Atomic number 58

Cerium

Ce
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Shells Orbitals 4f 5d 6s

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58 electrons in 6 shells: 2, 8, 18, 19, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Lanthanide
Period: 6
Block: f
Standard atomic weight: 140.12(1) Da
Electron configuration: [Xe] 4f¹ 5d¹ 6s²
Phase at 25 °C: solid
Melting point: 795 °C
Boiling point: 3443 °C
Density: 6.770 g/cm³
Electronegativity (Pauling): 1.12
Discovered: 1803, Martin Heinrich Klaproth; independently Jöns Jacob Berzelius and Wilhelm von Hisinger

Neighbours in the table

La Pr

Cerium carries the name of Ceres, the planetoid that Giuseppe Piazzi spotted in 1801, itself named for the Roman goddess of agriculture and harvest. Three men found the element in 1803. The German chemist Martin Heinrich Klaproth called his oxide ochroeite earth because of its yellow colour. Working independently at the same time, the Swedish chemist Berzelius and the mineralogist Wilhelm von Hisinger named theirs ceria.

The metal itself waited until 1875, when the Americans William Hillebrand and Thomas Norton prepared it. Los Alamos describes a lustrous, malleable metal. Hillebrand was a mineralogist as well as a chemist, CIAAW, the atomic-weights commission of IUPAC, the International Union of Pure and Applied Chemistry, records.

Beyond its xenon core a cerium atom holds four electrons, one each in 4f and 5d and two in 6s: [Xe] 4f¹ 5d¹ 6s². Its inner 4f level sits at almost the same energy as the outer electrons, Los Alamos explains. A small push therefore shifts electrons between them, and cerium shows two valence states, numbers of electrons used in bonding.

Cooled or compressed, the metal’s valence appears to move from about 3 towards 4, and its volume changes by about 10 per cent. Ceric salts, from the higher state, are orange-red or yellowish, while cerous salts, from the lower state, are usually white.

Where it is found

Among the rare-earth elements cerium is the most abundant, according to Jefferson Lab. Jefferson Lab puts it at 1.2 × 10⁻⁶ milligrams per litre of seawater. Four isotopes make up natural cerium, with cerium-140 at 88.450 per cent and cerium-142 at 11.114 per cent in CIAAW’s table.

Cerium turns up in allanite, also called orthite, and in monazite, bastnasite, cerite and samarskite, Los Alamos lists. Monazite and bastnasite are the main commercial sources. Monazite collects on the beaches of Travancore in India and in the river sands of Brazil, and bastnasite comes from Southern California. At Mountain Pass there, the US Geological Survey records bastnaesite, a rare-earth fluorocarbonate, mined as a primary product.

Monazite is a phosphate, (Ce, La, Th, Nd, Y)PO₄, and ion exchange now separates cerium from its sand, Jefferson Lab reports. The metal comes from reducing cerous fluoride with calcium, which gives high purity, or from electrolysis of molten cerous chloride.

Why it matters

Cerium oxide holds several everyday jobs. Glass polishers adopted it in place of rouge, Los Alamos notes, because it polishes much faster. A Welsbach gas mantle, thorium oxide with about 1 per cent cerium oxide, glows brilliantly when heated in a gas flame. Built into the walls of self-cleaning ovens, the oxide keeps cooking residues from collecting.

Glassmakers use cerium compounds as an ingredient and as a decoloriser, and the US Geological Survey lists cerium oxide as a colourant as well. Cerium sulfide replaces cadmium pigments, mostly in plastics, the Survey adds. Ceric sulfate serves as an oxidising agent in quantitative analysis, measuring how much of a substance a sample contains. Cerium also goes into carbon-arc lighting, especially for the film industry. Misch metal, a cerium-rich alloy, goes into the lighter flints of cigarette lighters.

Two isotopes put cerium to work in science and medicine. Lanthanum-138 decays 34.5 per cent to cerium-138 and 65.5 per cent to barium-138, in the International Atomic Energy Agency’s (IAEA) evaluation. Its half-life there is 1.03 × 10¹¹ years, and IUPAC notes that the ratio of cerium-138 to cerium-142 dates rocks over billions of years.

Cerium-144 has served in brachytherapy, treatment from a radioactive source placed inside the body. Its half-life is 284.886 days in IAEA’s evaluation.

Properties

Properties of cerium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	58
Standard atomic weight	140.12(1) Da	Da	IUPAC/CIAAW 2024
Melting point	795 °C	°C	Los Alamos: 795 °C
Boiling point	3443 °C	°C	Los Alamos: 3443 °C
Density	6.770 g/cm³	g/cm³
Electronegativity (Pauling)	1.12
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹ 4f¹
Phase at 25 °C	solid

Safety

Pure cerium metal is a flammable solid, and Los Alamos warns that it may ignite if scratched with a knife. Finely divided powder can catch fire by itself in air, and the metal reacts with water and moist air, releasing flammable gas. NITE, a Japanese government body, adds in its classification damage to the blood from a single exposure. Combined with other materials the metal can be used safely, Jefferson Lab notes, and the PubChem Laboratory Chemical Safety Summary for cerium covers handling.

Questions

What does cerium do in lighters?

Cerium is a main ingredient of misch metal, an alloy that the US Geological Survey prices in a grade of 65 per cent cerium and 35 per cent lanthanum. Los Alamos reports that misch metal goes widely into pyrophoric alloys, ones that ignite easily, for cigarette lighters. Ferrocerium, a related pyrophoric alloy, is traded in its own right: in 2024 the United States imported about 220 tonnes, counted as rare-earth-oxide equivalent, and exported 1,100, the US Geological Survey estimates.

Is cerium radioactive?

Cerium itself is not radioactive, Los Alamos states, but commercial grades that are not pure may carry traces of thorium, which is. Monazite, a main cerium ore, holds thorium alongside the rare earths. Of the natural isotopes, cerium-140 is listed as stable by the International Atomic Energy Agency (IAEA), while cerium-136, cerium-138 and cerium-142 carry only lower limits on their half-lives. Cerium-144, used in brachytherapy, is radioactive, with a half-life of 284.886 days in IAEA's evaluation.

What is cerium oxide used for?

Cerium oxide is increasingly used instead of rouge to polish glass, and Los Alamos says the reason is that it polishes much faster. Jefferson Lab names two oxides, Ce₂O₃ and CeO₂. Cerium also serves as a catalyst in petroleum refining, Los Alamos notes. In 2024 the oxide at 99.5 per cent purity sold for about one dollar per kilogram, the US Geological Survey estimates.

Is cerium a rare earth metal?

Yes. Cerium is a lanthanide, one of the f-block elements that Chem1 places between groups 3 and 4 of the periodic table, and it counts among the rare-earth metals. Of all the rare-earth elements it is the most abundant, Jefferson Lab reports. Monazite, one of its ores, holds lanthanum, neodymium, yttrium and thorium beside it.