cerium

Cerium is a soft, ductile, silvery-white metallic chemical element with symbol Ce and atomic number 58. It tarnishes rapidly when exposed to air and is soft enough to be cut with a knife. It is the second element in the lanthanide series, and while it often shows the +3 state characteristic of the series, it also exceptionally has a stable +4 state that does not oxidise water. It is also traditionally considered to be one of the rare earth elements. Cerium has no biological role, but is not very toxic either.
Cerium is the most common of the lanthanides, followed by neodymium, lanthanum, and praseodymium. It is the 26th most abundant element, making up 66ppm of the Earth's crust, half as much as chlorine and five times as much as lead. Despite always being found in combination with the other rare earth elements in minerals such as monazite and bastnäsite, cerium is easy to extract from its ores, as it can be distinguished among the lanthanides by its unique ability to be oxidised to the +4 state.
Cerium was discovered in Bastnäs, Sweden by Jöns Jakob Berzelius and Wilhelm Hisinger in 1803, and independently by Martin Heinrich Klaproth in Germany: it was the first of the lanthanides to be discovered. It was first isolated by Carl Gustaf Mosander in 1839. Today, cerium and its compounds have a variety of uses: for example, cerium(IV) oxide is used to polish glass and is an important part of catalytic convertors. Cerium metal is used in ferrocerium lighters for its pyrophoric properties.