Curium Electron Configuration

Reference for curium's electron configuration: [Rn] 5f⁷ 6d¹ 7s² (96 electrons). Includes orbital filling table, shell breakdown, and why Cm promotes one electron to 6d.

Z = 96 Cm Curium

Curium — Electron Configuration

Atomic number 96 · Actinide series · Period 7 · f-block (anomalous)

[Rn] 5f⁷ 6d¹ 7s² 96 electrons 10 outer e⁻ Anomalous 5f⁷ 6d¹

Why 5f⁷ 6d¹ and Not 5f⁸?

Strict Aufbau order predicts the 96th electron would enter 5f, giving [Rn] 5f⁸ 7s². In practice, a half-filled 5f⁷ arrangement — one electron in each of the seven 5f orbitals — is especially stable because all electrons have parallel spins, maximizing exchange energy and minimizing electron repulsion. Placing an 8th electron into a 5f orbital would pair spins and break this stability. The electron promotes to 6d instead, giving [Rn] 5f⁷ 6d¹ 7s². This mirrors gadolinium ([Xe] 4f⁷ 5d¹ 6s²) directly above in period 6.

Noble-Gas Shorthand

[Rn] 5f⁷ 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 5f⁷ 6d¹ 7s²

All 96 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
5f f orbitals, shell n=5 7 14 5f⁷
6d d orbitals, shell n=6 1 10 6d¹
7s s orbital, shell n=7 2 2 7s²
Total 96

Valence subshells (5f, 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 5f⁷ 25 / 50 electrons (50%)
Shell 6 (n=6) — 6s² 6p⁶ 6d¹ 9 / 32 electrons (28%)
Shell 7 (n=7) — 7s² 2 / 32 electrons (6%)

Summary

Reference for curium's electron configuration: [Rn] 5f⁷ 6d¹ 7s² (96 electrons). Includes orbital filling table, shell breakdown, and why Cm promotes one electron to 6d.

How it works

  1. Curium's 96 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, strict Aufbau predicts filling 5f then 7s, giving [Rn] 5f⁸ 7s².
  4. Instead, a half-filled 5f⁷ subshell (all 7 orbitals singly occupied) is unusually stable due to exchange energy.
  5. One electron promotes from 5f to 6d, giving the ground-state configuration [Rn] 5f⁷ 6d¹ 7s².
  6. This is analogous to gadolinium ([Xe] 4f⁷ 5d¹ 6s²) one period above — both gain stability via the half-filled f⁷ arrangement.

Use cases

  • Chemistry reference for students studying actinide or heavy-element configurations.
  • Compare curium's anomalous 5f⁷ 6d¹ filling with expected 5f⁸ filling.
  • Understand the exchange-energy stabilization of half-filled f subshells.
  • Verify quantum numbers and subshell counts for exam preparation.
  • Teaching aid for periodic trends, electron shielding, and f-block anomalies.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu