24 Cr 51.996

Chromium

Transition metal Chromium solid

[Ar] 3d⁵ 4s¹ · 2 · 8 · 13 · 1

← V Vanadium Mn Manganese →

The hardest of the pure metals and the one that makes steel stainless. The name means colour: chromium compounds span the rainbow, and it is chromium that makes ruby red and emerald green.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

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Atomic properties

Atomic number: 24
Atomic mass: 51.996 а.е.м.
Electron configuration: [Ar] 3d⁵ 4s¹
Electrons per shell: 2 · 8 · 13 · 1
Block: d-block
Group: Chromium group
Period: 4
Electronegativity (Pauling): 1.66
Electronegativity (Allen): 1.65
Atomic radius: 166 pm
Covalent radius: 139 pm
Van der Waals radius: 189 pm
Ionisation energy 1: 652.9 kJ/mol
Ionisation energy 2: 1 590.6 kJ/mol
Ionisation energy 3: 2 987 kJ/mol
Ionisation energy 4: 4 743 kJ/mol
Ionisation energy 5: 6 702 kJ/mol
Electron affinity: 64.3 kJ/mol
Common oxidation states: +3, +6
Oxidation states: -4, -2, -1, +1, +2, +3, +4, +5, +6

Physical properties

State at 25 °C: solid
Density: 7.19 g/cm³
Melting point: 2 180 K · 1 906.9 °C
Boiling point: 2 944 K · 2 670.9 °C
Speed of sound: 5 940 m/s

Thermal properties

Heat of fusion: 21 kJ/mol
Heat of vaporisation: 339.5 kJ/mol
Specific heat capacity: 0.449 J/(g·K)
Thermal conductivity: 93.9 W/(m·K)

Mechanical properties

Young's modulus: 279 GPa
Shear modulus: 115 GPa
Bulk modulus: 160 GPa
Poisson ratio: 0.21
Mohs hardness: 8.5
Brinell hardness: 1 120 MPa

Electrical and magnetic properties

Electrical resistivity: 125 nΩ·m
Magnetic ordering: antiferromagnetic
Curie point: no data
Néel point: 311 K
Superconducting point: no data

Crystal structure

Crystal structure: body-centred cubic (bcc)
Lattice constants: a = 291 pm

Abundance

In the crust: 102 mg/kg
In the ocean: 0.0003 mg/L
In the universe: 15 000 mg/kg
In the human body: 0.03 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
⁵⁰Cr	4.345 %	stable	—
⁵²Cr	83.789 %	stable	—
⁵³Cr	9.501 %	stable	—
⁵⁴Cr	2.365 %	stable	—

Chromium-53 in meteorites is the decay product of manganese-53 and serves as a chronometer for the Solar System's first few million years.

Discovery

Year of discovery: 1794
Discovered by: Louis Nicolas Vauquelin
Where: France
Origin of the name: from Greek chroma (colour): its salts span the whole rainbow

History

Louis Vauquelin isolated chromium in 1797–98 from Siberian crocoite brought from near Yekaterinburg. The brilliance of its salts suggested the name.

Where it occurs

102 mg/kg of the crust; essentially all industrial chromium comes from chromite, FeCr₂O₄. The main producers are South Africa, Kazakhstan and India.

How it is produced

Chromite is carbon-reduced to ferrochrome for steelmaking; pure chromium comes from aluminothermic reduction or electrolysis.

Role in living things

Trivalent chromium was long thought essential to glucose metabolism, but modern work casts doubt; no human deficiency has been described.

Safety

The oxidation state is everything: Cr(III) is largely harmless, while Cr(VI) is a potent carcinogen attacking lungs and kidneys. Hexavalent chromium was the contaminant in the case behind the film Erin Brockovich.

Uses

Stainless steel: above 10.5 % chromium the metal grows a self-healing oxide film
Electroplated chrome for decorative and wear-resistant finishes
Refractories for metallurgical furnaces
Pigments: chrome green, chrome yellow, chromium oxide
Leather tanning — most of the world's leather is chrome-tanned

Curiosities

The chrome in a browser name, a disc format and a bumper all trace back to the Greek for colour.
Ruby is aluminium oxide with a chromium trace; the same chromium in beryl gives emerald.
Stainless steel resists rust not because chromium refuses to oxidise, but because it oxidises instantly and tightly.
Chromium's Mohs hardness of 8.5 exceeds that of hardened steel.