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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Cobalt, Co

cobalt sample

Atomic number: 27
Atomic mass: 58.933200(9)
Natural abundance: 29 ppm
59Co (58.933200) - 100%

History and Production

From the German Kobald, an evil spirit or goblin. Although the element was not discovered until 1735 by G. Brandt, cobalt compounds have been used for thousands of years to give a blue color to glass and pottery. It was, however, identified it as an element by T.O. Bergman in 1780. Cobalt is often associated with other ores such as nickel, iron, copper and lead and is often extracted as a byproduct. These ores are usually roasted to produce a mixed metal oxides (speiss). Cobalt oxide can be extracted from the speiss, via some chemical treatments, and be reduced by heating with carbon (such as charcoal). Cobalt can be used as an alloy with iron, nickel to produce alloy of large magnetic strength. Cobalt is also alloyed with tungsten and chromium for use in high performance cutting tools. The compounds have found their uses in ceramics and pigments (as in paints and inks).

Physical Data

The metal is brittle, and silvery in color, resembling that of iron. It is, however, much less reactive than the iron. Cobalt tends to exist in two allotrope forms, alpha and beta. The former exists below 417C while the later is stable above this temperature. Nevertheless, the presence of small amount of iron may stabilized the beta form to exist at the room temperature. This makes precise determination of certain physical properties of the metal difficult. Cobalt is also quite rare, with the abundance of 25 ppm in the earth's crust. It exists in a variety of minerals of which smaltite (CoAs2), cobaltite (CoAsS) and linnaeite (Co3S4) are some of the important ores.

Interatomic distance: 250.6 pm
Melting point: 1495C
Boiling point: 2927C
Thermal conductivity/Wm-1K-1: 100.0 (27C)
Density/kgm-3: 8900 (20C), 7970 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 424.7 kJ/mol
Gibbs free energy of formation: 380.3 kJ/mol
Entropy: 179.5 J/mol K
Heat capacity: 23.0 J/mol K

Electronic data

Electronic configuration: [Ar] 3d7 4s2
Term symbol: 4F9/2
Electron affinity: 63.8733 kJ/mol   Electronegativity (Pauline): 1.88
Ionization energy (first, second, third): 760.401, 1648.26, 3232.26 kJ/mol

Chemical properties

Test for cobalt (as Co2+ ion):
(1) Blue precipitate is produced when a hydroxide solution is added into a slution containing cobalt(II) ion. The precipiate can dissolve in excess ammonia.
(2) Brown precipitate is formed with potassium cyanide solution. A deep brown solution is formed when excess potassium cyanide is used.
(3) Gives blue borax bead test.

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