Quick Facts
| Property | Value |
|---|---|
| Symbol | Cr |
| Atomic Number | 24 |
| Atomic Mass | 51.996 u |
| Category | Transition Metal |
| Period | 4 |
| Group | 6 |
| Block | d |
| Electron Configuration | [Ar] 3d⁵ 4s¹ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Solid |
| Density | 7.19 g/cm³ |
| Melting Point | 1907°C (2180 K) |
| Boiling Point | 2671°C (2944 K) |
| Appearance | Lustrous, steel-gray metal |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Ar] 3d⁵ 4s¹ |
| Electronegativity | 1.66 (Pauling scale) |
| First Ionization Energy | 652.9 kJ/mol |
| Atomic Radius | 128 pm |
| Covalent Radius | 139 pm |
| Van der Waals Radius | 189 pm |
History and Discovery
Discovered by: Louis Nicolas Vauquelin Year of Discovery: 1797 Location: Paris, France
Etymology
The name "chromium" comes from the Greek "chroma" meaning "color," due to the many colorful compounds it forms.
Discovery Story
Louis Nicolas Vauquelin discovered chromium in 1797 while analyzing crocoite (lead chromate) ore from Siberia. He isolated the metal in 1798 by heating chromium oxide with charcoal. The element was named for the variety of colors in its compounds.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ⁵⁰Cr | 4.35% | Stable | - |
| ⁵²Cr | 83.79% | Stable | - |
| ⁵³Cr | 9.50% | Stable | - |
| ⁵⁴Cr | 2.37% | Stable | - |
Occurrence
Natural Abundance
Chromium comprises about 102 ppm of Earth's crust, making it the 21st most abundant element. The primary ore is chromite (FeCr₂O₄). Major deposits are in South Africa, Kazakhstan, India, and Turkey. South Africa holds about 70% of known reserves.
Extraction and Production
- Aluminothermic Reduction: Of chromium oxide for pure metal
- Carbon Reduction: Of chromite for ferrochromium (main product)
- Sources: Chromite mining
- Global production: about 30 million tons of chromite ore annually
Applications and Uses
Metallurgical Applications
- Stainless steel (10-20% chromium)
- Chrome plating (decorative and protective)
- Superalloys for jet engines
- Tool steels
Chemical Applications
- Chromium compounds for pigments
- Leather tanning (chromium sulfate)
- Wood preservation
- Catalysts
Refractory Applications
- Chromite bricks for furnace linings
- High-temperature applications
- Glass and ceramic colorant
Other Uses
- Chrome yellow and chrome green pigments
- Magnetic recording tape
- Anodizing aluminum
Biological Role
Chromium(III) is considered an essential trace nutrient for humans, though its exact role is debated.
In the Human Body
- About 6 mg in adult body
- May enhance insulin action
- Involved in carbohydrate metabolism
- Adequate intake: 25-35 μg/day
Dietary Sources
Broccoli, grape juice, whole grains, brewer's yeast, and meat are sources of chromium.
Safety and Hazards
Toxicity
Chromium(III) is relatively safe and possibly essential. Chromium(VI) compounds are highly toxic, carcinogenic, and can cause severe damage to respiratory system, skin, and kidneys.
Handling Precautions
- Hexavalent chromium (Cr⁶⁺) is a known carcinogen
- Strict industrial controls for Cr(VI) compounds
- Respiratory protection required for Cr(VI) exposure
- Chrome plating and leather tanning require special precautions
Environmental Impact
Hexavalent chromium contamination is a serious environmental concern (as depicted in the film "Erin Brockovich"). Proper disposal and treatment of chromium waste is essential.
Interesting Facts
- Ruby and emerald get their red and green colors from chromium impurities
- Chrome plating was developed in the 1920s and became hugely popular for automobile parts
- The electron configuration [Ar] 3d⁵ 4s¹ is anomalous (expected would be 3d⁴ 4s²)
- Stainless steel was invented in 1913 by Harry Brearley in Sheffield, England
- The word "chrome" is often used for shiny metallic finishes even when not chromium