Iridium
- This article is about the chemical element. For the satellite phone service, see Iridium (satellite).
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General | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Name, Symbol, Number | iridium, Ir, 77 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Chemical series | transition metals | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Group, Period, Block | 9, 6, d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | silvery white | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Miscellaneous | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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 apparatus, electrical contacts, and as a hardening agent for platinum alloys.
Occurrence
Iridium is rare on Earth, but relatively common in meteorites. The element 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.
History
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 was 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 Yucatán 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.
Notable characteristics
A platinum group 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.
It is used in high-strength alloys that can withstand high temperatures.
The measured density of this element is only slightly lower than that of osmium, which is often listed as the most dense 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.
Isotopes
There are two natural isotopes of iridium, and many radioisotopes, the most stable radioisotope 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.
Alloys
- 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 is used as tips for 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 fountain pen nibs, surgical equipment, and other high-wear devices.
Applications
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 compass bearings.
- Iridium is commonly used in complexes like Ir(mppy)3 and other complexes in polymer LED technology to increase the efficiency from 25% to almost 100% due to triplet harvesting.
- Used in high-dose-radiation therapy for the treatment of prostate and other forms of cancer
- 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 used to tip some early twentieth century fountain pen nibs. The tip material in modern pens is still conventionally called "iridium," although there is seldom any iridium in it.
Precautions
Iridium metal is mostly non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.
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