Osmium is a transition metal in the iron subgroup of Group VIIIB (Group 8) and the 6th period. Other members of the group are iron (Fe) and ruthenium (Ru). It is one of the six platinum group elements: ruthenium, osmium (Os), rhodium (Rh), iridium (Ir), palladium (Pd), and platinum (Pt).
The element was discovered, along with iridium, in 1803 by S. Tennant in the black residue left after dissolving crude platinum in aqua regia.
He named the element after the Latin word for "odor" because of the characteristic and pungent odor of the volatile oxide, OsO4. (NOTE: This oxide is very toxic.)
Osmium is exceedingly rare in the earth's crust (0.005 ppm), but 50 times more abundant than its neighbor ruthenium. (Note the contrast between these abundances and that of iron, the other member of this subgroup.) As a consequence, Os is an expensive element, costing about $7200 per 100 grams (in 1991), a price that makes it one of the most expensive elements. Like rhodium and iridium, osmium is a by-product of the nickel processing industry.
In the solid state, the metal has a hexagonal closest packed structure.
Os has oxidation numbers ranging from 0 to 8, but it is usually +3, +4, +6, or +8 in its compounds.
Although iridium is thought to be the densest known substance (22.65 g/cc), the density of osmium is only slightly less (22.61 g/cc).
The metal is bluish white, extremely hard but brittle, and difficult to fabricate. It is attacked by air at high temperatures to give toxic OsO4.
Osmium is used almost entirely to make very hard alloys with other platinum group metals to make fountain pen tips, instrument bearings, phonograph needles, and electrical contacts.