Chromium (Cr)

A ruby is red and an emerald is green, and the ion responsible for both is identical. Chromium in the +3 state, substituting for aluminum at a few tenths of a per cent, colours corundum red and colours beryl green. Nothing about the chromium differs. What differs is the crystal it is sitting in.

Why the same impurity gives opposite colours

A chromium ion in a crystal is surrounded by oxygen atoms, and their electric field splits the energy of its d orbitals into groups. The size of that splitting determines which wavelengths of light the ion absorbs when an electron jumps between them, and the splitting depends on how close the surrounding oxygens sit.

In corundum, the aluminum site is small and the oxygens are held tight, so the splitting is large. Ruby absorbs strongly in the violet and the yellow-green and transmits the red. In beryl, the site is roomier, the field is weaker, the absorption bands shift, and the transmitted window moves to green. Alexandrite sits so precisely on the boundary that it looks green in daylight and red under incandescent light, because the two transmission windows are nearly balanced and the illuminant decides.

Chromium in ruby does something else as well: after absorbing light it re-emits, sharply, at 694 nanometres. That fluorescence is why a ruby glows under ultraviolet, and it is why the first laser ever operated was a ruby laser. Theodore Maiman built it at Hughes Research Laboratories and made it work on 16 May 1960, pumping a small ruby rod with a photographic flashlamp. Physical Review Letters rejected his paper; he published a short note in Nature instead, and it is now among the most consequential few hundred words in twentieth-century physics.

A pigment that Van Gogh could not have known would fade

Louis-Nicolas Vauquelin isolated the element in 1797 from crocoite, a striking orange-red mineral from the Beryozovskoye mine in the Urals then known as Siberian red lead. He named it from chroma — colour — because every compound he made from it was vividly coloured, which is an unusually descriptive name for an element and an accurate one.

The most commercially important of those compounds was lead chromate, chrome yellow, which became the standard bright yellow pigment of the nineteenth century and was used heavily by the Impressionists and Post-Impressionists. It is not stable. Exposed to light, the chromium is slowly reduced from the +6 to the +3 state, and the pigment browns.

This is now documented rather than suspected. Synchrotron X-ray studies of paint microsamples from Van Gogh's works, including the sunflower paintings, have identified reduced chromium concentrated in the surface layers of the degraded areas, and have shown that the effect is worse where the chrome yellow was mixed with certain white pigments. The paintings were more brilliant when they left the easel than any living person has seen them.

Ten and a half per cent

Chromium's industrial importance rests on a threshold. Add chromium to iron and at somewhere around 10.5% the alloy stops rusting, because the surface forms a film of chromium oxide a few nanometres thick that is dense, adherent, transparent and self-repairing. Scratch it in air and it reforms immediately. Below the threshold, the film is not continuous and the steel corrodes normally.

Priority for the discovery is genuinely contested and cannot be resolved to one name. Léon Guillet in France published systematic studies of iron-chromium alloys between 1904 and 1906 without identifying corrosion resistance as the point. Philip Monnartz in Germany established in 1911 that the effect had a sharp compositional threshold and attributed it to passivation, which is arguably the key scientific insight. Benno Strauss and Eduard Maurer at Krupp patented austenitic chromium-nickel steels in 1912. Harry Brearley, in Sheffield in 1913, was looking for an erosion- resistant steel for gun barrels, noticed that his rejected test samples were not rusting, and had the commercially decisive idea of making cutlery out of it. Elwood Haynes in Indiana filed in the United States and eventually shared rights with Brearley.

Brearley is the name most often given, and what he uniquely contributed was the application rather than the alloy.

The oxidation state is the whole hazard

Very few elements have a toxicity that depends so completely on their oxidation state. Chromium in the +3 state is poorly absorbed and of low toxicity; it is the form in leather tanning and in dietary supplements, and its status as an essential human nutrient is disputed but its danger is not. Chromium in the +6 state, as chromate and dichromate, is a confirmed human carcinogen.

The reason is a case of mistaken identity at the cell membrane. The chromate ion has almost the same size, shape and charge as sulfate, so the sulfate transport channels that every cell operates carry it inside without objection. Once in the cytoplasm it is reduced through reactive intermediates that damage DNA directly. Trivalent chromium has no such passport and largely stays out.

Hexavalent chromium was used as a corrosion inhibitor in the cooling towers at Pacific Gas and Electric's compressor station at Hinkley, California, and leaked into the groundwater. The resulting litigation, driven by the legal clerk Erin Brockovich, produced a $333 million settlement in 1996 and later a film. The compound remains under tight regulatory pressure: chromium trioxide is subject to authorisation under the European REACH regime, which has forced the plating industry into a long and difficult substitution.

What a half-filled shell does and does not explain

Chromium's ground-state electron arrangement puts one electron in the 4s orbital and five in the 3d rather than the two-and-four that the filling order predicts. This is one of the standard exceptions taught alongside copper.

The usual explanation — that a half-filled d subshell is especially stable — is a serviceable mnemonic and not really the mechanism. What is actually going on is that the 4s and 3d levels lie very close together at this point in the row, so the arrangement is decided by smaller effects: electrons in different orbitals with parallel spins avoid one another and repel less, and the exchange interaction rewards keeping as many spins aligned as possible. The energy differences are small enough that the "rule" fails to predict the analogous cases further down the table, which is a reasonable warning about how much weight to put on it.

The consequence that matters practically is that chromium can lose electrons from both levels, and it forms stable compounds at +2, +3 and +6 with quite different chemistry at each.

Sediment that records the rise of oxygen

Chromium isotope ratios have become a tool for reconstructing the oxygenation of Earth's atmosphere, and they work on a different principle from the other proxies.

Chromium is released from rock in the +3 state. Converting it to the mobile +6 state requires strong oxidants — in practice, manganese oxides that only form when there is free oxygen around — and that oxidation step fractionates the isotopes, enriching the dissolved chromium that washes into the ocean in the heavier ones. Marine sediments therefore record an isotopic offset from the crustal value only when the land surface was oxidising.

The record shows essentially no fractionation for most of the Proterozoic and then clear excursions in the Neoproterozoic, which is used to argue for a second major rise in atmospheric oxygen shortly before animals appear. The interpretation is actively debated — how much oxygen the signal actually requires is not settled — but it is one of the few proxies that speaks to oxygen levels on land rather than in water.