Iridium

Vapor pressure
77	osmium ← iridium → platinum
	periodic table
General
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)
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

Previous (Irfan Pathan)

Next (Iris Murdoch)

This article is about the chemical element. For the satellite phone service, see Iridium (satellite).

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 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.

History

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% platinum and 10% 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

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.

References
ISBN links support NWE through referral fees

Los Alamos National Laboratory - Iridium

External links

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