A chemical element, Ce, atomic number 58, atomic weight 140.12. It is the most abundant metallic element of the rare-earth group in the periodic table. The naturally occurring element is made up of the isotopes 136Ce, 138Ce, 140Ce, and 142Ce. A radioactive α-emitter, 142Ce has a half-life of 5 × 1015 years. Cerium occurs mixed with other rare earths in many minerals, particularly monazite and blastnasite, and is found among the products of the fission of uranium, thorium, and plutonium. See also: Periodic table
Although the common valence of cerium is 3, it also forms a series of quadrivalent compounds and is the only rare earth which occurs as a quadrivalent ion in aqueous solution. Although it can be separated from the other rare earths in high purity by ion-exchange methods, it is usually separated chemically by taking advantage of its quadrivalent state.
Cerium is a minor (1–2%) addition to some nonconsumable tungsten electrodes to improve electron thermal emission, as well as an innoculant for some types of nodular cast iron and for rare-earth magnets.