Cerium – facts and properties

Position in the periodic table

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Occurrence and extraction

Cerium is about as common in the Earth's crust as copper or zinc, which makes it the most abundant lanthanide. It is found in bastnaesite and monazite, where it accounts for just under half of the rare-earth content. Because so much of it is produced, it is the cheapest of them.

One peculiarity helps in separating it: cerium is the only lanthanide that is stable in aqueous solution when oxidised to oxidation state plus four. The tetravalent compounds behave differently from the trivalent ones of its neighbours and, for example, precipitate at a lower acidity. Cerium can therefore be removed before the harder separation of the remaining lanthanides begins.

The element was discovered in 1803, at almost the same time in two places. In Sweden, Jöns Jakob Berzelius and Wilhelm Hisinger found it in the mineral cerite from a mine near Bastnäs, and in Berlin Martin Heinrich Klaproth found it independently. Two years earlier the celestial body Ceres had been discovered, named after the Roman goddess of agriculture. Berzelius and Hisinger named the new element after it.

Properties and reactions

Cerium is in period 6 and is the second lanthanide. In that row lanthanum lies to its left and praseodymium to its right. Its electron configuration is [Xe] 4f¹ 5d¹ 6s² and its electronegativity is 1.12. The metal has a density of 6.77 g/cm³, melts at 798 °C and boils at 3424 °C. It is soft and pliable, a base metal, and covers itself with an oxide layer in air.

The tetravalent state can be explained by the electron configuration. If cerium gives up all four outer electrons, the configuration of the noble gas xenon is left, and that is particularly stable. Cerium(III) is colourless, whereas cerium(IV) in solution is yellow to orange and a strong oxidising agent. In analysis, cerium(IV) solutions serve as a standard solution for redox titrations, for instance to determine iron(II).

Cerium dioxide, CeO₂, is a pale yellow to white solid with a useful property: it can take up and release oxygen as cerium switches between plus three and plus four. Finely divided metal is easy to ignite. Shavings catch fire with slight warming or through friction, and that is exactly what is exploited.

Uses in everyday life and technology

The lighter flint is the best-known use. It is an alloy of mischmetal with iron. Mischmetal is a mixture of the light lanthanides in which cerium dominates, alongside lanthanum, neodymium and praseodymium. Carl Auer von Welsbach developed the alloy at the start of the twentieth century. When the wheel is turned it scrapes off fine, hot particles that burn in air and fly off as sparks.

Ceria is one of the most important polishing agents for glass. It polishes spectacle lenses, camera lenses, mirrors and screens, and is also used to planarise oxide layers on silicon wafers in chip making. Added to glass, it removes the greenish tint of iron traces and holds back ultraviolet light.

In the car's catalytic converter, ceria acts as an oxygen store. The exhaust gas swings between rich in oxygen and poor in oxygen. With a surplus the ceria takes up oxygen, and with a shortage it releases it. That lets the converter oxidise carbon monoxide and unburnt hydrocarbons while reducing nitrogen oxides at the same time.

Cerium and its compounds are regarded as having low toxicity. The greater danger is fire from metal shavings and dust.

Molecules containing this element

• Cerium(IV) oxide CeO₂View in 3D

Quick question

What is the atomic number of cerium (Ce)?

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