What is the most common oxidation state of chromium?

The +3 state is the most thermodynamically stable and most commonly encountered in everyday compounds. Cr³⁺ forms a stable [Ar] 3d³ configuration and is the dominant species in compounds like Cr₂O₃, CrCl₃, and Cr(OH)₃. It is also the form found in biological systems as a trace nutrient.

Why is the +6 oxidation state of chromium so oxidizing?

Cr(VI) has a highly electron-deficient chromium center with no d electrons remaining, making it a strong electron acceptor. Compounds like dichromate (Cr₂O₇²⁻) and chromate (CrO₄²⁻) are strong oxidizing agents in acidic or neutral conditions, being reduced to Cr³⁺ in the process. CrO₃ and its derivatives are used in organic oxidations for exactly this reason.

Why is chromium's electron configuration [Ar] 3d⁵ 4s¹ and not [Ar] 3d⁴ 4s²?

The [Ar] 3d⁵ 4s¹ configuration is anomalous relative to the Aufbau prediction of [Ar] 3d⁴ 4s². It arises because a half-filled d subshell (3d⁵) is particularly stable due to exchange energy — each d electron can exchange with every other same-spin d electron, maximizing electron exchange stabilization. Promoting one electron from 4s to 3d recovers enough exchange energy to overcome the 4s→3d promotion cost.

What is the difference between chromate and dichromate?

Both contain Cr in the +6 state. Chromate (CrO₄²⁻) is the stable form at neutral to basic pH, giving a yellow color. Dichromate (Cr₂O₇²⁻) is the dominant species in acidic solution, giving an orange-red color. The equilibrium 2 CrO₄²⁻ + 2 H⁺ ⇌ Cr₂O₇²⁻ + H₂O shifts with pH.

Is Cr(III) or Cr(VI) more toxic?

Cr(VI) is classified as a carcinogen and is far more toxic than Cr(III). Cr(VI) compounds are easily absorbed through skin and mucous membranes, where they are reduced to Cr(III) intracellularly — this reduction process generates reactive intermediates that damage DNA. Cr(III) at trace levels is considered an essential nutrient. Industrial uses of Cr(VI) (plating, pigments) require strict exposure controls.

What are the negative oxidation states of chromium?

Chromium can exhibit -1 and -2 oxidation states in carbonyl compounds such as [Cr(CO)₅]²⁻ (-2 in chromate anions of organometallics) and related low-valent organometallic species. These are stabilized by strong π-accepting ligands like CO that accept electron density from the metal via back-bonding. These states are rare and confined to organometallic chemistry.

Chromium Oxidation States

Name: Chromium Oxidation States
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Author: Nham Vu

Reference for chromium oxidation states: Cr spans -2 to +6, with +3 most stable and +6 strongly oxidizing. Includes key compounds, anomalous electron configuration, and a formula analyzer.

Atomic # 24 Cr Chromium

Atomic Mass

51.9961 u

Group

6 (VIB)

Period

Block

d-block

Electronegativity

1.66 (Pauling)

Oxidation States

-2 to +6

Chromium exhibits oxidation states from -2 to +6, spanning the full range possible for a Group 6 d-block metal. The +3 state is the most stable under ordinary conditions, giving the stable d³ configuration. The +6 state is a strong oxidizing agent (chromate, dichromate, CrO₃). Positive states from +1 to +5 and negative states (-1, -2) exist mainly in specific coordination or organometallic contexts.

State	d-count	Stability	Context & Examples
-2	d⁸	Rare	Organometallic carbonylates; [Cr(CO)₄]²⁻ anion. Strong π-acceptor ligands (CO) stabilize via back-bonding.
-1	d⁷	Rare	Low-valent carbonyl anions; [Cr(CO)₅]⁻. Confined to organometallic chemistry under inert conditions.
0	d⁶	Elemental only	Pure chromium metal and zero-valent carbonyls like Cr(CO)₆. Lustrous, hard metal; Cr(CO)₆ is a volatile solid used in CVD.
+1	d⁵	Rare	Known in some organometallic and cyclopentadienyl complexes. Not found in simple ionic chemistry.
+2	d⁴	Moderate (reducing)	CrCl₂, CrO, CrSO₄·7H₂O. Cr²⁺ is a reducing agent — readily oxidized to Cr³⁺ in air. Pale blue solution.
+3	d³	Most stable	Cr₂O₃ (green), CrCl₃ (violet/dark green), Cr(OH)₃, chrome alum KCr(SO₄)₂·12H₂O. Kinetically inert; half-filled t₂g set.
+4	d²	Uncommon	CrO₂ (magnetic recording tape), CrF₄. Intermediate oxidation, typically disproportionates or requires stabilizing ligands.
+5	d¹	Rare	CrF₅, some peroxo complexes. Unstable, typically a transient intermediate in Cr(VI)→Cr(III) reductions.
+6	d⁰	Strongly oxidizing	CrO₃, CrO₄²⁻ (chromate, yellow), Cr₂O₇²⁻ (dichromate, orange), CrO₂Cl₂ (chromyl chloride). Potent oxidizer; carcinogenic.

Why +3 is most stable: Cr³⁺ has a d³ configuration with three electrons occupying the lower-energy t₂g orbitals in an octahedral field. This half-filled t₂g set is kinetically inert — ligand exchange is slow, making Cr³⁺ complexes exceptionally stable. Thermodynamically, the +3 state sits in a stability well between the reducing +2 and the strongly oxidizing +6.

Chromium compounds span a wide range of colors and reactivities reflecting the different d-electron counts at each oxidation state. The color of Cr³⁺ complexes in particular (violet, green, dark purple) depends on the ligand field splitting of the d³ configuration.

Compound	Formula	Cr State	Notes
Chromium(III) Oxide	Cr₂O₃	+3	Green powder; the most stable Cr oxide. Used as a green pigment, abrasive, and in chromite refining. Amphoteric.
Chromium(III) Chloride	CrCl₃	+3	Violet or dark green solid depending on hydration. Used in dyeing, organic catalysis. Kinetically inert in solution.
Chromium(III) Hydroxide	Cr(OH)₃	+3	Gray-green gelatinous precipitate. Amphoteric — dissolves in both acid and excess base (forming [Cr(OH)₄]⁻).
Chromite	FeCr₂O₄	+3	Primary Cr ore (spinel structure). Fe²⁺ and Cr³⁺ in an inverse spinel. Mined for ferrochrome production.
Chrome Alum	KCr(SO₄)₂·12H₂O	+3	Violet crystals; a common lab salt. Used in leather tanning, mordant dyeing, and as a Cr³⁺ source.
Chromium(II) Chloride	CrCl₂	+2	White solid; strong reducing agent. Readily oxidized to Cr³⁺ in air. Used in organometallic synthesis.
Chromium Hexacarbonyl	Cr(CO)₆	0	Volatile colorless solid; Cr(0) stabilized by six CO π-acceptor ligands. CVD precursor and organometallic reagent.
Chromium Trioxide	CrO₃	+6	Dark red crystalline solid; highly oxidizing and corrosive. Used in chromic acid oxidations; Cr(VI) carcinogen.
Potassium Chromate	K₂CrO₄	+6	Yellow crystals; stable in neutral/basic solution. Used in analytical chemistry, dyeing, and corrosion inhibition.
Potassium Dichromate	K₂Cr₂O₇	+6	Orange-red crystals; strong oxidizer in acidic solution. Widely used as a lab oxidizing agent and in titrations.
Chromyl Chloride	CrO₂Cl₂	+6	Dark red volatile liquid; reacts violently with water. Used in Etard oxidation of methylarenes.
Chromium Dioxide	CrO₂	+4	Black magnetic solid; used as the active layer in magnetic recording tape (now largely replaced by metal particles).

Ground State Configuration

Core

Core (completes [Ar])

Half-filled d — anomalous stability

Only 1 electron — anomalous (expected 4s²)

Notation

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

Noble gas: [Ar] 3d⁵ 4s¹

Expected: [Ar] 3d⁴ 4s²

Cr³⁺: [Ar] 3d³

Cr⁶⁺: [Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶

Ionization Energies

[Ar] 3d⁵ 4s¹

Neutral — anomalous half-filled d⁵

Cr⁺

[Ar] 3d⁵

IE₁ = 652.9 kJ/mol — remove 4s¹

Cr²⁺

[Ar] 3d⁴

IE₂ = 1591 kJ/mol — remove one 3d

Cr³⁺

[Ar] 3d³

IE₃ = 2987 kJ/mol — half-filled t₂g (most stable)

Cr⁶⁺

[Ar] 3d⁰

Total IEs 1–6 — all d and 4s electrons removed

The anomaly explained: Chromium prefers [Ar] 3d⁵ 4s¹ over [Ar] 3d⁴ 4s² because placing five electrons in five separate d orbitals — all with parallel spins — maximizes exchange energy (10 exchange pairs vs 6 for d⁴). The energy recovered exceeds the cost of promoting one 4s electron to 3d.

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Summary

Reference for chromium oxidation states: Cr spans -2 to +6, with +3 most stable and +6 strongly oxidizing. Includes key compounds, anomalous electron configuration, and a formula analyzer.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, Formula Analyzer, or Physical Props — to explore each area.
The Oxidation States panel lists all known states (-2 through +6) with stability and chemical context.
The Compounds panel lists key chromium compounds with formulas, Cr oxidation state assignments, and notes.
The Electron Config panel shows the anomalous [Ar] 3d⁵ 4s¹ orbital filling and ionization data.
The Formula Analyzer accepts a formula like CrCl₃ or K₂Cr₂O₇ and calculates the oxidation state of Cr.
The Physical Props panel provides atomic and material data for quick reference.

Use cases

Students studying d-block transition metal chemistry and variable oxidation states.
Chemistry students learning about oxidizing agents, dichromate/chromate equilibria.
Researchers needing a quick reference for Cr compounds and their oxidation states.
Anyone studying industrial chemistry, electroplating, or corrosion science involving chromium.
Students preparing for exams covering Period 4 or Group 6 transition metal chemistry.

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Last updated: 2026-07-08 · Reviewed by Nham Vu