Iridium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

77 osmium ← iridium → platinum Rh ↑ Ir ↓ Mt Periodic Table - Extended 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·cm−3
Liquid density at m.p.	19 g·cm−3
Melting point	2719  K (2446 ° C, 4435 ° F)
Boiling point	4701 K (4428 ° C, 8002 ° F)
Heat of fusion	41.12 kJ·mol−1
Heat of vaporization	231.8 kJ·mol−1
Heat capacity	(25 °C) 25.10 J·mol−1·K−1
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
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 47.1 nΩ·m
Thermal conductivity	(300 K) 147 W·m−1·K−1
Thermal expansion	(25 °C) 6.4 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 4825 m/s
Young's modulus	528 GPa
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
Selected 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 192 mIr 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
References

Iridium ( IPA: /ɪˈɹɪdiəm/) is a chemical element in the periodic table that has the symbol Ir and atomic number 77. A dense, 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 significance in the determination of the probable cause of the demise, by a meteorite strike, of the dinosaurs. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.

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.

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.

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 in tips of ballpoint pens.
• 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.

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. Some linguists have claimed the word-root is derived from "irid", which means "seven" in the Lezghi Language presently spoken in Azerbaijan and Daghestan.

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.

Occurrence

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.

Iridium is rare on Earth, but relatively common in meteorites.

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.

Precautions

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