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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.

Platinum, Pt

platinum sample

Atomic number: 78
Atomic mass: 195.078(2)
Natural abundance: 0.01 ppm
190Pt (189.959930) - 0.01%
192Pt (191.961035) - 0.79%
194Pt (193.962664) - 32.9%
195Pt (194.964774) - 33.8%
196Pt (195.964935) - 25.3%
198Pt (197.967876) - 7.2%

History and Production

From Spanish platina, meaning silver. Its use as ornamental artifacts have been realized by the Indians of Ecuador well before the Spanish conquest. However, its elemental nature was first discovered by A. de Ulloa, a Spanish astronomer, in 1736 in the gold mines of what is now Colombia. His report on the discovery was not published until 1748, due to the unfortunate incident of his ship being fallen victim to privateers and finally captured by the British Navy. Meanwhile, in 1741, C. Wood discovered the metal from a sample brought from Colombia. It became known as 'white gold' but the term in now used to refer to Au/Pd alloy. It is recovered commercially from anode slimes in the electrolytic refinement of copper and nickel. It is also recovered from sperrylite (PtAs2) from Sudbury, Ontario. It is used in a variety of applications where corrosion resistance is of importance such as wires, electrical contacts and laboratory vessels. It is used as jewelry and also in dentistry. It alloys with cobalt to make powerful magnets. It is also an important industrial catalyst for sulfuric acid productions and petroleum cracking.

Physical Data

It is silvery-white in appearance, malleable and ductile. It has a very small coefficient of expansion similar to that of soda-glass. It is usually found (0.01 ppm) as native with other platinum metals in nickel and copper ores.

Interatomic distance: 274.6 pm
Melting point: 1768.4C
Boiling point: 3825C
Thermal conductivity/Wm-1K-1: 71.6 (27C)
Density/kgm-3: 21450 (20C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 565.3 kJ/mol
Gibbs free energy of formation: 520.5 kJ/mol
Entropy: 192.4 J/mol K
Heat capacity: 25.5 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d9 6s1
Term symbol: 3D3
Electron affinity: 205.3209 kJ/mol   Electronegativity (Pauline): 2.20
Ionization energy (first, second, third): 864.384, 1791.06, - kJ/mol


M.C. Usselman, 'A secret history of platinum', Chemistry in Britain, Vol. 37, December 2001.

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