Thorium (Th)
Actinide · f-block · period 7
Soft, silvery-white metal that tarnishes grey then black in air.
Thorium (symbol Th) is chemical element number 90, a actinide in the actinide series (f-block), period 7 of the periodic table. Its standard atomic weight is 232.04 and its electron configuration is [Rn] 6d2 7s2. At room temperature it is a solid, and it is radioactive. Named after Thor, the Norse god of thunder.
Properties
| Atomic number | 90 |
|---|---|
| Atomic mass | 232.04 |
| Category | Actinide |
| Block | f-block |
| Group · Period | f-block series · 7 |
| Electron configuration | [Rn] 6d2 7s2 |
| Electrons per shell | 2, 8, 18, 32, 18, 10, 2 |
| State at 25 °C | solid |
| Melting point | 2023 K |
| Boiling point | 5061 K |
| Density | 11.7 g/cm³ |
| Electronegativity (Pauling) | 1.3 |
| Radioactive | Yes — no stable isotope |
| Origin | primordial |
| Discovered | 1829 · Jöns Jacob Berzelius |
Key facts about Thorium
- Named after Thor, the Norse god of thunder.
- About as abundant as lead, and a possible fuel for safer nuclear reactors.
- Old gas lamp mantles glowed brilliantly because they were soaked in thorium.
- Only weakly radioactive, with a half-life close to the age of the universe.
What Thorium is used for
Proposed thorium nuclear fuel cycle, high-quality optical glass, historically gas mantles and welding electrodes.
Where it sits in the periodic table
Thorium is in the actinide series, period 7, of the f-block. Its electron configuration [Rn] 6d2 7s2 means its outermost shell holds 2 electrons, which is what places it here and explains much of its chemistry. See Thorium on the interactive table →
Thorium — frequently asked questions
What is Thorium used for?
Proposed thorium nuclear fuel cycle, high-quality optical glass, historically gas mantles and welding electrodes.
Is Thorium radioactive?
Yes. Thorium has no stable isotope, so every form of it is radioactive.
What is the electron configuration of Thorium?
Thorium has the electron configuration [Rn] 6d2 7s2, which is 2, 8, 18, 32, 18, 10, 2 electrons per shell.
What group and period is Thorium in?
Thorium is in the actinide series (f-block), period 7, in the f-block. Its outer shell holds 2 electrons.
Who discovered Thorium?
Thorium was discovered in 1829 by Jöns Jacob Berzelius.