Chromium Element Properties
Complete reference for Chromium (Cr, element 24): atomic data, electron configuration, oxidation states, physical constants, and chemical behavior.
Chromium
Transition Metal — Period 4, Group 6
Atomic Identity
Electron Configuration
The expected configuration [Ar] 3d⁴ 4s² is overridden because a half-filled 3d subshell (five electrons, one per orbital) provides maximum exchange energy, lowering the total energy. Promoting one 4s electron into the 3d subshell achieves this half-filled stability.
6 unpaired electrons — strongly paramagnetic
Physical Properties
Chemical Properties
Oxidation States in Detail
| State | Key Compounds | Color | Notes |
|---|---|---|---|
| +6 | CrO₄²⁻ (chromate), Cr₂O₇²⁻ (dichromate), CrO₃ | Yellow / Orange-red | Strongly oxidizing; toxic and carcinogenic (Cr VI) |
| +3 | Cr₂O₃, CrCl₃, Cr(OH)₃, Cr(H₂O)₆³⁺ | Green / Violet | Most stable state; essential trace nutrient (Cr III) |
| +2 | CrCl₂, CrO, CrSO₄ | Blue | Reducing agent; readily oxidized to +3 |
| 0 | Cr(CO)₆ (chromium hexacarbonyl) | White/colorless | Organometallic; volatile |
| −2 | Na₂[Cr(CO)₅] (anionic carbonyl) | Varies | Rare; low-valent organometallics |
Key Facts About Chromium
Hardest Pure Metal
Chromium has a Mohs hardness of 8.5, making it the hardest of all pure metallic elements. Its BCC structure resists scratching and deformation.
Stainless Steel Alloy
Adding at least 10.5% chromium to iron creates stainless steel. Chromium forms a thin, self-healing Cr₂O₃ oxide layer that prevents rust and corrosion.
Anomalous Configuration
Chromium's [Ar] 3d⁵ 4s¹ configuration defies the Aufbau principle. The half-filled 3d shell maximizes exchange energy, providing greater stability.
Chrome Plating
Electrodeposition of chromium from a Cr(VI) bath (chromic acid) creates hard, reflective coatings on car parts, tools, and industrial machinery.
Gem Colorant
Trace amounts of Cr³⁺ are responsible for the red color of rubies (Cr in Al₂O₃) and the green color of emeralds (Cr in Be₃Al₂Si₆O₁₈).
Discovered 1798
French chemist Nicolas Louis Vauquelin isolated chromium from crocoite (PbCrO₄) in 1798. The name comes from Greek chroma (color), reflecting its vivid compounds.
Summary
Complete reference for Chromium (Cr, element 24): atomic data, electron configuration, oxidation states, physical constants, and chemical behavior.
How it works
- Browse the atomic identity section for symbol, atomic number, and standard atomic weight.
- Check the electron configuration panel for the anomalous [Ar] 3d⁵ 4s¹ notation and its explanation.
- Review the physical properties panel for density, melting point, boiling point, and crystal structure.
- Consult the chemical properties section for electronegativity, ionization energies, and oxidation states.
- Explore the oxidation states panel to understand +3 and +6 chromium chemistry.
- Copy any value with one click for use in reports or calculations.
Use cases
- Look up chromium constants for chemistry homework or exams.
- Verify atomic data when writing lab reports on transition metals.
- Reference electron configuration for understanding anomalous d-block behavior.
- Check physical properties for materials science and metallurgy applications.
- Understand chromium oxidation states for inorganic or analytical chemistry.
- Confirm electronegativity and ionization energy for bond analysis.
- Reference chromium data for environmental chemistry and toxicology studies.
- Teach or learn d-block chemistry using chromium as a key example.