Cobalt
. Cobalt (element), symbol Co, silvery-white, magnetic, metallic element used chiefly for making alloys . The atomic number of cobalt is 27; it is one of the transition elements of the periodic table (see Periodic Law ). Cobalt was discovered in 1735 by the Swedish chemist George Brandt. It has a relatively low strength and little ductility at normal temperatures, but is ductile at high temperatures. Cobalt melts at about 1495° C (about 2723° F), boils at about 2870° C (about 5198° F), and has a specific gravity of 8.9; the atomic weight of cobalt is 58.933. Of several known cobalt isotopes, the radioactive cobalt-60 is the most important. It has a half-life of 5.7 years and produces intensive gamma radiation. Cobalt-60 is used extensively in industry and in radioisotope therapy. Cobalt is about the 30th most abundant element in crustal rocks. Cobalt occurs as the arsenide, known as smaltite or speiss cobalt; as cobalt sulfarsenide, known as cobalt glance or cobaltite ; and as a hydrat