Full configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²
In the short form, [Ar] stands for the 18 electrons of argon, the noble gas before cobalt. Only the electrons after it are written out.
Orbital diagram
Outer subshells only. Each box is an orbital; electrons fill singly first (Hund's rule), then pair up with opposite spins.
What the configuration tells you
Cobalt is in group 9, period 4, in the d-block. It has 3 unpaired electrons in the ground state, so the atom is paramagnetic.
Configuration of cobalt ions
| Ion | Configuration | Unpaired | Moment (BM) |
|---|---|---|---|
| Co²⁺ | [Ar] 3d⁷ | 3 | 3.87 |
| Co³⁺ | [Ar] 3d⁶ | 4 | 4.90 |
Moment is the spin-only magnetic moment μ = √(n(n + 2)) Bohr magnetons, where n is the number of unpaired electrons. Positive ions lose electrons from the outermost shell first, so transition metals lose their 4s or 5s electrons before d.
Nearby elements
| Z | Element | Configuration |
|---|---|---|
| 24 | Chromium | [Ar] 3d⁵ 4s¹ |
| 25 | Manganese | [Ar] 3d⁵ 4s² |
| 26 | Iron | [Ar] 3d⁶ 4s² |
| 28 | Nickel | [Ar] 3d⁸ 4s² |
| 29 | Copper | [Ar] 3d¹⁰ 4s¹ |
| 30 | Zinc | [Ar] 3d¹⁰ 4s² |
Questions students ask
What is the electronic configuration of cobalt?
[Ar] 3d⁷ 4s², or in full, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s². Cobalt has atomic number 27.
How many electrons are in each shell of cobalt?
2, 8, 15, 2 (K, L, M, N… shells in order).
How many unpaired electrons does cobalt have?
3 in the ground state, so cobalt atoms are paramagnetic.
What is the electronic configuration of Co²⁺?
[Ar] 3d⁷, with 3 unpaired electrons.