Cobalt is a transition metal in Group VIIIB (Group 9) and the 4th period. Other members of the cobalt group are rhodium (Rh) and iridium (Ir).

Georg Brandt first described metallic cobalt in 1735. The name is derived from the German word, Kobold, for "goblin" or "evil spirit", since the presence of cobalt in ores of other metals made smelting difficult. The symbol Co is an abbreviation of the name.

Cobalt is a brittle, hard metal. In bulk form it is silvery white, lustrous, closely resembling iron and nickel in appearance. In the solid state, the metal has a hexagonal closest packed structure.

Cobalt occurs in the minerals cobaltite, smaltite, and erythrite, and is often associated with nickel, silver, lead, copper, and iron ores, from which it is most frequently obtained as a by-product. It is also present in meteorites.

It reacts with oxygen to give cobalt(III) and cobalt(II) oxide. With acids it gives highly colored Co(II) and Co(III) salts.

Cobalt metal has a magnetic permeability about two-thirds that of iron. It is alloyed with iron, nickel, and other metals to make Alnico, an alloy of unusual magnetic strength with many important uses. Stellite alloys, containing cobalt, chromium, and tungsten, are used for high-speed, heavy-duty, high-temperature cutting tools, and for dies. Cobalt is also used in other magnet steels and stainless steels, and in alloys used in jet turbines and turbo-superchargers. The metal is used in electroplating because of its appearance, hardness, and resistance to oxidation.

The salts have been used for centuries for the production of brilliant and permanent blue colors in porcelain, glass, pottery, tiles and enamels.

Cobalt chloride can be used as an invisible ink. It is almost colorless in dilute solution when applied to paper; upon heating it undergoes dehydration and turns blue, becoming colorless again when the heat is removed and water is taken up by the molecule. Many compounds of cobalt are highly colored.

Cobalt carefully used in the form of the chloride, sulfate, acetate, or nitrate has been found effective in correcting a certain mineral deficiency disease in animals. Vitamin B-12 is a complex of cobalt with an organic molecule. Soils should contain 0.13 to 0.30 parts per million. of cobalt for proper animal nutrition.

Cobalt--60, an artificial isotope, is an important gamma ray source, and is extensively used as a radiotherapeutic agent. Single compact sources of Cobalt--60 are readily available and have a gamma ray output equivalent to thousands of grams of radium.