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Element Fact
Cesium is used in atomic clock. As in 1999, the best clock is accurate to a second in 20 million years.

Iridium, Ir

iridium sample

Atomic number: 77
Atomic mass: 192.217(3)
Natural abundance: 0.001 ppm
Isotopes:
191Ir (190.960591) - 37.3%
193Ir (192.962924) - 62.7%

History and Production

From Latin iris, or violet. Discovered in 1803, by S. Tennant after crude platinum has been dissolved in aqua regia. The name was given because of the variety of colors of its compound. It is obtained from native osmiridium or iridiosmium in Alaska and South Africa. It can also be obtained from anode slime, produced during electrolytc refinement of copper and nickel. It is used with other platinum metals for making hard alloys for instrument pivots and with platinum in making electrodes for spark plugs. It is also used to make crucibles and electrical contacts with high corrosion resistance even at high temperatures. Iridium is also used in making the standard meter bar of Paris (90% Pt, 10% Ir), as a fundamental unit of length.

Physical Data

It is silvery-white in appearance with a slight yellowish cast. It is the most corrosion-resistant metal known, very hard and brittle. It is not attacked by any acids but is attacked by molten salts (such as NaCl). It is the densest (if not the second, after osmium) of all elements. Its natural abundance is very low, about 0.001 ppm, usually associate with other platinum metals in native form in copper and nickel sulfide ores.

Interatomic distance: 271.4 pm
Melting point: 2446C
Boiling point: 4428C
Thermal conductivity/Wm-1K-1: 147 (27C)
Density/kgm-3: 22560 (20C), 20000 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 665.3 kJ/mol
Gibbs free energy of formation: 617.9 kJ/mol
Entropy: 193.6 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d7 6s2
Term symbol: 4F9/2
Electron affinity: 150.8839 kJ/mol   Electronegativity (Pauline): 2.20
Ionization energy (first, second, third): 865.18, -, - kJ/mol

Chemical properties


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