cerium (sērˈēəm) [key] [from the asteroid Ceres], metallic chemical element; symbol Ce; at. no. 58; at. wt. 140.116; m.p. 799°C; b.p. 3,426°C; sp. gr. 6.77 at 25°C; valence +3 or +4. Cerium is a soft, malleable, ductile, iron-grey metal with hexagonal or cubic crystalline structure. It is slightly harder than lead. It is the most abundant of the rare-earth metals of Group 3 of the periodic table. It does not tarnish rapidly in dry air but quickly loses its luster in moist air. It oxidizes slowly in cold water and rapidly in hot water. It is attacked by solutions of alkalis and by concentrated or dilute acids. When heated it burns with a brilliant flame to form the oxide (ceria) that exhibits incandescence and is used in making lamp mantles (see Welsbach mantle). The metal is used as a core for the carbon electrodes of arc lamps. The element forms alloys with other metals. An alloy of cerium and iron is used as the flint in cigarette and gas lighters. Minute particles of this alloy ignite in the air when scratched from the surface of the larger mass. Cerium is prepared by electrolysis of the chloride or by reduction of the fused fluoride with calcium. Cerium was recognized in 1803 in the oxide (ceria) as a new metal by M. H. Klaproth and by J. J. Berzelius and Wilhelm Hisinger; it was named for the asteroid Ceres, which had been discovered only two years earlier. The metal was obtained in a very impure state by C. G. Mosander and by Friedrich Wöhler some thirty years later; the nearly pure metal was not obtained until 1875 by W. F. Hillebrand and T. H. Norton.