iridium
Hard, brittle, silver-white, metallic element, atomic number 77, relative atomic mass 192.2. It is resistant to tarnish and corrosion. Together with the elements ruthenium, rhodium, palladium, osmium, and platinum, it forms the group of platinum metals which have similar physical and chemical properties. It occurs in platinum ores and as a free metal (native metal) with osmium in osmiridium, a natural alloy that includes traces of the other platinum metals.
It is alloyed with platinum for jewellery and used for watch bearings and in scientific instruments. It was named in 1804 by English chemist Smithson Tennant (1761–1815) for the iridescence of its salts in solution.
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