Chromium

Chromium is the shiny metal that stops stainless steel from rusting and gives sports cars their gleaming chrome bumpers. Its name comes from the Greek word for colour, and with good reason, as its compounds include some of the most vivid pigments in the history of art: chrome yellow, chrome green and chrome red.

  • Atomic Number2424 protons, 24 electrons
  • Atomic Mass51.996 uAbout 52× heavier than hydrogen
  • State at Room TempSolidshiny, hard silvery metal
  • Density7.15 g/cm³Denser than iron
  • Melting / Boiling1906.8°C / 2670.8°CMelts at 1,907°C
  • Discovered1797Louis-Nicolas Vauquelin, 1797

How does chromium compare to its neighbours in Period 4?

Chromium sits just before manganese and iron in Period 4, steadily increasing in mass.

Atomic Mass Comparison
Vanadium50.9 u
Chromium52.0 u
Manganese54.9 u
Iron55.8 u
Cobalt58.9 u

Chromium (52 u) is very close in mass to vanadium (50.9 u) and manganese (54.9 u). The atomic masses of the first-row transition metals rise very gradually as each extra proton is added across Period 4.

What is chromium?

Chromium is a transition metal in Group 6 of the periodic table. It has 24 protons and is the hardest naturally occurring metal, harder even than iron. Like titanium, it forms a very thin, tightly bonded oxide layer when exposed to air. This passive layer makes chromium extremely resistant to corrosion, even when scratched. It can have multiple oxidation states: the most common being +3 (green compounds) and +6 (vivid orange-yellow compounds, but highly toxic).

Chromium gets its name from the Greek word chroma, meaning colour, because almost all chromium compounds are intensely coloured. The mineral from which it was extracted, crocoite (lead chromate), is a striking orange-red. The French chemist Louis-Nicolas Vauquelin isolated chromium in 1798 and named it appropriately. The symbol Cr comes from the name.

Fact Rubies and emeralds both owe their colour to chromium. A ruby is aluminium oxide with a trace of chromium(III), just 0.1% is enough to turn colourless corundum a deep red. An emerald is beryl with chromium giving it its vivid green. The same element, in the same oxidation state, produces strikingly different colours in different host minerals.

Where you find chromium

On Earth

Chromium is the 21st most abundant element in the Earth's crust, found mainly in the ore mineral chromite.

  • Chromite ore (FeCr₂O₄). This iron-chromium oxide is the only commercially important chromium ore. South Africa holds approx. 55% of the world's chromite reserves, with Kazakhstan, India and Zimbabwe also significant.
  • Crocoite. Lead chromate is a rare but spectacular orange-red mineral, found notably in Tasmania, Australia. It was the first chromium mineral to be analysed.
  • Ultramafic rocks. Chromite is commonly found in peridotite and other rocks from the Earth's mantle, brought to the surface by geological processes.

How we use chromium

  • Stainless steel. Adding 10.5-30% chromium to steel creates stainless steel, an alloy that resists rust because the chromium forms a self-repairing oxide layer. Over 85% of all chromium goes into making stainless steel.
  • Chrome plating. A thin layer of chromium deposited by electrolysis gives a hard, shiny, corrosion-resistant surface to other metals. Car bumpers, taps, bathroom fittings and tool parts are chrome-plated.
  • Pigments. Chrome yellow (lead chromate) and viridian (chromium oxide green) are historic pigments used by artists for centuries, giving bright, stable colours.
  • Tanning leather. Chromium(III) sulfate is the most common tanning agent for leather, making it soft, supple and water-resistant. About 80% of all leather worldwide is chrome-tanned.
Did you know? Chrome yellow: the vivid yellow pigment made from lead chromate, was Vincent van Gogh's favourite yellow colour. He used it extensively in paintings like "The Sunflowers". However, chrome yellow darkens over time as the chromium is slowly reduced. Conservation scientists believe this is why the sunflower yellows in some of Van Gogh's paintings have become duller and darker than he originally intended.

How it was discovered

Chromium was discovered in 1797 by the French chemist Louis-Nicolas Vauquelin, who analysed the striking orange-red mineral crocoite found in Siberia. By treating the mineral with acids, he extracted a new oxide, then reduced it with charcoal to produce the metal. He named it chromium from the Greek for colour because of the many brilliant colours of its compounds. Vauquelin also discovered beryllium in the same year, an outstanding year for chemistry.

Deeper dive: chromium chemistry: oxidation states and stainless steel

Chromium's chemistry is dominated by its +3 and +6 oxidation states. Chromium(III) compounds are generally stable and form green or violet solutions and complexes. Chromium is essential in trace amounts for human health, chromium(III) helps regulate blood sugar by enhancing the action of insulin. Chromium(VI) compounds, by contrast, are highly toxic and carcinogenic. Hexavalent chromium: the "Cr(VI)" made famous by the Erin Brockovich case, causes cancer when inhaled or ingested, and its disposal is tightly regulated.

Stainless steel was discovered in 1913 by Harry Brearley in Sheffield, England, who was looking for a corrosion-resistant steel for gun barrels. He found that adding at least 10.5% chromium created a steel that could be left in an acid solution for weeks without rusting. The chromium reacts with oxygen in air to form a microscopically thin layer of chromium oxide. Unlike rust on iron, this layer is dense, tightly bonded to the steel and self-repairing, if scratched, it re-forms instantly in air. The discovery transformed the food industry, medical equipment, architecture and everyday life.

Chromium is the metal of shine and colour, from gleaming chrome bumpers to vivid pigments and rust-proof steel. Moving to 25 protons brings us to manganese, an essential metal in steel making and a surprising ingredient in human brain chemistry.