Osmium

Osmium was discovered in 1803 by the English chemist Smithson Tennant, in the dark residue left when crude platinum was dissolved in acid, the same residue that yielded iridium. He named it from the Greek osme, a smell, after the sharp, acrid odour of its volatile oxide. Osmium is a hard, brittle, bluish-white member of the platinum group and the densest of all naturally occurring elements, used in tiny quantities to harden alloys for fountain-pen nibs and instrument pivots. The densest of all naturally occurring elements, osmium is hard and brittle and used only in small amounts to harden platinum alloys, in the tips of fine pen nibs, instrument pivots and electrical contacts, while its volatile oxide is a valued stain in microscopy.

Osmium tetroxide is also a standard fixative and stain in electron microscopy, since it binds preferentially to fats and lipids in biological tissue, rendering cell membranes visible at the extreme magnifications the technique requires, a use that makes this pungent, toxic compound as familiar in biology laboratories as the metal itself is scarce in industry. Its notorious volatility and toxicity mean elemental osmium is handled far more cautiously than its platinum-group relatives, almost always alloyed rather than used in pure metallic form.

Osmium is named, almost uniquely, for a smell. Its tetroxide is volatile and intensely irritating, with a penetrating chlorine-like odour that gave the element its Greek name, osme; the same compound is a hazard to the eyes and lungs and a useful stain in microscopy. The element has no mythological or alchemical tradition, being a nineteenth-century discovery among the platinum metals, but it holds a physical distinction that lends it a quiet fame: it is the densest substance the Earth naturally provides, so that a fist-sized lump is startlingly, almost unbelievably heavy. Discovered in the same platinum residues by the same chemist as iridium, osmium is the pungent, ponderous twin of the rainbow-named element.

Like its platinum-group neighbour iridium, osmium is a siderophile element that sank overwhelmingly into the Earth's molten core as the planet formed, leaving the crust unusually depleted in it relative to meteorites and asteroids; geochemists accordingly use osmium isotope ratios, alongside iridium, as a further chemical fingerprint for identifying ancient asteroid and comet impacts in the geological record. The two elements, discovered by the same chemist in the same platinum residue, thus ended up telling much the same story about catastrophe from space more than a century and a half later.