Chromium (Cr)

Chromium (symbol Cr) is the chemical element with atomic number 24: a transition metal, solid at 0 °C, with an atomic mass of 51.996 u. Discovered in 1797 (Louis-Nicolas Vauquelin).

Chromium is a hard, shiny metal whose Greek name means “color,” because its compounds are vividly colored. Added to steel, it forms an invisible, self-healing oxide layer that makes it stainless. It is also behind the red of ruby and the green of emerald. Its hexavalent form, however, is highly toxic.

Key facts

Atomic number: 24
Symbol: Cr
French name: Chrome
Atomic mass: 51.996 u
Molar mass: 51.996 g/mol
Family: Transition metals
Position: period 4, group 6, d-block
State at 0 °C: Solid
Melting point: 1,906.9 °C
Boiling point: 2,670.9 °C
Density: 7.15 g/cm³
Electronegativity (Pauling): 1.66
First ionization energy: 653 kJ/mol
Atomic radius: 140 pm
Electron configuration: [Ar] 3d⁵ 4s¹
Abundance in Earth's crust: 102 ppm (21st most abundant element)
Indicative price: 9 $/kg (order of magnitude)
Radioactive: no
Discovery: 1797, Louis-Nicolas Vauquelin

Origin of the name

From the Greek chrōma, color, for its brightly colored compounds.

What does chromium look like?

A bluish silvery-gray metal, very shiny when polished, and one of the hardest pure metals.

Where is chromium found?

About 100 ppm of Earth's crust. It is mined almost exclusively as chromite, concentrated in ancient igneous rocks, notably the Bushveld Complex in South Africa.

Natural sources

Chromite
Crocoite

How is chromium produced?

Chromite is smelted with coke in an electric furnace to produce ferrochrome, which goes straight into steelmaking. About 40 million tonnes of chromite are mined each year, nearly half of it in South Africa. Pure chromium is obtained by aluminothermic reduction or electrolysis.

Top producers

South Africa
Kazakhstan
Turkey
India

What is chromium used for?

Stainless steel (at least 10.5% chromium).
Decorative and wear-resistant chrome plating.
Leather tanning (most leather).
Yellow, green and orange pigments.
Refractory bricks and high-temperature alloys.

In everyday life

“18/10” stainless steel cutlery and sinks (18% chromium, 10% nickel)
A chrome-plated faucet
Leather shoes and bags
A ruby or an emerald

Role in living things

Trivalent chromium was long thought essential to sugar metabolism, but the European Food Safety Authority concluded in 2014 that there was insufficient evidence of this need. It remains present in trace amounts in food.

Hazards of chromium

Hexavalent chromium (chromium VI) is carcinogenic when inhaled and a skin sensitizer: it causes eczema in masons (cement) and tanners. Metallic chromium and chromium III are far less dangerous.

Cosmic origin

White dwarfs: Produced by the thermonuclear explosion of a white dwarf (type Ia supernova).
Supernovae: Forged in the cores of massive stars, then scattered when they explode.

History of chromium

In 1797, French chemist Louis-Nicolas Vauquelin studied crocoite, a reddish-orange mineral from Siberia, and found a new element in it, which he isolated the following year. He named it chrome, from the Greek khrôma, “color,” because of the variety of hues of its salts.

Did you know?

The same chromium gives ruby its red (in aluminium oxide) and emerald its green (in beryl).
Chromium VI pollution in the town of Hinkley, California, inspired the film Erin Brockovich (2000).
The yellow of American school buses was historically called “chrome yellow.”

Isotopes of chromium

Four stable isotopes, with chromium-52 dominant. Radioactive chromium-51 was long used to measure how long red blood cells survive.

chromium-50: 4.345% of natural chromium, stable
chromium-51: radioactive, half-life 27.7 days. Red blood cell label and a measure of kidney filtration.
chromium-52: 83.789% of natural chromium, stable
chromium-53: 9.501% of natural chromium, stable. Its variations, inherited from manganese-53, are used to date meteorites.
chromium-54: 2.365% of natural chromium, stable

Related elements

Same family (transition metals): Scandium (Sc), Titanium (Ti), Vanadium (V), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Cadmium (Cd), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re), Osmium (Os), Iridium (Ir), Platinum (Pt), Gold (Au), Mercury (Hg), Rutherfordium (Rf), Dubnium (Db), Seaborgium (Sg), Bohrium (Bh), Hassium (Hs).

Same column (group 6): Molybdenum (Mo), Tungsten (W), Seaborgium (Sg).