Cobalt is a hard, gray, ferromagnetic metal.    It is relatively unreactive and is stable in air.    Cobalt is insoluble to water and has a melting point of 1495°C and a boiling point of 2927°C.    It is frequently used to produce magnetic and high strength superalloys. Cobalt (atomic symbol: Co, atomic number: 27) is a Block D, Group 9, Period 4 element with an atomic weight of 58.933195.  Cobalt Bohr ModelThe number of electrons in each of cobalt's shells is 2, 8, 15, 2 and its electron configuration is [Ar]3d7 4s2.  The cobalt atom has a radius of 125 pm and a Van der Waals radius of 192 pm. In its elemental form, cobalt has a lustrous gray appearance. Cobalt is found in cobaltite, erythrite, glaucodot and skutterudite ores. Elemental CobaltCobalt produces brilliant blue pigments which have been used since ancient times to color paint and glass.   Co-60, a commercially important radioisotope, is useful as a radioactive tracer and gamma ray source.