What is the electron configuration of iron?

Iron (Fe, Z=26) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Its 26 electrons fill the argon core and then distribute 8 more across the 3d and 4s subshells.

What is the noble-gas shorthand for iron?

The abbreviated (noble-gas) notation is [Ar] 3d⁶ 4s², where [Ar] represents the filled argon core 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons).

How many valence electrons does iron have?

Iron has 8 valence electrons: six in 3d and two in 4s. These are the electrons available for bonding and oxidation-state changes.

Why does iron form +2 and +3 ions?

Fe²⁺ (ferrous) loses both 4s electrons first, giving [Ar] 3d⁶. Fe³⁺ (ferric) also loses one 3d electron, giving [Ar] 3d⁵ — the half-filled d subshell adds extra stability via exchange energy.

How many unpaired electrons does iron have?

Iron has four unpaired electrons in its 3d subshell. Hund's rule fills the five 3d orbitals with one paired orbital and four singly-occupied orbitals, so 4 remain unpaired.

Why is iron magnetic?

Iron is ferromagnetic due to its four unpaired 3d electrons. In metallic iron, these unpaired spins align collectively in magnetic domains, producing a strong permanent magnetic moment.

Does 4s fill before 3d in iron?

Yes. In the free atom, the 4s orbital has lower energy than 3d and fills first by the Aufbau principle. However, when electrons are removed (ionization), 4s electrons are lost before 3d electrons because the energy ordering reverses in the positive ion.

Iron Electron Configuration

Name: Iron Electron Configuration
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Author: Nham Vu

Reference tool for iron's electron configuration (1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²), orbital box diagram, quantum numbers, and key atomic properties.

Z = 26 Fe Iron

Iron — Electron Configuration

Atomic number 26 · Transition metal · Period 4, Group 8 · d-block

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² [Ar] 3d⁶ 4s² 26 electrons 8 valence e⁻

Subshell Breakdown

Subshell	Type	Electrons	Max Capacity	Notation
1s	s orbital, shell n=1	2	2	1s²
2s	s orbital, shell n=2	2	2	2s²
2p	p orbitals, shell n=2	6	6	2p⁶
3s	s orbital, shell n=3	2	2	3s²
3p	p orbitals, shell n=3	6	6	3p⁶
4s	s orbital, shell n=4 (fills before 3d)	2	2	4s²
3d	d orbitals, shell n=3 (fills after 4s)	6	10	3d⁶
Total		26

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²

All subshells written explicitly; 26 electrons total.

Noble-Gas Shorthand

[Ar] 3d⁶ 4s²

[Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶ (argon core, 18 electrons).

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 3d⁶ 14 / 18 electrons (78%)

Shell 4 (n=4) — 4s² 2 / 32 electrons (6%)

Shell 3 can hold up to 18 electrons (3s + 3p + 3d). Iron uses 14 of those 18 slots. Shell 4 can hold 32 electrons; iron has only the 4s² pair so far.

The Argon Core — [Ar]

The first 18 electrons of iron match the complete electron configuration of argon (Z=18):

[Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶

The remaining 8 electrons — the valence configuration 3d⁶ 4s² — govern iron's chemistry, oxidation states, and magnetic properties.

Orbital Box Diagram

2 electrons (paired) — filled

6 electrons across 3 orbitals — filled

2 electrons (paired) — filled

6 electrons across 3 orbitals — filled (part of [Ar] core)

2 electrons (paired) — fills before 3d in free atom

6 electrons: 1 paired orbital + 4 unpaired (Hund's rule) — 4 unpaired spins

Each box = one orbital. Opposing arrows = paired electrons (+½ and −½ spin). Single arrow = unpaired electron (spin up by convention).

The 3d⁶ configuration has one doubly-occupied orbital plus four singly-occupied orbitals (Hund's rule maximizes total spin). This gives iron 4 unpaired electrons and makes it strongly paramagnetic.

Quantum Numbers for All 26 Electrons

Electron #	n	l	mₗ	mₛ	Subshell
1	1	0	0	+½	1s
2	1	0	0	−½	1s
3	2	0	0	+½	2s
4	2	0	0	−½	2s
5	2	1	-1	+½	2p
6	2	1	-1	−½	2p
7	2	1	0	+½	2p
8	2	1	0	−½	2p
9	2	1	1	+½	2p
10	2	1	1	−½	2p
11	3	0	0	+½	3s
12	3	0	0	−½	3s
13	3	1	-1	+½	3p
14	3	1	-1	−½	3p
15	3	1	0	+½	3p
16	3	1	0	−½	3p
17	3	1	1	+½	3p
18	3	1	1	−½	3p
19	4	0	0	+½	4s
20	4	0	0	−½	4s
21	3	2	-2	+½	3d
22	3	2	-2	−½	3d
23	3	2	-1	+½	3d
24	3	2	0	+½	3d
25	3	2	1	+½	3d
26	3	2	2	+½	3d

n = principal quantum number; l = azimuthal (0→s, 1→p, 2→d); mₗ = magnetic; mₛ = spin. Electrons 21–22 are the paired 3d orbital; electrons 23–26 are the four unpaired 3d electrons (all spin +½ per Hund's rule).

Summary