ElementsWhat is thorium?
Thorium is a weakly radioactive, silvery metal. It is about three times more common than uranium in the Earth's crust, and India has large reserves in its beach sands.
What is thorium used for?
A possible fuel for nuclear reactors. In the past it was used in gas lamp mantles and some welding rods.
Where the name comes from
From Thor, the Norse god of thunder.
Key facts
- Symbol
- Th
- Atomic number
- 90
- Atomic mass
- 232.038 u
- Family
- Actinide
- Period
- 7
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 1,750 °C (2,023 K, 3,182 °F)
- Boiling point
- 4,788 °C (5,061 K, 8,650 °F)
- Density
- 11.72 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 18, 10, 2
- Electron configuration
- [Rn]7s2 6d2
- Common oxidation states
- +4
- Electronegativity (Pauling)
- 1.3
- First ionisation energy
- 6.08 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 237 pm
- Discovered
- 1828
Place in the periodic table
Thorium is element number 90. It sits in period 7 (row 7) of the periodic table, in the separate row of actinides drawn below the main table, and belongs to the f-block. Just before it comes actinium (89), and just after it comes protactinium (91).
Family: Actinides.
Physical properties
At room temperature thorium is a solid. It melts at 1,750 °C and boils at 4,788 °C, so it stays liquid over a range of 3,038 degrees. Its density is 11.72 grams per cubic centimetre, so a one-litre block would weigh about 11.72 kilograms. By density it ranks 21st among the 96 elements with a measured value.
Inside the atom
An atom of thorium has 90 protons in its nucleus and, when neutral, 90 electrons around it. The electrons are arranged in 7 shells: 2, 8, 18, 32, 18, 10, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Rn]7s2 6d2.
Its electronegativity on the Pauling scale is 1.3. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 6.08 electronvolts (about 587 kilojoules per mole). One mole of thorium weighs 232.038 grams, so a single gram contains about 2.60 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains thorium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.