Osmium (Os)

Which element is the densest was an open question for most of two centuries, and the two candidates were never in doubt. Osmium and iridium are neighbours, they were discovered together in the same residue by the same chemist, and their densities differ by about one part in a thousand.

That margin is smaller than the errors in any classical measurement. Weighing a sample and measuring its volume gives an answer contaminated by microscopic voids in the metal, by traces of the other platinum-group elements that are almost impossible to remove, and by surface oxidation. Nineteenth and early twentieth century determinations put first one and then the other ahead, several times over.

How the question was actually settled

The answer came from not weighing anything. X-ray diffraction gives the dimensions of the unit cell — the repeating box of the crystal lattice — and if you know how many atoms sit in that box and what they weigh, the density follows by arithmetic, with impurities and voids excluded by construction.

Run that calculation on the best modern lattice constants and the two metals finally separate by more than their error bars. John Arblaster's assessments for Johnson Matthey — iridium in 2010, osmium in 2013, the osmium figures revised again in 2023 — give iridium 22.562 ± 0.011 grams per cubic centimetre and osmium 22.589 ± 0.005 at 20 °C. The gap is 0.027, about a tenth of a per cent, and roughly twice the two uncertainties added together. That is what closed the question. It was not a bigger number for osmium; it was an error bar small enough for a tenth of a per cent to mean anything.

Two qualifications survive, and both are sharp rather than hand-waving. Below about 150 K the thermal expansion data are not good enough to keep the two apart, so the ranking down there is formally undecided. And the order genuinely reverses under pressure: above roughly 3 GPa at room temperature, where both metals pass through about 22.75 g/cm³, iridium is the denser of the two. Everywhere at ordinary pressure from room temperature upward, osmium holds the title, and the margin widens as the sample gets hotter — by iridium's melting point it is more than twenty times what it is on the bench.

That reversal is the flip side of osmium's other extreme property: it is exceptionally hard to compress, so it gains less density per gigapascal than iridium does and is eventually overtaken. Its bulk modulus has been measured at values in the region of 400 to 460 gigapascals, and a 2002 diamond-anvil study put it above diamond's own figure. That claim is still argued over, since the comparison depends on the pressure scale used, but osmium is at minimum in the same class as the hardest known material at resisting uniform squeezing — while being far softer than diamond in the ordinary sense of resisting a scratch.

Both metals are denser than any other element by a comfortable margin — including gold, uranium and plutonium — because the lanthanide contraction squeezes the sixth-row transition metals into a smaller volume than their atomic masses would otherwise occupy.

Tennant's residue

When crude platinum ore is dissolved in aqua regia, most of it goes into solution and a black powder is left behind. Through the eighteenth century that powder was generally written off as graphite.

Smithson Tennant did not accept this. Working through it in 1803 he showed that the insoluble residue contained two previously unknown metals, and he named both. His colleague and business partner William Hyde Wollaston was simultaneously working the soluble fraction from the same batches of ore and pulled palladium and rhodium out of it. Four elements came out of one commercial platinum-refining operation in the space of two years, split cleanly between two men by whether the metal dissolved.

Tennant did not have long to enjoy it. He was killed in 1815 when a bridge collapsed under his horse near Boulogne.

A smell that names an element

Tennant's name for osmium came from the Greek osmē, smell, and it refers not to the metal but to its tetroxide. Osmium tetroxide is a pale yellow solid that sublimes readily at room temperature, and its vapour has a sharp, penetrating odour that the nose detects at concentrations of parts per billion — below the level at which it does harm, which is an unusual and useful arrangement.

It is harmful. Osmium tetroxide attacks the cornea, and severe exposures have caused temporary blindness; the powdered metal slowly converts to the tetroxide in air, which is why osmium is one of the few noble metals with a serious toxicity classification attached to it.

The compound that made cells visible

That same reactivity gave osmium the most consequential job it has. Osmium tetroxide reacts with carbon-carbon double bonds, and cell membranes are built from unsaturated lipids full of them. Treating a biological specimen with it deposits osmium — a very heavy atom, opaque to electrons — precisely along membranes.

Transmission electron microscopy of cells is therefore, in practice, osmium microscopy. The mitochondrial cristae, the nuclear envelope, the Golgi stacks and the endoplasmic reticulum in essentially every classic electron micrograph of a cell are visible because osmium bound to their lipids. It fixes the tissue at the same time, stopping the structures from moving while they are stained.

Synthetic chemists use the same reaction for a different purpose: adding two hydroxyl groups across a double bond, on the same face, to give a cis-diol. Because osmium tetroxide is expensive and hazardous, the practical version uses a trace of it with a cheap reoxidant to regenerate it in the flask. Adding chiral ligands makes the addition pick a face, and the resulting Sharpless asymmetric dihydroxylation was part of the work recognised by the 2001 Nobel Prize in Chemistry.

Almost nobody uses the metal

Pure osmium is barely used, and the reasons are practical rather than exotic: it is brittle even at high temperature, it is difficult to machine, it oxidises to a toxic vapour, and there is very little of it. World production runs to well under a tonne a year, entirely as a byproduct of nickel and platinum refining.

Where osmium does appear, it is alloyed and it is doing the same job in every case — resisting wear at a tiny contact point. Osmiridium, the natural alloy of osmium and iridium, tipped fountain pen nibs for the better part of a century, and turned up in phonograph styli, compass bearings and instrument pivots. Those markets have largely gone to other materials or gone away entirely.

A small market has appeared in the other direction. Since the 2010s crystallised osmium has been sold as a collectable investment metal, on the strength of its scarcity and its distinctive crystal surface. Whether that is a commodity market or a curiosity is a matter of opinion; the underlying scarcity is real enough.

Seven isotopes and a clock in a sulfide

Osmium's isotopic composition is where it earns its keep in the earth sciences, because two of its isotopes are decay products of other elements.

Osmium-187 accumulates from the beta decay of rhenium-187, and the rhenium-osmium chronometer built on that pair is unusual in an important way. Most radiometric systems use elements that prefer silicate minerals, so they date the rocks around an ore body rather than the ore. Rhenium and osmium both prefer sulfides and metal, so Re-Os dates molybdenite, pyrite, organic-rich shales and iron meteorites directly. Osmium-186 similarly grows in from the alpha decay of platinum-190, giving a second, much less used, chronometer.

Seawater carries a dissolved osmium isotope signature that reflects the balance between osmium weathered off continents and osmium delivered by cosmic dust and impacts, so the marine sedimentary record of osmium isotopes preserves a history of both — including a sharp excursion at the Cretaceous-Palaeogene boundary, alongside the iridium anomaly found in the same clay.