Thorium (Th)
Element 90 · Actinide · Solid at 25 °C · Period 7 (f-block)
Thorium is several times more abundant than uranium and has long been studied as an alternative nuclear fuel; it once lit gas lamps in glowing mantles.
On the periodic table, thorium sits in the f-block in period 7. It is one of 15 elements classified as an actinide, and at room temperature (25 °C) it is a solid.
The actinides fill the 5f orbitals in the bottom row of the table, and every one of them is radioactive. Only thorium and uranium occur on Earth in significant quantities; the rest are made in reactors or particle accelerators. Their nuclear properties — fission above all — shaped the twentieth century.
Properties of thorium
| Atomic number (Z) | 90 |
|---|---|
| Symbol | Th |
| Atomic mass | 232.04 |
| Category | Actinide |
| Group | — (f-block, associated with group 3) |
| Period | 7 |
| Block | f-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Rn] 6d2 7s2 |
| Electronegativity (Pauling) | 1.3 |
| Melting point | 1,750 °C (2,023.15 K / 3,182 °F) |
| Boiling point | 4,788 °C (5,061.15 K / 8,650.4 °F) |
| Density | 11.72 g/cm³ |
Position in the periodic table
Thorium belongs to the actinides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 7, and are pulled out into their own row only to keep the table a manageable width.
Physical properties
Thorium melts at 1,750 °C (2,023.15 K / 3,182 °F) and boils at 4,788 °C (5,061.15 K / 8,650.4 °F), giving it a liquid range of about 3,038 °C at standard pressure.
Its density is 11.72 g/cm³.
See also: the interactive periodic table, how to read the periodic table, periodic trends.