Iridium Information
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Iridium Element
Iridium is a chemical element with the symbol Ir and atomic
number 77. A very hard, brittle, silvery-white transition metal of the platinum
group, iridium is considered to be the second-densest metal (after osmium) with
a density of 22.56 g/cm3 as defined by experimental X-ray crystallography.
However, at room temperature and standard atmospheric pressure, iridium has
been calculated to have a density of 22.65 g/cm3, 0.04 g/cm3 higher than osmium
measured the same way.[5] Still, the experimental X-ray crystallography value
is considered to be the most accurate, as such iridium is considered to be the
second densest element.[6] It is the most corrosion-resistant metal, even at
temperatures as high as 2000 °C. Although only certain molten salts and
halogens are corrosive to solid iridium, finely divided iridium dust is much
more reactive and can be flammable.
Iridium was discovered in 1803 among insoluble impurities in
natural platinum. Smithson Tennant, the primary discoverer, named iridium after
the Greek goddess Iris, personification of the rainbow, because of the striking
and diverse colors of its salts. Iridium is one of the rarest elements in
Earth's crust, with annual production and consumption of only three tonnes.
191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as
well as the only stable isotopes; the latter is the more abundant.
The most important iridium compounds in use are the salts and
acids it forms with chlorine, though iridium also forms a number of
organometallic compounds used in industrial catalysis, and in research. Iridium
metal is employed when high corrosion resistance at high temperatures is
needed, as in high-performance spark plugs, crucibles for recrystallization of
semiconductors at high temperatures, and electrodes for the production of
chlorine in the chloralkali process. Iridium radioisotopes are used in some radioisotope
thermoelectric generators.
The element was discovered in 1803 in the acid-insoluble
residues of platinum ores by the English chemist Smithson Tennant; the French
chemists H.-V. Collet-Descotils, A.-F. Fourcroy, and N.-L. Vauquelin identified
it at about the same time. The name iridium, derived from the Greek word iris
(“rainbow”), refers to the various colours of its compounds. Natural iridium
consists of a mixture of two stable isotopes, iridium-191 (37.3 percent) and
iridium-193 (62.7 percent). The chemistry of iridium centres on oxidation
states of +1, +3, and +4, though compounds of all states from 0 to +6 are known
with perhaps the exception of +2. Complexes in oxidation state +1 chiefly
contain carbon monoxide, olefins, and phosphines as ligands. The anions
hexachloroiridate, [IrCl6]2−, and hexabromoiridate, [IrBr6]2−, are the only
notable chemical species containing iridium in the +4 oxidation state. Iridium
is somewhat more reactive than ruthenium and osmium.
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