Full configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
In the short form, [Ar] stands for the 18 electrons of argon, the noble gas before iron. 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
Iron loses its two 4s electrons first to form Fe²⁺ (3d⁶), then one 3d electron to form Fe³⁺ (3d⁵). Fe³⁺ is more stable because 3d⁵ is half-filled.
Configuration of iron ions
| Ion | Configuration | Unpaired | Moment (BM) |
|---|---|---|---|
| Fe²⁺ | [Ar] 3d⁶ | 4 | 4.90 |
| Fe³⁺ | [Ar] 3d⁵ | 5 | 5.92 |
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 |
|---|---|---|
| 23 | Vanadium | [Ar] 3d³ 4s² |
| 24 | Chromium | [Ar] 3d⁵ 4s¹ |
| 25 | Manganese | [Ar] 3d⁵ 4s² |
| 27 | Cobalt | [Ar] 3d⁷ 4s² |
| 28 | Nickel | [Ar] 3d⁸ 4s² |
| 29 | Copper | [Ar] 3d¹⁰ 4s¹ |
Questions students ask
What is the electronic configuration of iron?
[Ar] 3d⁶ 4s², or in full, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Iron has atomic number 26.
How many electrons are in each shell of iron?
2, 8, 14, 2 (K, L, M, N… shells in order).
How many unpaired electrons does iron have?
4 in the ground state, so iron atoms are paramagnetic.
What is the electronic configuration of Fe²⁺?
[Ar] 3d⁶, with 4 unpaired electrons.