Iridium (Ir)

Stable isotopes of iridium available from ISOFLEX

Isotope Z(p) N(n) Atomic Mass Natural Abundance Enrichment Level Chemical Form
Ir-191  77  114  190.960591 37.30%  >93.00% Metal
Ir-193  77  116  192.962923 62.70%  98.40% Metal

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Other isotopes of iridium available from ISOFLEX

Isotope Z(p) N(n) Atomic Mass Half-life Specific Activity  Form
Ir-192  77  115   191.962602 73.83 days ~550-600 curies/g at calibration  Iridium metal discs

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Iridium was discovered in 1803 by French chemists Victor Collet-Descotils, Antoine François Comte de Fourcroy and Louis Nicolas Vauquelin, and British scientist Smithson Tennant. It is named for Iris, the Greek winged goddess of the rainbow and messenger of the Olympic gods.

A silver-white metal with low ductility and close-packed cubic crystals, iridium does not tarnish in air. On heating strongly, a slightly volatile oxide is formed. It is insoluble in acid and slowly soluble in aqua regia and in fused alkalis. It is the most corrosion-resistant element and is highly resistant to chemical attack at ordinary temperatures. At elevated temperatures (near 600 ºC), iridium metal combines with oxygen to form a coating of iridium dioxide. Similarly, the metal reacts with halogens only at elevated temperatures. Iridium forms alloys with several metals, mostly platinum group metals. Iridium does not react with concentrated acids or with molten alkalis.

The most important use of iridium is as an alloying metal for platinum and palladium. Iridium enhances the resistance of platinum to chemical attack and corrosion, as well as its enhancing hardness and tensile strength. Such alloys are used for jewelry, decorative purposes, electrical contacts, thermocouples, crucibles, electrodes, hypodermic needles and medical accessories. The radioisotope Iridium-192 is used in the examination of ferrous welds and in other radiographic applications.

Properties of Iridium

Name Iridium 
Symbol Ir 
Atomic number 77 
Atomic weight 192.22 
Standard state Solid at 298 ºK 
CAS Registry ID 7439-88-5 
Group in periodic table
Group name Precious metal or platinum group metal 
Period in periodic table
Block in periodic table d-block 
Color Silvery white 
Classification Metallic 
Melting point 2410 °C
Boiling point 4130 °C
Vaporization point 4130 °C
Thermal conductivity 147 W/(m·K) at 298.2 °K
Electrical resistivity 5.3 µΩ·cm at 20 °C 
Electronegativity 2.2 
Specific heat 0.133 kJ·mol-1 at 20 °C 
Heat of vaporization 560 kJ·mol-1 at 4130 °C
Heat of fusion 26 kJ·mol-1
Density of liquid 19 g/cm3 at 2410 °C
Density of solid 22.42 g/cm3 (highest among metals) 
Mohs hardness scale 6.0-6.5
Electron configuration [Xe]4f145d76s2 
Oxidation states 0, +1, +2, +3, +4, +5, +6 
Common oxidation states +3, +4

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