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Iridium                                                                                                                          

Standard Sate: solid at 298K 

Color: silvery white                   

The name iridium is appropriate, for its salts are highly coloured. Iridium is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion resistant metal known, and was used in making the standard metre bar of Paris, which is a 90% platinum-10% iridium alloy.

(Ir), chemical element, one of the platinum metals of Group VIII of
the periodic table. It is very dense and rare and is used in platinum
alloys. A precious, silver-white metal, iridium is hard and brittle, but it
becomes ductile and can be worked at a white heat, from 1,200 to
1,500 C (2,200 to 2,700 F). It is one of the densest terrestrial
substances. In the massive state the metal is practically insoluble in
acids and is not attacked even by aqua regia. It can be dissolved in
concentrated hydrochloric acid in the presence of sodium perchlorate
at 125 to 150 C (257 to 302 F).

Because of difficulties in preparation and fabrication, the pure metal
has few applications. Iridium is chiefly used in the form of platinum
alloys. Platinum-iridium alloys (5 to 10 percent iridium) are readily
workable metals that are much harder and stiffer and more resistant
to chemical attack than the soft pure platinum. Such alloys are used
for jewelry, pen points, surgical pins and pivots, and electrical contacts
and sparking points. The international prototype standard kilogram of
mass is made from an alloy containing 90 percent platinum and 10
percent iridium.

Pure iridium probably does not occur in nature; its abundance in the
Earth's crust is very low, about 0.001 parts per million. Though rare,
iridium does occur in natural alloys with other noble metals: in
iridosmine up to 77 percent iridium, in platiniridium up to 77 percent,
in aurosmiridium 52 percent, and in native platinum up to 7.5 percent.
Iridium generally is produced commercially along with the other
platinum metals as a by-product of nickel or copper production.

Iridium-containing ores are found in South Africa and Alaska, U.S.,
as well as in Myanmar (Burma), Brazil, Russia, and Australia. In the
late 20th century South Africa was the world's major producer of
iridium.

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. atomic number 77 atomic
weight 192.2 melting point 2,410 C (4,370 F) boiling point 4,527 C
(8,181 F) specific gravity 22.4 (20 C) valence 1, 3, 4 electronic
config. 2-8-18-32-17 or (Xe)4f 145d9

"iridium" Encyclop�dia Britannica Online.

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