Actinium Electron Configuration

Reference for actinium's electron configuration: [Rn] 6d¹ 7s² (full: 1s²…6p⁶ 6d¹ 7s²). Includes orbital filling table, shell breakdown, and why Ac prefers 6d over 5f.

Z = 89 Ac Actinium

Actinium — Electron Configuration

Atomic number 89 · Actinide series · Period 7 · d-block (anomalous)

[Rn] 6d¹ 7s² 89 electrons 3 valence e⁻ Anomalous 6d¹

Why 6d¹ and Not 5f¹?

Strict Aufbau order predicts the 89th electron would enter 5f, giving [Rn] 5f¹ 7s². In practice, at Z=89 the 6d sublevel sits slightly lower in energy than 5f — relativistic contraction of inner shells shifts the 6d-5f crossover point. Actinium therefore adopts [Rn] 6d¹ 7s², analogous to lanthanum ([Xe] 5d¹ 6s²) in the preceding period. Thorium (Z=90) onward begins filling 5f in earnest, making Ac the conventional start of the actinide row even though its own differentiating electron is in 6d.

Noble-Gas Shorthand

[Rn] 6d¹ 7s²

[Rn] = 86-electron radon core.

Full Expanded Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²

All 89 electrons written by subshell.

Subshell Breakdown

Subshell Type Electrons Capacity Notation
1s s orbital, shell n=1 2 2 1s²
2s s orbital, shell n=2 2 2 2s²
2p p orbitals, shell n=2 6 6 2p⁶
3s s orbital, shell n=3 2 2 3s²
3p p orbitals, shell n=3 6 6 3p⁶
3d d orbitals, shell n=3 10 10 3d¹⁰
4s s orbital, shell n=4 2 2 4s²
4p p orbitals, shell n=4 6 6 4p⁶
4d d orbitals, shell n=4 10 10 4d¹⁰
4f f orbitals, shell n=4 14 14 4f¹⁴
5s s orbital, shell n=5 2 2 5s²
5p p orbitals, shell n=5 6 6 5p⁶
5d d orbitals, shell n=5 10 10 5d¹⁰
6s s orbital, shell n=6 2 2 6s²
6p p orbitals, shell n=6 6 6 6p⁶
6d d orbitals, shell n=6 1 10 6d¹
7s s orbital, shell n=7 2 2 7s²
Total 89

Valence subshells (6d and 7s) are highlighted. All others form the [Rn] core.

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)
Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)
Shell 3 (n=3) — 3s² 3p⁶ 3d¹⁰ 18 / 18 electrons (100%)
Shell 4 (n=4) — 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 32 / 32 electrons (100%)
Shell 5 (n=5) — 5s² 5p⁶ 5d¹⁰ 18 / 32 electrons (56%)
Shell 6 (n=6) — 6s² 6p⁶ 6d¹ 9 / 32 electrons (28%)
Shell 7 (n=7) — 7s² 2 / 32 electrons (6%)

Summary

Reference for actinium's electron configuration: [Rn] 6d¹ 7s² (full: 1s²…6p⁶ 6d¹ 7s²). Includes orbital filling table, shell breakdown, and why Ac prefers 6d over 5f.

How it works

  1. Actinium's 89 electrons fill orbitals in order of increasing energy following the Aufbau principle.
  2. Radon ([Rn], Z=86) provides the filled core: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶.
  3. After the [Rn] core, the expected filling would be 5f¹, but at Z=89 the 6d sublevel is slightly lower in energy than 5f.
  4. The 89th electron therefore occupies 6d¹, giving Ac the ground-state configuration [Rn] 6d¹ 7s².
  5. This anomaly is similar to lanthanum ([Xe] 5d¹ 6s²) one period above — actinium is the d-block precursor to the actinide series.
  6. Valence electrons are 6d¹ 7s², giving Ac a +3 common oxidation state (losing all three outer electrons).

Use cases

  • Chemistry reference for students studying actinide or heavy-element configurations.
  • Compare actinium's anomalous 6d filling with the expected 5f filling pattern.
  • Understand why actinium is classified as a d-block element despite heading the actinide series.
  • Verify quantum numbers and subshell counts for exam preparation.
  • Teaching aid for periodic trends, electron shielding, and energy-level crossings.

Frequently Asked Questions

Last updated: 2026-07-08 · Reviewed by Nham Vu