Metallic chemical element , one of the transition element s, chemical symbol Co, atomic number 27.
Widely dispersed in small amounts in many minerals and ores, this magnetic, silvery white metal with a faint bluish tinge is used mostly for special alloy s (e.g., alnico, tool steel ) with exacting applications. At valence 2 or 3 it forms numerous coordination complexes. One is vitamin B 12 (cyanocobalamin; see vitamin B complex ). Cobalt and its compounds are used in electroplating and colouring ceramics and glass and as lamp filaments, catalysts, a trace element in fertilizers, and paint and varnish driers. The pigment cobalt blue has a variable composition, roughly that of cobalt oxide plus alumina. A radioactive isotope of cobalt emits penetrating gamma rays that are used in radiation therapy.