Thorium
actinide · period 7 · group 3 · f-block
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and has a high melting point.
What the numbers say
At standard temperature and pressure Thorium is a solid, sitting in period 7, group 3 of the f-block.
Its melting point of 2023 K (1750 °C) is the 19th highest of the 110 elements with a measured value — mid-range among the elements.
At 11.724 g/cm³ it is the 22nd densest of 96 elements with a quoted density.
Pauling electronegativity is 1.3. Paired with oxygen (3.44) that is a difference of 2.14, which puts the bond predominantly ionic — expect a stable oxide ceramic.
In the solid state it takes a face-centered cubic structure — FCC packing gives 12 independent slip systems, which is why FCC metals are the ductile, formable ones.
It is reported in 3 oxidation states (2, 3, 4), which sets how many distinct compounds and phases it can form.
First identified in 1829.
Crystal structure
Solid Thorium adopts a Face-centered Cubic arrangement, in the Cubic (isometric) system — axes a = b = c, angles α = β = γ = 90°. Symmetry controls the anisotropy of stiffness and thermal expansion, and the available slip systems set how far the metal deforms before it fractures.
Check it against computed structures
The structure search queries the Materials Project, OQMD and the Open Materials Database for computed crystal structures containing Th — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 32 · O 18 · P 10 · Q 2
Properties
| Atomic number | 90 |
|---|---|
| Atomic mass | 232.03774 u |
| Series | actinide |
| Group | 3 |
| Period | 7 |
| Block | f-block |
| Phase at STP | Solid |
| Density | 11.724 g/cm³ |
| Melting point | 2023 K (1750 °C) |
| Boiling point | 5061 K (4788 °C) |
| Electronegativity (Pauling) | 1.3 |
| Electron configuration | [Rn] 6d² 7s² |
| Crystal structure | Face-centered Cubic |
| Oxidation states | 2,3,4 |
| Discovered | 1829 |
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See Th in context with all 118 elements.
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