Cobalt is a ferromagnetic metal with a specific gravity of 8.9. The Curie temperature is 1115 °C and the magnetic moment is 1.6–1.7 Bohr magnetons per atom. Cobalt has relative two thirds that of iron. Metallic cobalt occurs as two crystallographic structures: hcp and fcc. The ideal transition temperature between the hcp and fcc structures is 450 °C, but in practice, the energy difference is so small that random intergrowth of the two is common.
Cobalt is a weakly reducing metal that is protected from oxidation by a passivating oxide film. It is attacked by halogens and sulfur. Heating in oxygen produces Co3O4 which loses oxygen at 900 °C to give the monoxide CoO. The metal reacts with Fluorine gas (F2) at 520 K to give CoF3; with chlorine (Cl2), bromine (Br2) and iodine (I2), the corresponding binary halides are formed. It does not react with hydrogen gas (H2) or nitrogen gas (N2) even when heated, but it does react with boron, carbon, phosphorus, arsenic and sulphur. At ordinary temperatures, it reacts slowly with mineral acids, and very slowly with moist, but not with dry, air.
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