## History Cerium was named for the asteroid Ceres, which was discovered in 1801. The element was discovered two years later in 1803 by Klaproth and by Berzelius and Hisinger. In 1875 Hillebrand and Norton prepared the metal. ## Sources Cerium is the most abundant so-called rare-earths metal. It is found in a number of minerals including allanite (also known as orthite), monazite, bastnasite, certie, and samarskite. Monazite and bastnasite are presently the two more important sources of cerium. Large deposits of monazite (found on the beaches of Travancore, India and in river sands in Brazil), allanite (in the western United States), and bastnasite (in Southern California) will supply cerium, [thorium](/research/charts/periodic-table-of-elements/thorium), and the other rare-earth metals for many years to come. Metallic cerium is prepared by metallothermic reduction techniques, such as reducing cerous [fluoride](/research/charts/periodic-table-of-elements/flourine) with [calcium](/research/charts/periodic-table-of-elements/calcium), or using electrolysis of molten cerous chloride or others processes. The metallothermic technique produces high-purity cerium. ## Properties 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 percent occurs when cerium is subjected to high pressures or low temperatures. Cesium's valence appears to change from about 3 to 4 when it is cooled or compressed. The low temperature behavior of cerium is complex. Cerium is an iron-gray lustrous metal. It is malleable, and oxidizes very readily at room temperature, especially in moist air. Except for [europium](/research/charts/periodic-table-of-elements/europium), cerium is the most reactive of the _rare-earth_ metals. It decomposes slowly 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 slats are orange red or yellowish; cerous salts are usually white. ## Uses Cerium is a component of misch metal, which is extensively used in the manufacture of pyrophoric alloys for cigarette lighters. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. The oxide is an important constituent of incandescent gas mantles and is emerging as a hydrocarbon catalyst in _self cleaning_ ovens. In this application it can be incorporated into oven walls to prevent the collection of cooking residues. As ceric 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. The oxide is finding increased use as a glass polishing agent instead of rouge, for it polishes much faster than rouge. Cerium, with other rare earths, is used in carbon-arc lighting, especially in the motion picture industry. It is also finding use as an important catalyst in petroleum refining and in metallurgical and nuclear applications.