1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰f¹ 5s²p⁶d¹ 6s²

History

Cerium was discovered, in 1803, in Sweden by Jöns Jakob Berzelius and Wilhelm von Hisinger, and independently in Germany by Martin Heinrich Klaproth, but not isolated as a metal until 1875 by Hillebrand and Norton.  Cerium was so named by Berzelius after the dwarf planet Ceres, discovered two years earlier in 1801.

Martin Heinrich Klaproth

Characteristics

Cerium is a silvery metallic element, belonging to the lanthanide group.  It is used in some rare-earth alloys.  It resembles iron in color and luster, but is soft, and both malleable and ductile.  It tarnishes readily in the air.  Only europium is more reactive than cerium among rare earth elements.   Alkali solutions and dilute and concentrated acids attack the metal rapidly.   The pure metal is likely to ignite if scratched with a knife. Cerium oxidizes slowly in cold water and rapidly in hot water.

Although cerium belongs to chemical elements group called rare earth metals, it is not rare at all.  Cerium is available in relatively large quantities (68 ppm in Earth’s crust); in fact it is more common than lead.

Cerium in the +3 oxidation state is referred to as cerous, while the metal in the +4 oxidation state is called ceric.

Cerium (IV) salts are orange red or yellowish, whereas cerium (III) salts are usually white.

Occurrence

Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight. It is found in a number of minerals including allanite (also known as orthite)—(Ca, Ce, La, Y)₂(Al, Fe)₃(SiO₄)₃(OH), monazite (Ce, La, Th, Nd, Y)PO₄, bastnasite (Ce, La, Y)CO₃F, hydroxylbastnasite (Ce, La, Nd)CO₃(OH, F), rhabdophane (Ce, La, Nd)PO₄-H₂O, zircon (ZrSiO₄), and synchysite Ca(Ce, La, Nd, Y)(CO₃)₂F. Monazite and bastnasite are presently the two most important sources of cerium.

Metallic cerium is prepared by metallothermic reduction techniques, such as by reducing cerous fluoride with calcium, or by electrolysis of molten cerous chloride or other cerous halides.  The metallotherrnic technique is used to produce high-purity cerium. Cerium is especially interesting because of its variable electronic structure.  The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relative occupancy of these electronic levels. This gives rise to dual valency states.  For example, a volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures.  It appears that the valence changes from about 3 to 4 when it is cooled or compressed.  The low temperature behavior of cerium is complex.  Four allotropic modifications are thought to exist: cerium at room temperature and at atmospheric pressure is known as gamma cerium.   Upon cooling to -16^oC, gamma cerium changes to beta cerium.  The remaining gamma cerium starts to change to alpha cerium when cooled to -172^oC, and the transformation is complete at -269^oC.  Alpha Cerium has a density of 8.16; delta cerium exists above 726^oC.  At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point. Except for europium, cerium is the most reactive of the "rare-earth" metals.  It slowly decomposes in cold water, and rapidly in hot water. Alkali solutions and dilute and concentrated acids attack the metal rapidly.   The pure metal is likely to ignite if scratched with a knife.  Ceric salts are orange red or yellowish; cerous salts are usually white. 

Large deposits of monazite found on the beaches of Travancore, India, in river sands in Brazil, and deposits of allanite in the western United States, and bastnasite in Southern California will supply cerium, thorium, and the other rare-earth metals for many years to come. 

Applications

• In metallurgy:
  • Cerium is used in making aluminum alloys.
  • Adding cerium tocast irons opposes graphitization and produces a malleable iron.
  • In steels, cerium degasifies and can help reduce sulfides and oxides.
  • Cerium is used in stainless steel as a precipitation hardening agent.
  • 3 to 4% cerium added to magnesium alloys, along with 0.2 to 0.6% zirconium, helps refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings.
  • Cerium is used in alloys that are used to make permanent magnets.
  • Cerium is used as an alloying element in tungsten electrodes for gas tungsten arc welding. 
  • Cerium is a major component of ferrocerium, also known as "lighter flint". Although modern alloys of this type generally use Mischmetal rather than purified cerium, it still is the most prevalent constituent.
  • Cerium is used in carbon-arc lighting, especially in the motion picture industry.
• Cerium (IV) Oxide 
  • The oxide is used in incandescent gas mantles, such as the Welsbach mantle, where it was combined with Thorium, Lanthanum, Magnesium or Yttrium oxides.
  • The oxide is emerging as a hydrocarbon catalyst in self-cleaning ovens, incorporated into oven walls.
  • Cerium (IV) oxide has largely replaced Rouge in the glass industry as a polishing abrasive.
  • Cerium (IV) oxide is finding use as a petroleum cracking catalyst in petroleum refining.
  • In glass, cerium (IV) oxide allows for selective absorption of ultraviolet light. 
• Cerium (IV) Sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis.
• Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer.
• Cerium compounds are used for the coloring of enamel.
• Cerium (III) and cerium (IV) compounds such as ceriium (III) chloride have uses as catalysts in organic synthesis. 

Compounds

Cerium has two common oxidation states, +3 and +4. The most common compound of cerium is cerium (IV) oxide (CeO₂), which is used as "jeweller's rouge" as well as in the walls of some self-cleaning ovens.  Two common oxidizing agents used in titrations are ammonium cerium (IV) sulfate (ceric ammonium sulfate, (NH₄)₂Ce(SO₄)₃ and ammonium cerium (IV) nitrate (ceric ammonium nitrate or CAN, (NH₄)₂Ce(NO₃)₆). Cerium also forms a chloride, CeCl₃ or cerium (III) chloride, used to facilitate reactions at carbonyl groups in organic chemistry.  Other compounds include cerium (III) carbonate Ce₂(CO₃)₃, cerium (III) fluoride (CeF₃), cerium (III) oxide (Ce₂O₃), as well as cerium (IV) sulfate (ceric sulfate, Ce(SO₄)₂ and cerium(III) triflate Ce(OSO₂CF₃)₃.

Isotopes

Naturally occurring cerium is composed of 3 stable isotopes and 1 radioactive isotope; ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce, and ¹⁴²Ce with ¹⁴⁰Ce being the most abundant (88.48% natural abundance).  30 radioisotopes have been characterized with the most {abundant and/or stable} being ¹⁴²Ce with a half-life of greater than 5×10¹⁶ years, ¹⁴⁴Ce with a half-life of 284.893 days, ¹³⁹Ce with a half-life of 137.640 days, and ¹⁴¹Ce with a half-life of 32.501 days.  All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes.  This element also has 2 meta states.

The isotopes of cerium range in atomic weight from 123 u  (¹²³Ce) to 152 u (¹⁵²Ce).

Precautions

Cerium, like all rare earth metals, is of low to moderate toxicity.   Cerium is a strong reducing agent and ignites spontaneously in air at 65 to 80°C.   Fumes from cerium fires are toxic.

Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Animals injected with large doses of cerium have died due to cardiovascular collapse.

It is much more reactive than iron, however, readily oxidizing in moist air and releasing hydrogen from boiling water. Friction from abrading a sample can cause it to ignite.

Cerium (IV) oxide is a powerful oxidizing agent at high temperatures and will react with combustible organic materials. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. Cerium serves no known biological function.

*Mischmetal (from German : Mischmetall - "mixed metals") is an alloy of rare earth elements in various naturally-occurring proportions. It is also called cerium mischmetal, rare earth mischmetal or misch metal. A typical composition includes approximately 50% cerium and 45% lanthanum, with small amounts o neodymium and praseodymium.  Its most common use is in the "flint" ignition device of many lighters and torches, although an alloy of only rare-earth elements would be too soft to give good sparks. For this purpose, it is blended with iron oxide and magnesium oxide to form a harder material known as ferrocerium.

Mischmetal is used in the preparation of virtually all rare earth elements. This is because such elements are nearly identical in most chemical processes, meaning that ordinary extraction processes do not distinguish them. Highly specialized processes, such as those developed by Carl Auer von Welsbach, exploit subtle differences in solubility to separate mischmetal into its constituent elements, with each step producing only an incremental change in composition. Such processes later informed Marie Curie in her search for new elements.

Mischmetal is widely applied in steel foundaries for making FeSiMg alloy and it is used to remove free oxygen and sulfur by forming stable oxysulfides and by tying up undesirable trace elements, such as lead and antimony.

Mischmetal can be further processed to various shapes of ingots, wires, slabs, rods, and discs.