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.

Use the Actinium Electron Configuration

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

Actinium (Ac) has atomic number 89 and the ground-state electron configuration [Rn] 6d¹ 7s², not the [Rn] 5f¹ 7s² that the Aufbau principle predicts. At Z=89, the 6d orbital sits slightly lower in energy than 5f, so the single outer electron drops into 6d. This page shows the full expanded configuration, a shell-by-shell orbital filling table, and key element facts.

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: 2026-09-30 · Reviewed by Nham Vu