Electron Configuration of Iron (Fe) — Step by Step
Question
Write the complete electron configuration of iron (Fe, Z = 26). Also write the abbreviated (noble gas) form and show the 3d and 4s electrons clearly.
Solution — Step by Step
Iron has atomic number 26, meaning it has 26 electrons to distribute across orbitals.
We fill orbitals following three rules:
- Aufbau Principle: Fill orbitals in order of increasing energy
- Pauli Exclusion Principle: Each orbital holds at most 2 electrons with opposite spins
- Hund's Rule: Within a subshell, place one electron in each orbital before pairing
Step 1: Know the Aufbau Filling Order
The correct energy order for filling: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → …
💡 Expert Tip
The 4s orbital fills before 3d because it has slightly lower energy in the empty atom. This is why Fe's configuration has 4s² before 3d⁶, not the other way around.
Step 2: Fill All 26 Electrons
| Subshell | Electrons | Running total |
|---|---|---|
| 1s | 2 | 2 |
| 2s | 2 | 4 |
| 2p | 6 | 10 |
| 3s | 2 | 12 |
| 3p | 6 | 18 |
| 4s | 2 | 20 |
| 3d | 6 | 26 ✓ |
Step 3: Write the Full Configuration
Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
Step 4: Write the Abbreviated (Noble Gas) Form
The nearest noble gas before Fe (Z = 26) is Argon (Ar, Z = 18): 1s² 2s² 2p⁶ 3s² 3p⁶
We replace the Ar core with [Ar] and write only the outer electrons:
Fe: [Ar] 3d⁶ 4s²
📌 Note
In the abbreviated form, it's conventional to write the d subshell before 4s when using the noble gas core: [Ar] 3d⁶ 4s². NCERT writes it as [Ar] 4s² 3d⁶ in some places. Both are accepted — the electrons are the same either way.
Why This Works
The Aufbau principle is based on orbital energies. The (n+l) rule states that the orbital with lower (n + l) value fills first. When (n + l) values are equal, the orbital with lower n fills first.
For 3d: n = 3, l = 2, so n + l = 5 For 4s: n = 4, l = 0, so n + l = 4
Since 4s has a lower (n + l) value, it fills before 3d. That's why iron has 4s² filled before 3d⁶.
The 3d⁶ configuration places 6 electrons in 5 d-orbitals. By Hund's Rule:
- 5 d-orbitals, first 5 electrons go one per orbital (all ↑)
- The 6th electron must pair with one of the existing electrons
So Fe has 4 unpaired electrons in 3d, making it paramagnetic.
Alternative Method — Using the Periodic Table
Iron is in Period 4, Group 8 of the d-block. The d-block starts at Group 3 (Sc), so:
- Sc is the 1st d-block element (3d¹ 4s²)
- Fe is the 6th d-block element → 3d⁶ 4s²
Count across: Sc(3d¹), Ti(3d²), V(3d³), Cr(3d⁴*), Mn(3d⁵), Fe(3d⁶) → confirmed.
(*Cr is an exception: 3d⁵ 4s¹ instead of expected 3d⁴ 4s² — half-filled d subshell stability)
Common Mistake
⚠️ Common Mistake
Mistake: Writing iron's configuration as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ — forgetting to fill 4s before 3d.
Why it's wrong: 4s has lower energy than 3d in an empty atom. The 4s² must be filled before the 3d subshell is populated. The correct configuration ends with 4s² 3d⁶, not 3d⁸.
Another common error: Writing Fe²⁺ configuration as [Ar] 3d⁴ 4s² — this is wrong. When Fe is ionised, 4s electrons are removed first (they become higher in energy than 3d once the atom becomes a cation). Fe²⁺ = [Ar] 3d⁶; Fe³⁺ = [Ar] 3d⁵.
Quick Reference
| Species | Configuration | Unpaired electrons |
|---|---|---|
| Fe (Z=26) | [Ar] 3d⁶ 4s² | 4 |
| Fe²⁺ | [Ar] 3d⁶ | 4 |
| Fe³⁺ | [Ar] 3d⁵ | 5 |