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

Name: Chromium Electron Configuration
Availability: InStock
Author: Nham Vu

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

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: 2026-07-08 · Reviewed by Nham Vu