactive), 230Th 0.02% (radioactive) |
| Known isotopes | 32 |
| Abundance in Earth's crust | 9.6 mg/kg |
| Abundance in seawater | 0.000001 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Thorium electron configuration
Thorium has 90 electrons. Written in short form, its ground-state configuration is [Rn] 6d2 7s2 — the electrons of the noble gas radon, plus 6d2 7s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d2 7s2.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f2, 6d0, but thorium actually has 5f0, 6d2. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1828 — Jöns Jacob Berzelius · Sweden.
Thor, the Norse god of thunder.
Known for
About three times more abundant than uranium. Studied as a nuclear fuel; thorium oxide was used in gas-lamp mantles.
Isotopes of thorium
Naturally occurring thorium is a mix of 2 isotopes. Each has 90 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 232Th | 90 | 142 | 99.98% | radioactive · half-life 14 billion years |
| 230Th | 90 | 140 | 0.02% | radioactive · half-life 75.4 thousand years |
32 isotopes of thorium are known in our data set (half-lives as published in nuclear data tables).
Thorium and its neighbours in the actinide series
Neighbouring elements in the same f-block series:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Actinium | 89 | [227] | 1,050 °C | 3,200 °C | 10 g/cm³ | 1.1 |
| Thorium | 90 | 232.04 | 1,750 °C | 4,785 °C | 11.7 g/cm³ | 1.3 |
| Protactinium | 91 | 231.04 | 1,572 °C | — | 15.4 g/cm³ | 1.5 |
| Uranium | 92 | 238.03 | 1,135 °C | 4,131 °C | 19.1 g/cm³ | 1.38 |
| Neptunium | 93 | [237] | 644 °C | — | 20.2 g/cm³ | 1.36 |