Full configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
In the short form, [Ar] stands for the 18 electrons of argon, the noble gas before chromium. 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.
Why it's an exception
Chromium is an exception. Instead of 3d⁴ 4s², one 4s electron moves to 3d to make a half-filled 3d⁵, which is extra stable because of exchange energy.
Configuration of chromium ions
| Ion | Configuration | Unpaired | Moment (BM) |
|---|---|---|---|
| Cr²⁺ | [Ar] 3d⁴ | 4 | 4.90 |
| Cr³⁺ | [Ar] 3d³ | 3 | 3.87 |
| Cr⁶⁺ | [Ar] | 0 | 0.00 |
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 |
|---|---|---|
| 21 | Scandium | [Ar] 3d¹ 4s² |
| 22 | Titanium | [Ar] 3d² 4s² |
| 23 | Vanadium | [Ar] 3d³ 4s² |
| 25 | Manganese | [Ar] 3d⁵ 4s² |
| 26 | Iron | [Ar] 3d⁶ 4s² |
| 27 | Cobalt | [Ar] 3d⁷ 4s² |
Questions students ask
What is the electronic configuration of chromium?
[Ar] 3d⁵ 4s¹, or in full, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹. Chromium has atomic number 24.
How many electrons are in each shell of chromium?
2, 8, 13, 1 (K, L, M, N… shells in order).
How many unpaired electrons does chromium have?
6 in the ground state, so chromium atoms are paramagnetic.
What is the electronic configuration of Cr²⁺?
[Ar] 3d⁴, with 4 unpaired electrons.
Why is chromium an exception?
Chromium is an exception. Instead of 3d⁴ 4s², one 4s electron moves to 3d to make a half-filled 3d⁵, which is extra stable because of exchange energy.