Thorium (Th): Electron Configuration, Properties and Uses
Thorium (Th): Electron Configuration, Properties and Uses
Electron configuration [Rn] 7s2 6d2 · f-block, actinide series · period 7 · Ar 232.04
Thorium data
- Atomic number
- 90
- Relative atomic mass
- 232.04
- Group
- – (actinide series)
- Period
- 7
- Block
- f
- Category
- Actinide
- Electron configuration
- [Rn] 7s2 6d2
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d2
- Electrons per shell
- 2, 8, 18, 32, 18, 10, 2 (K L M N O P Q)
- Valence electrons
- 2 (in the outer shell)
- Electronegativity (Pauling)
- 1.3
- First ionisation energy
- 6.08 eV
- Atomic radius (van der Waals)
- 237 pm
- State at room temperature
- Solid
- Melting point
- 1750 °C
- Boiling point
- 4788 °C
- Density
- 11.72 g/cm³
- Common oxidation states
- +4
- Discovered
- 1828
Source: PubChem (NIH, accessed 13 Nov 2026); atomic mass: IUPAC standard atomic weight.
Electron shell diagram
K 2 · L 8 · M 18 · N 32 · O 18 · P 10 · Q 2
Orbital diagram (outer electrons)
About thorium
Thorium is a chemical element with the symbol Th and atomic number 90. It belongs to the actinide series in period 7 (f-block), usually shown below the main table. Thorium was first discovered in 1828. Thorium is a solid at room temperature, melting at 1750 °C and boiling at 4788 °C.
Thorium electron configuration
The electron configuration of thorium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d2, or in short [Rn] 7s2 6d2. Its electrons occupy 7 shells: K = 2, L = 8, M = 18, N = 32, O = 18, P = 10, Q = 2. As an actinide, thorium fills the 5f subshell. Its outer 7s subshell holds 2 electrons; valence electrons for this series are not assigned by the usual group rule. This is an exception to the Aufbau rule: the usual filling order predicts [Rn] 7s2 5f2, but the 5f and 6d subshells are so close in energy that electrons occupy 6d. When it forms cations, electrons leave the outermost subshell first: Th4+ = [Rn] (like radon).
Filling order: 1s2(2) → 2s2(4) → 2p6(10) → 3s2(12) → 3p6(18) → 4s2(20) → 3d10(30) → 4p6(36) → 5s2(38) → 4d10(48) → 5p6(54) → 6s2(56) → 4f14(70) → 5d10(80) → 6p6(86) → 7s2(88) → 6d2(90)
Uses of thorium
The main uses of thorium include thoriated tungsten welding electrodes and a candidate fuel for thorium reactors; formerly gas lamp mantles.
Safety
Thorium is radioactive. Using, storing or transporting radioactive material requires a licence from the national regulator (BAPETEN in Indonesia). Always read the safety data sheet (SDS) before working with a chemical.
Other actinides
Frequently asked questions
What is the electron configuration of thorium?
The electron configuration of thorium (Th, atomic number 90) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d2, or [Rn] 7s2 6d2 in short. By shell: 2, 8, 18, 32, 18, 10, 2.
How many valence electrons does thorium have?
Thorium has 2 electrons in its outer shell. For lanthanides and actinides, valence electrons are not assigned by the usual group rule.
Which group and period is thorium in?
Thorium is in the actinide series, period 7, f-block.
What is the atomic mass of thorium?
The relative atomic mass of thorium is 232.04. Use it to calculate molar mass with the molar mass calculator.
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