Actinium (Ac): Electron Configuration, Properties and Uses
Actinium (Ac): Electron Configuration, Properties and Uses
Electron configuration [Rn] 7s2 6d1 · f-block, actinide series · period 7 · Ar [227]
Actinium data
- Atomic number
- 89
- Relative atomic mass
- [227]
- Group
- – (actinide series)
- Period
- 7
- Block
- f
- Category
- Actinide
- Electron configuration
- [Rn] 7s2 6d1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1
- Electrons per shell
- 2, 8, 18, 32, 18, 9, 2 (K L M N O P Q)
- Valence electrons
- 2 (in the outer shell)
- Electronegativity (Pauling)
- 1.1
- First ionisation energy
- 5.17 eV
- Atomic radius (van der Waals)
- 260 pm
- State at room temperature
- Solid
- Melting point
- 1051 °C
- Boiling point
- 3198 °C
- Density
- 10.07 g/cm³
- Common oxidation states
- +3
- Discovered
- 1899
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 9 · Q 2
Orbital diagram (outer electrons)
About actinium
Actinium, symbol Ac, is element number 89 in the periodic table. It belongs to the actinide series in period 7 (f-block), usually shown below the main table. The element was discovered in 1899. Actinium is a solid at room temperature, melting at 1051 °C and boiling at 3198 °C.
Actinium electron configuration
The electron configuration of actinium is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1, or in short [Rn] 7s2 6d1. Its electrons occupy 7 shells: K = 2, L = 8, M = 18, N = 32, O = 18, P = 9, Q = 2. As an actinide, actinium 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 5f1, 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: Ac3+ = [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) → 6d1(89)
Uses of actinium
Well-known applications of actinium include targeted alpha therapy with Ac-225 and actinium-beryllium neutron sources.
Safety
Actinium 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 actinium?
The electron configuration of actinium (Ac, atomic number 89) is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1, or [Rn] 7s2 6d1 in short. By shell: 2, 8, 18, 32, 18, 9, 2.
How many valence electrons does actinium have?
Actinium 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 actinium in?
Actinium is in the actinide series, period 7, f-block.
What is the atomic mass of actinium?
Actinium has no stable isotope, so the mass number of its longest-lived isotope, [227], is given.
Prepare solutions with analytical-grade reagents
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