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77 osmiumiridiumplatinum


Name, Symbol, Number iridium, Ir, 77
Chemical series transition metals
Group, Period, Block 9, 6, d
Appearance silvery white
Iridium foil.jpg
Atomic mass 192.217(3) g/mol
Electron configuration [Xe] 4f14 5d7 6s2
Electrons per shell 2, 8, 18, 32, 15, 2
Physical properties
Phase solid
Density (near r.t.) 22.65 g/cm³
Liquid density at m.p. 19 g/cm³
Melting point 2719 K
(2446 °C, 4435 °F)
Boiling point 4701 K
(4428 °C, 8002 °F)
Heat of fusion 41.12 kJ/mol
Heat of vaporization 231.8 kJ/mol
Heat capacity (25 °C) 25.10 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 2713 2957 3252 3614 4069 4659
Atomic properties
Crystal structure cubic face centered
Oxidation states 2, 3, 4, 6
(mildly basic oxide)
Electronegativity 2.20 (Pauling scale)
Ionization energies 1st: 880 kJ/mol
2nd: 1600 kJ/mol
Atomic radius 135 pm
Atomic radius (calc.) 180 pm
Covalent radius 137 pm
Magnetic ordering no data
Electrical resistivity (20 °C) 47.1 nΩ·m
Thermal conductivity (300 K) 147 W/(m·K)
Thermal expansion (25 °C) 6.4 µm/(m·K)
Speed of sound (thin rod) (20 °C) 4825 m/s
Speed of sound (thin rod) (r.t.) 528 m/s
Shear modulus 210 GPa
Bulk modulus 320 GPa
Poisson ratio 0.26
Mohs hardness 6.5
Vickers hardness 1760 MPa
Brinell hardness 1670 MPa
CAS registry number 7439-88-5
Notable isotopes
Main article: Isotopes of iridium
iso NA half-life DM DE (MeV) DP
189Ir syn 13.2 d ε 0.532 189Os
190Ir syn 11.8 d ε 2.000 190Os
191Ir 37.3% Ir is stable with 114 neutrons
192Ir syn 73.83 d β 1.460 192Pt
ε 1.046 192Os
192mIr syn 241 y IT 0.155 192Ir
193Ir 62.7% Ir is stable with 116 neutrons
194Ir syn 19.3 h β< 2.247 194Pt
195Ir syn 2.5 h β< 1.120 195Pt

Iridium (chemical symbol Ir, atomic number 77) is a dense, hard, brittle, silvery-white transition metal of the platinum family. It occurs in natural alloys with platinum or osmium and is notable for being the most corrosion-resistant element known. Researchers have associated it with a meteorite strike that is thought to have caused the demise of dinosaurs. It is used in high-temperature apparatuses and electrical contacts, and as a hardening agent for platinum alloys. It is a catalyst for certain reactions in organic chemistry, and radioactive iridium may be used in radiation therapy for some types of cancer.



Iridium is rare in the Earth's crust, but it is found at higher concentrations in some volcanic flows, suggesting that the Earth's core is richer in this element. Also, iridium is relatively common in meteorites.

The element is found 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. Iridium is recovered commercially as a byproduct from nickel mining and processing.


Iridium was discovered in 1803 by Smithson Tennant, while working with William Hyde Wollaston in London, England. They were looking for a way to purify platinum by dissolving native platinum ore in aqua regia (a mixture of concentrated hydrochloric and nitric acids). A large amount of insoluble black powder remained as a byproduct of this operation. Wollaston focused on analyzing the soluble portion and discovered palladium (in 1802) and rhodium (in 1804), while Tennant examined the insoluble residue. In the summer of 1803, Tennant identified two new elements—osmium and iridium. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.

The element was named after the Latin word iris, meaning rainbow, because many of its salts are strongly colored.

An alloy of 90 percent platinum and 10 percent iridium was used in 1889 to construct the standard meter bar and kilogram mass, kept by the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) near Paris. In 1960, the meter bar was replaced as the definition of the fundamental unit of length (see krypton), but the kilogram prototype is still the international standard of mass.

Iridium has also been linked to evidence for what is known as the "Cretaceous-Tertiary extinction event" ("KT event") of 65 million years ago, at the temporal boundary between the Cretaceous and Tertiary eras. In 1980, a team led by Luis Alvarez found a thin stratum of iridium-rich clay near what is now Yucatán Peninsula. They attributed this iridium to an asteroid or comet impact, and theorized that this impact was responsible for the demise of the dinosaurs. This theory is widely accepted by scientists. On the other hand, 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.

Notable characteristics

Iridium is a transition metal that lies between osmium and platinum in period six of the periodic table. It is thus a member of the platinum group of metals. In addition, it is located in group nine (former group 8B), just below rhodium.

Like platinum, iridium is white, but it has a slight yellowish cast. On account of its extreme hardness and brittle properties, iridium is difficult to machine, form, or work. Yet it can be used to make high-strength alloys that withstand high temperatures.

Iridium is the most corrosion-resistant metal known. It cannot be attacked by any acid, including aqua regia, but it can be attacked by molten salts such as sodium chloride (NaCl) and sodium cyanide (NaCN).

The measured density of this element is only slightly lower than that of osmium, which is often listed as the densest element known. On the other hand, when the density is calculated based on the space lattice structures of these elements, one obtains a density of 22,650 kilograms per cubic meter (kg/m³) for iridium, versus 22,610 kg/m³ for osmium. Based on these data, it is currently not possible to arrive at a firm conclusion about which of them is denser.


Iridium has two stable, naturally occurring isotopes: 191Ir and 193Ir. In addition, it has many radioisotopes, of which Ir-192 has the longest half-life (73.83 days). Ir-192 beta decays into platinum-192, while most of the other radioisotopes decay into osmium.


  • Iridiosmium, iridosmium, or osmium iridian (Os, Ir): This is an alloy of osmium and iridium. It occurs naturally as small, extremely hard, flat metallic grains with hexagonal crystal structure, and sometimes contains traces of platinum, palladium, rhodium, and ruthenium. Iridiosmium has been used in making fountain pen nibs.
  • Osmiridium: This is an alloy of osmium and iridium, with traces of platinum and rhodium. It is found in small amounts in nature, in mines of other platinum group metals. It can also be made artificially. It can be isolated by adding a piece to aqua regia, which has the ability to dissolve gold and platinum but not osmiridium. This alloy is used in making surgical equipment and other high-wear devices. It was once used for fountain pen nibs.


  • Iridium is mainly used as a hardening agent in platinum alloys.
  • It is also used to make crucibles and other items that need to withstand high temperatures.
  • It is used in electrical contacts, such as platinum/iridium spark plugs.
  • Osmium/iridium alloys are used for compass bearings.
  • Iridium is commonly used in complexes (such as Ir(mppy)3) for polymer LED (PLED) technology.
  • It is a catalyst for certain organic reactions, such as to produce acetic acid from methanol.
  • Radioactive isotopes of iridium are used in high-dose radiation therapy for the treatment of prostate and other forms of cancer.

Platinum/iridium alloy was once used in bushing the vents of heavy ordnance. A finely powdered material called iridium black was used for painting porcelain black. In the twentieth century, iridium was used to tip some fountain pen nibs. The tip material in modern pens is still conventionally called "iridium," although there is seldom any iridium in it.


Iridium metal is relatively unreactive and therefore mostly nontoxic. Iridium compounds, however, should be considered highly toxic.


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