Electron Configuration of Iron (Fe, Z = 26)

Given

• Iron: atomic number Z = 26, so 26 electrons in the neutral atom

• Aufbau (building-up) ordering of subshells

• Pauli exclusion: at most 2 electrons per orbital; Hund's rule: spread out in degenerate orbitals before pairing

Find

The ground-state electron configuration of Fe (Z = 26).

Principle

Electrons fill subshells from lowest to highest energy (the Aufbau order), with at most two electrons per orbital and degenerate orbitals filled singly before pairing. Because the 4s orbital sits slightly lower in energy than 3d, the 4s subshell fills first even though its principal quantum number is larger.

Formula

Solution

1. Neutral Fe has 26 electrons. Fill in Aufbau order: 1s takes 2, 2s takes 2, 2p takes 6, 3s takes 2, 3p takes 6. That accounts for 18 electrons, reaching the argon core.

2. Remaining electrons: 26 - 18 = 8. The next subshell is 4s, which fills before 3d, so 4s takes 2.

3. Remaining: 8 - 2 = 6 electrons go into 3d: 3d6. All 26 electrons are placed.

4. Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d6. Check: 2+2+6+2+6+2+6 = 26 electrons.

5. Noble-gas shorthand: the first 18 electrons equal [Ar], so Fe = [Ar] 4s2 3d6.

Unit check

The configuration is a bookkeeping check rather than a calculation: every electron in the neutral atom must appear exactly once, and the superscripts must sum to the atomic number, 26.

What this means physically

The 4s2 3d6 valence pattern explains iron's chemistry: it can lose its two 4s electrons to form Fe2+, and additionally one 3d electron to form Fe3+ — the two oxidation states that make iron central to metallurgy, rust, and hemoglobin.