Osmium

Osmium is the densest naturally occurring element, at about 22.59 g/cm3, just ahead of iridium. Its configuration is [Xe]4f14 5d6 6s2. The lanthanide contraction makes its atoms barely larger than those of ruthenium above it, while its atomic mass is nearly twice as great, so a great deal of mass is packed into a small volume. The bluish-white metal is extremely hard and brittle, so it is used mainly in alloys where wear resistance matters.

Osmium can use all eight of its valence electrons, reaching the +8 oxidation state in osmium tetroxide, OsO4. This volatile, pale yellow solid gives off a sharp, chlorine-like smell, which is the origin of the element's name. OsO4 adds cleanly across carbon–carbon double bonds to make diols, a reaction central to the asymmetric synthesis that earned K. Barry Sharpless part of the 2001 Nobel Prize in Chemistry. It is also a stain in electron microscopy, where it binds to fats in cell membranes.

Discovery

English chemist Smithson Tennant discovered osmium in 1803 in London, along with iridium. When crude platinum was dissolved in aqua regia, a black insoluble residue remained, which most chemists assumed was graphite. Tennant showed that it contained two new metals. He named one osmium, from the Greek osme, 'smell', because of the pungent odour of its volatile oxide.