Cerium is the most abundant of the rare earth elements.
The energy of cerium's inner 4f level is almost the same as the valence electrons, causing cerium to have dual valency states. Cerium is an iron-gray, malleable metal that oxidizes very readily at room temperature. Except for europium, cerium is the most reactive of the rare-earth metals. It decomposes in water, is attacked rapidly by alkali solutions and concentrated acids, and the metal ignites when scratched by a knife.