Chromium – facts and properties
- Symbol
- Cr
- Atomic number
- 24
- Atomic mass
- 51.996 u
- Period
- 4
- Group
- 6
- Category
- transition metal
- Electron configuration
- [Ar] 3d⁵ 4s¹
- Electronegativity
- 1.66
- Atomic radius
- 166 pm
- Valence electrons
- 6
- State at standard conditions
- solid
- Melting point
- 1907 °C
- Boiling point
- 2671 °C
- Density
- 7.15 g/cm³
- Discovered
- 1797
Chromium is a steel-grey metal with a bluish shine and atomic number 24. It is the hardest of all pure metals and the reason why stainless steel does not rust. Its name comes from the Greek word for colour, because its compounds are yellow, orange, green or violet. The red of ruby and usually the green of emerald are also caused by traces of this element, in emerald sometimes together with vanadium.
Position in the periodic table
Occurrence and extraction
In nature chromium is found almost only in compounds. The one ore of economic importance is chromite, a black iron chromium oxide. The largest deposits are in South Africa, followed by Kazakhstan, Turkey and India.
The element was discovered in a different, strikingly orange-red mineral: crocoite, a lead chromate from Siberia. The French chemist Louis-Nicolas Vauquelin analysed it and identified a new element in 1797. A year later he obtained the metal by heating the oxide with charcoal. The element was named for the many colours of its compounds.
For steelmaking, chromite is reduced with coke in an electric arc furnace. This gives ferrochrome, an iron–chromium alloy that is added straight to molten steel. Pure chromium is made from chromium(III) oxide by reduction with aluminium, an example of the thermite process, or by electrolysis of chromium salt solutions.
Properties and reactions
Chromium is in period 4 and group 6. Its electron configuration, [Ar] 3d⁵ 4s¹, is a well-known exception. Instead of two electrons in the 4s orbital and four in the 3d subshell, chromium has a single 4s electron and five d electrons, because a half-filled d subshell is considered especially stable. This is a rule of thumb that does not hold for every element. Its electronegativity is 1.66.
The metal has a density of 7.15 g/cm³, melts at 1907 °C and boils at 2671 °C. It is very hard but brittle. In air it immediately forms an invisible, dense oxide layer that protects it from further attack, a state known as passivation. That is why chrome-plated metal keeps its shine for decades.
In compounds, chromium mainly has the oxidation states plus three and plus six. Chromium(III) compounds are usually green or violet, and chromium(III) oxide is a durable green pigment. Chromium(VI) compounds are yellow or orange: chromates are yellow and dichromates orange. They are strong oxidising agents. A classic example is the old breathalyser, in which orange potassium dichromate turns green as alcohol reduces chromium(VI) to chromium(III). Chromium(VI) compounds are toxic and carcinogenic, whereas chromium(III) is much less of a concern.
Uses in everyday life and technology
By far the largest share of chromium goes into stainless steel. Stainless steel contains at least about one tenth chromium, often more. Like the pure metal, the alloy forms a protective chromium oxide layer on its surface. Cutlery, saucepans, sinks, washing machine drums and equipment in hospitals and food factories are made of it for that reason.
In chrome plating, a thin layer of chromium is deposited electrochemically on metal or plastic. Decorative chrome gives a mirror finish to taps and motorbike parts, while hard chrome protects piston rods and rollers from wear. Because the plating baths traditionally contained chromium(VI), the process is strictly regulated in the EU, and companies are increasingly switching to chromium(III) baths.
Other uses include pigments such as chromium oxide green and, in the past, chrome yellow, heat-resistant chromite bricks for industrial furnaces in steelworks and glassworks, and tanning leather with chromium(III) salts, which makes much of the world's leather soft and durable. Chromium(III) was long thought to be an essential trace element for sugar metabolism, but the European Food Safety Authority now sees no convincing evidence for this.
Molecules containing this element
- Chromium(II) oxide CrO
- Chromium(VI) oxide CrO3
- Chromium(III) oxide Cr2O3
Quick question
Why is stainless steel often much more corrosion resistant than regular steel?
FAQ
Is chromium toxic?
That depends on its oxidation state. Chromium metal and chromium(III) compounds are of low toxicity. Chromium(VI) compounds such as chromates and dichromates are toxic, irritate the skin and airways and can cause cancer. People with a chromium allergy may react to leather goods that contain traces of chromium(VI).
Why doesn't stainless steel rust?
Stainless steel contains a lot of chromium. The chromium forms an extremely thin, dense oxide layer on the surface that keeps water and oxygen away from the iron. If the layer is scratched, it re-forms by itself in air. That is why stainless steel does not rust in everyday use.
Why is a ruby red?
A ruby is aluminium oxide in which a small fraction of the aluminium ions has been replaced by chromium(III) ions. These chromium ions absorb green and violet light, so mainly red light is left. Pure aluminium oxide would be colourless. In emerald, chromium, sometimes together with vanadium, sits in a different crystal and produces a green colour instead.