What is the electron configuration of chromium?

Chromium (Cr, Z=24) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹, or in noble-gas shorthand [Ar] 3d⁵ 4s¹. This is anomalous — the Aufbau principle would predict [Ar] 3d⁴ 4s².

Why is chromium's electron configuration anomalous?

A half-filled 3d subshell (3d⁵) is especially stable because each orbital holds exactly one electron, minimizing electron-electron repulsion and maximizing exchange energy. This stability gain is large enough that one 4s electron shifts into 3d, overriding the normal Aufbau filling order.

What is the noble-gas shorthand for chromium?

The shorthand is [Ar] 3d⁵ 4s¹, where [Ar] represents the filled argon core (1s² 2s² 2p⁶ 3s² 3p⁶, Z=18).

What are chromium's common oxidation states?

Chromium most commonly exhibits +2 (Cr²⁺, chromous), +3 (Cr³⁺, chromic — the most stable), and +6 (Cr⁶⁺, as in chromate CrO₄²⁻ and dichromate Cr₂O₇²⁻). The +3 state is dominant in most chromium chemistry.

How many valence electrons does chromium have?

Chromium has 6 valence electrons — one in 4s and five in 3d. All six can participate in bonding, which explains the wide range of oxidation states from +1 to +6.

Which other elements have anomalous electron configurations like chromium?

The most common examples are copper (Z=29, [Ar] 3d¹⁰ 4s¹ instead of [Ar] 3d⁹ 4s²) and molybdenum (Z=42, [Kr] 4d⁵ 5s¹). Both involve stability associated with half-filled or completely filled d subshells.

What block and period is chromium in?

Chromium is in Period 4, Group 6 (VIB), and belongs to the d-block (transition metals) of the periodic table. It is the first element in Group 6.

Chromium Electron Configuration

Chromium (Cr, Z=24) has the anomalous electron configuration [Ar] 3d⁵ 4s¹ — not [Ar] 3d⁴ 4s² as Aufbau predicts. The half-filled 3d subshell provides extra stability, making chromium a classic exception to the Aufbau principle.

Anomalous configuration — Aufbau exception

Chromium does not follow the Aufbau principle. The actual ground state is [Ar] 3d⁵ 4s¹, not the predicted [Ar] 3d⁴ 4s². A half-filled 3d subshell provides extra stability through maximized exchange energy.

Z = 24 Cr Chromium

Chromium — Electron Configuration

Atomic number 24 · Transition metal · Period 4, Group 6 · d-block

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ [Ar] 3d⁵ 4s¹ 24 electrons 6 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
3d	d orbitals, shell n=3 (half-filled)	5	10	3d⁵
4s	s orbital, shell n=4 (valence)	1	2	4s¹
Total		24

Full Configuration (actual)

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

All subshells written explicitly. Total: 24 electrons.

Noble-Gas Shorthand (actual)

[Ar] 3d⁵ 4s¹

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

Aufbau Prediction (incorrect)

[Ar] 3d⁴ 4s²

The Aufbau rule predicts this, but the actual ground state differs.

Common Oxidation States

+2 (Cr²⁺) +3 (Cr³⁺) +6 (Cr⁶⁺)

+3 is the most stable; +6 is strongly oxidizing (chromate, dichromate).

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 3d⁵ 13 / 18 electrons (72%)

Shell 4 (n=4) — 4s¹ 1 / 32 electrons (3%)

Shell 3 can hold up to 18 electrons (3s + 3p + 3d). Chromium places 13 here — 3d holds 5 of its possible 10 (the half-filled state that drives the anomaly).

Orbital Box Diagram

2 electrons (paired)

6 electrons across 3 orbitals (all paired)

2 electrons (paired)

6 electrons across 3 orbitals (argon core ends here)

5 unpaired electrons — half-filled 3d⁵ (stability maximum)

1 electron (unpaired, spin up) — lone 4s¹ valence electron

Each box = one orbital. Opposing arrows = paired electrons (±½ spin). The five 3d orbitals are each singly occupied (spin up), representing the maximum-spin half-filled state. The single 4s¹ electron is the first lost when chromium is oxidized to Cr²⁺ or Cr³⁺.

Quantum Numbers for All 24 Electrons

e⁻ #	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	3	2	-2	+½	3d
20	3	2	-1	+½	3d
21	3	2	0	+½	3d
22	3	2	1	+½	3d
23	3	2	2	+½	3d
24	4	0	0	+½	4s

n = principal quantum number; l = azimuthal (l=0 s, l=1 p, l=2 d); mₗ = magnetic; mₛ = spin. Electrons 19–24 (highlighted) are the valence-region electrons — 3d⁵ all spin-up, 4s¹ single.

Why Chromium Breaks the Aufbau Principle

Source	Configuration	3d electrons	4s electrons
Aufbau prediction	[Ar] 3d⁴ 4s²	4	2
Actual ground state	[Ar] 3d⁵ 4s¹	5 (half-filled)	1

Exchange Energy and Half-Filled Stability

Electrons in the same subshell with parallel spins can exchange positions without violating the Pauli exclusion principle. Each such exchange lowers the energy of the system by a small amount called the exchange energy. The number of possible exchanges scales with the number of electron pairs with parallel spins — a half-filled set of five d orbitals (3d⁵, all spin-up) maximizes these exchanges (10 pairs), yielding the highest possible exchange-energy gain for a partially filled d subshell.

The Energy Trade-off

Normally, the 4s orbital sits slightly lower in energy than 3d for transition-metal atoms. Promoting an electron from 4s to 3d costs energy. For chromium, the exchange-energy gain from achieving 3d⁵ (half-filled) is larger than the cost of that promotion. The net result is a lower-energy state with 3d⁵ 4s¹ rather than 3d⁴ 4s².

Hund's Rule Connection

Hund's first rule states that electrons occupy orbitals singly before pairing, all with the same spin direction, to maximize total spin. The half-filled 3d⁵ state perfectly satisfies this rule across five orbitals: each has one spin-up electron, giving S = 5/2. This maximum-spin configuration is the most stable arrangement within the 3d subshell.

The Same Pattern in Copper

Copper (Z=29) shows a parallel anomaly: the expected [Ar] 3d⁹ 4s² is overridden by [Ar] 3d¹⁰ 4s¹, because a completely filled 3d¹⁰ subshell is also unusually stable. Chromium and copper are the two clearest examples of 4s→3d promotion in Period 4 transition metals.

Chromium Oxidation States

Cr²⁺

+2 state — chromous

Loses the 4s¹ electron and one 3d electron. Configuration: [Ar] 3d⁴. Less stable; oxidized to Cr³⁺ in air. Found in CrO (chromous oxide) and CrCl₂.

Cr³⁺

+3 state — chromic (most stable)

Loses 4s¹ and two 3d electrons. Configuration: [Ar] 3d³. The dominant oxidation state in most chemistry: Cr₂O₃ (green pigment), CrCl₃, chromium alums, chrome tanning agents, and the passivating oxide layer on stainless steel.

Cr⁶⁺

+6 state — strongly oxidizing

All six 3d+4s valence electrons removed. Configuration: [Ar]. Found as chromate (CrO₄²⁻, yellow) and dichromate (Cr₂O₇²⁻, orange). Powerful oxidizing agents used in organic synthesis; highly toxic and carcinogenic — regulated in industrial use.

Summary

Chromium (Cr) has atomic number 24 and a ground-state configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹. This deviates from the Aufbau-predicted [Ar] 3d⁴ 4s² because a half-filled 3d⁵ subshell achieves extra electrostatic stability — one electron from 4s shifts into 3d, giving five singly occupied d orbitals (Hund's rule maximum). This page provides the full orbital diagram, subshell table, quantum numbers for all 24 electrons, and chromium's common oxidation states and physical properties.

How it works

The Aufbau principle normally fills 4s before 3d, predicting [Ar] 3d⁴ 4s² for chromium.
Instead, one 4s electron migrates to 3d, yielding [Ar] 3d⁵ 4s¹ — a half-filled d subshell.
A half-filled subshell (all five 3d orbitals singly occupied) is unusually stable due to reduced electron-electron repulsion and maximized exchange energy.
This exchange-energy gain outweighs the small energy cost of promoting a 4s electron.
The result: 24 electrons arranged as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹.
Copper (Z=29) shows the same pattern — one 4s electron shifts to complete the 3d¹⁰ shell.

Use cases

Quickly verify chromium's actual ground-state configuration vs. the Aufbau prediction.
Understand why chromium is a classic exception to the Aufbau principle.
Study the concept of exchange energy and half-filled subshell stability.
Compare chromium's anomaly with the copper (Z=29) anomaly in the same period.
Review oxidation states (+2, +3, +6) and how the d⁵ configuration relates to chromium's chemistry.
Use the quantum-number table for all 24 electrons in exam preparation.
Teaching aid for introductory inorganic chemistry courses covering electron configuration exceptions.
Explore the d-block and transition metal electronic structure patterns.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu

Chromium Electron Configuration

Chromium — Electron Configuration

Subshell Breakdown

Shell Fill Summary

Orbital Box Diagram

Quantum Numbers for All 24 Electrons

Why Chromium Breaks the Aufbau Principle

Exchange Energy and Half-Filled Stability

The Energy Trade-off

Hund's Rule Connection

The Same Pattern in Copper

Chromium Oxidation States

Identity

Physical Properties

Electronic Properties

Uses and Context

Summary

How it works

Use cases

Frequently Asked Questions

Chromium Electron Configuration

Use the Chromium Electron Configuration

Chromium — Electron Configuration

Subshell Breakdown

Shell Fill Summary

Orbital Box Diagram

Quantum Numbers for All 24 Electrons

Why Chromium Breaks the Aufbau Principle

Exchange Energy and Half-Filled Stability

The Energy Trade-off

Hund's Rule Connection

The Same Pattern in Copper

Chromium Oxidation States

Identity

Physical Properties

Electronic Properties

Uses and Context

Summary

How it works

Use cases

Frequently Asked Questions