Iridium is a very hard, brittle and dense metal and is also very rare.
Credit: Images of Elements
Iridium is the most corrosion-resistant element on the Periodic Table
of Elements. It also has the highest density of all the elements.
Because it resists corrosion, it is used to set standards in weights and
measures. But because it is so dense and brittle, it is hard to
machine, form or work it unless it is heated to extreme temperatures.
Properties
Iridium is a member of the platinum family and is white in color with a
yellowish hue. It has a density of 22.65 grams per cubic centimeter. By
comparison, the density of lead is 11.34 g/cm3 and the density of iron is 7.874 g/cm3.
Iridium is not affected by acids, bases, or most other strong chemicals, according to Chemistry Explained. That property makes it useful in making objects that are exposed to such materials.
Atomic number (number of protons in its nucleus): 77
Atomic symbol (on the Periodic Table of Elements): Ir
Atomic weight (average mass of the atom): 192.217
Melting point: 4,435 F (2,446 C)
Boiling point: 8,002.4 F (4,428 C)
Stable isotopes: 2, which are iridium-191 (37.3 percent) and iridium-193 (62.7 percent)
History
Several chemists may have discovered iridium about the same time in 1803, according to an article in the journal Platinum Metals Review.
English chemist Smithson Tennant, French chemists H.V.
Collet-Descotils, A.F. Fourcroy and N.L. Vauquelin all are said to have
found iridium in the acid-insoluble residues of platinum ores. Tennant
usually gets the credit, though.
Tennant discovered iridium by dissolving crude platinum in diluted aqua
regia (a mixture of nitric and hydrochloric acids), then by treating
the black residue left behind in turn with alkalis and acids, according
to the Royal Society of Chemistry.
After this treatment, the residue separated into two new elements. At
the Royal Institution in London he announced his findings and named one
element iridium and the other osmium. The name iridium comes from the
Latin word iris, which means rainbow. Though the metal itself isn't
rainbow colored, it is called this because of its multi-colored
compounds.
Because iridium is very resistant to corrosion, the standard meter bar
was made of 90 percent platinum and 10 percent iridium. This bar was
replaced as the definition of a meter in 1960, though. The meter was
redefined in terms of the orange-red spectral line of krypton. However,
the international prototype kilogram, which defines a kilogram, also made of a platinum and platinum-iridium alloy, is still in use around the world.
Sources
Today, iridium is commercially recovered as a byproduct of copper or
nickel mining. Ore containing iridium is found in Brazil, the United
States, Myanmar, South Africa, Russia and Australia.
Pure iridium is so rare on the Earth's crust that there is only about 2
parts per billion located in the crust, according to Chemistry
Explained.
"Iridium is one of the densest and rarest of Earth's natural elements.
It is so dense that it mainly exists in the Earth's core, rather than
crust," said Amanda Simson, an assistant professor of chemical
engineering at the University of New Haven.
But some iridium exists in the crust. In 1980 scientist Luis Alvarez
and his son Water Alvarez found significant amounts of iridium in a
certain part of the Earth's crust, spread out all over the Earth's
surface. "They speculated that it was caused by a meteor and linked this
to the extinction of dinosaurs 66 million years prior," explained
Simson.
Uses
Though brittle, iridium can be worked if heated to a white heat of
2,200 to 2,700 degrees Fahrenheit (1,200 to 1,500 degrees Celsius),
according to Encyclopedia Britannica. Iridium's principal use is to
harden platinum by making a platinum alloy.
It is also used to make devices needed for high temperatures and in
electrical contacts. It is also used on some optical lenses to reduce
glare. A compound of osmium and iridium, called osmiridium, is used in
fountain pen tips and compass bearings. Super-strong jewelry is also
made of an iridium and platinum alloy.
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