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Alternate meaning: Iridium (satellite)
Osmium - Iridium - Platinum


Name, Symbol, Number Iridium, Ir, 77
Chemical series transition metals
Group, Period, Block 9, 6, d
Density, Hardness 22650 kg/m3, 6.5
Appearance silvery white
Atomic properties
Atomic weight 192.217 amu
Atomic radius (calc.) 135 (180) pm
Covalent radius 137 pm
van der Waals radius no data
Electron configuration [Xe]4f14 5d7 6s2
e- 's per energy level 2, 8, 18, 32, 15, 2
Oxidation states (Oxide) 2, 3, 4, 6 (mildly basic)
Crystal structure Cubic face centered
Physical properties
State of matter solid (__)
Melting point 2739 K (4471 °F)
Boiling point 4701 K (8002 °F)
Molar volume 8.52 ×10-6 m3/mol
Heat of vaporization 604 kJ/mol
Heat of fusion 26.1 kJ/mol
Vapor pressure 1.47 Pa at 2716 K
Speed of sound 4825 m/s at 293.15 K
Electronegativity 2.20 (Pauling scale)
Specific heat capacity 130 J/(kg*K)
Electrical conductivity 19.7 106/m ohm
Thermal conductivity 147 W/(m*K)
1st ionization potential 880 kJ/mol
2nd ionization potential 1600 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP
191Ir 37.3% Ir is stable with 114 neutrons
Meta {syn.} 241 y IT 0.155
193Ir 62.7% Ir is stable with 116 neutrons
SI units & STP are used except where noted.

Iridium is a chemical element in the periodic table that has the symbol Ir and atomic number 77. A heavy, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion resistant element known and for its association with the demise of the dinosaurs. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.


Notable characteristics

A platinum family metal, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittle properties, iridium is difficult to machine, form, or work. Iridium is the most corrosion-resistant metal known. Iridium cannot be attacked by any acids or by aqua regia, but it can be attacked by molten salts, such as NaCl and NaCN.

The measured density of this element is only slightly lower than that of osmium, which is therefore often listed as the heaviest element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m³ for iridium versus 22610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time.


The principal use of iridium is as a hardening agent in platinum alloys. Other uses:

  • For making crucibles and devices that require high temperatures.
  • Electrical contacts (notable example: Pt/Ir sparkplugs).
  • Osmium/iridium alloys are used for tipping fountain pen nibs and for compass bearings.
  • Iridium is used as a catalyst for carbonylation of methanol to produce acetic acid

At one time iridium, as an alloy with platinum, was used in bushing the vents of heavy ordnance and, in a finely powdered condition (iridium black), for painting porcelain black.


Iridium was discovered in 1803 by Smithson Tennant in London, England along with osmium in the dark-colored residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). The element was named after the Latin word for rainbow (iris; iridium means "of rainbows") because many of its salts are strongly colored.

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the standard metre bar and kilogram mass, kept by the International Bureau of Weights and Measures near Paris. The metre bar has been replaced as the definition of the fundamental unit of length in 1960 (see krypton), but the kilogram prototype is still the international standard of mass.

The KT event of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary eras of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez (1980) proposed an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact near what is now Yucatan Peninsula. Their theory is widely accepted to explain the demise of the dinosaurs. Dewey M. McLean and others argue that the iridium may have been of volcanic origin instead. The Earth's core is rich in iridium, and Piton de la Fournaise on Réunion, for example, is still releasing iridium today.


Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridiosmium , both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.


There are two natural isotopes of iridium, and many radioisotopes, the most stable being Ir-192 with a half-life of 73.83 days. Ir-192 beta decays into platinum-192, while most of the other radioisotopes decay into osmium.


Iridium metal is generally non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.


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Last updated: 05-13-2005 07:56:04