Chemistry in its element: chromium


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Meera Senthilingam

This week an element that adds sparkle and value to minerals, through the colourful characteristics of its compounds.

Christopher Blanford

In the Western world, the colourful history of chromium begins, suitably enough, at the far end of the visible spectrum with a red-orange mineral that was named "Siberian red lead" by its discoverer, the 18th-century geologist Johann Lehmann. Although Mendeleev's periodic table was still almost a century away at this time, scientists around the world were rapidly discovering new elements - 30% of the naturally occurring elements were first isolated between 1775 and 1825. It was in the middle of this surge of discovery, over 35 years after Siberian red lead was first found that the French chemist Louis Vauquelin showed that this mineral, now known as crocoite, contained a previously unknown chemical element.

It took Vauquelin several steps to isolate chromium. First he mixed the crocoite solution with potassium carbonate to precipitate out the lead. Then he decomposed the lemon yellow chromate intermediate in acid, and finally removed the compounded oxygen by heating with carbon - leaving behind elemental chromium.

The name for this new element was debated among his friends, who suggested "chrome" from the Greek word for colour because of the colouration of its compounds. Although he objected to this name at first because the metal itself had no characteristic colour, his friends' views won out.

When Vauquelin exhibited his pale grey metal to the French Academy of Sciences, he commented on the metal's brittleness, resistance to acids and incapability of being melted. He thought these properties made it overly difficult to work with and thus limited its applications as a metal. He did suggest, however, that chromium's compounds would be widely used as beautiful, brilliantly coloured pigments. A browse through images of chromium compounds on Wikipedia shows a whole spectrum of colours: dark red chromium(VI) oxide, orange-red lead chromate, bright yellow sodium chromate, brilliant chrome green (that's chrome(III) oxide), light blue chromium(II) chloride, and violet anhydrous chromium(III) chloride. The last of these compounds shows an amazing property when hydrated. Its colour changes between pale green, dark green and violet depending on how many of the chromium ion's six coordination sites are occupied by chloride rather than water.

Of all these pigments, one of them stands out. I'm a chemist who was born, raised and schooled in the Midwestern United States, so the iconic yellow school buses in North America were familiar sights. Chrome yellow, also known as "school bus yellow", was adopted in 1939 for all U.S. school buses to provide high contrast and visibility in twilight hours. However, the presence of both toxic lead and hexavalent chromium of Erin Brockovitch fame has led to it being largely replaced by a family of azo dyes, known as Pigment Yellows, though chrome yellow is still used in some marine and industrial applications.

Of all chromium's natural occurrences, my favourites are gemstones, where a trace of the element adds a blaze of colour. As corundum, beryl, and crysoberyl, these metal oxides are colourless and obscure minerals. But add a dash of chromium, and they become ruby, emerald and alexandrite.

The chemist's tool of crystal-field theory, which models the electronic structure of transition metal complexes, provides a surprisingly accurate way of describing and predicting the source and variability of colour in chromium's compounds. In ruby - which is aluminium oxide with a few parts per thousand of the aluminium ions are replaced by chromium(III) ions - the chromium atoms are surrounded by six oxygen atoms. This means that the chromium atoms strongly absorb light in the violet and yellow-green regions. We see this as mainly red with some blue, giving, in the best cases, the characteristic pigeon-blood colour of the finest rubies.

The Cr3+ ion is about 26% bigger than the Al3+ ion it replaces. So, when more chromium is added to aluminium oxide, the octahedral environment around the chromium becomes distorted and the two bands of absorption shift towards the red. In aluminium oxide in which 20 to 40% of the atoms of aluminium have swapped to chromium, the absorbed and transmitted colours swap and we see this complex as green, transforming a synthetic ruby into a green sapphire.

My next gem, the emerald, in an oxide of silicon, aluminium and beryllium. It has the same substitution of a chromium ion for an aluminium ion and a similar distorted octahedral arrangement of oxygen around chromium, giving emeralds their characteristic green colour, like that from green sapphires.

Of the chromium gemstones, alexandrite is the most fascinating to me. Its stones are strongly pleochroic. That is, they absorb different wavelengths depending on the direction and polarisation of the light that's hitting them. So, depending on a gem's orientation, alexandrite's colour ranges from red-orange to yellow and emerald green. Its colour also changes depending on whether it is viewed in daylight or under the warm red tones of candlelight. When moved from daylight to candlelight, the best specimens turn from a brilliant green to a fiery red. Lesser gems turn from dull green to a turbid blood red.

Outside this rainbow of chromium compounds, chromium helps prevent a particularly undesirable colour: rust brown. In corrosion-resistant, or "stainless", steels, at least 11% of its mass is chromium. The alloyed chromium reacts with oxygen to form a transparent nanoscopic layer of oxide that forms a barrier to further oxygen penetration and so prevents the ruddy, flaky products of iron oxidation.

Given these widespread uses of chromium complexes, it should come as no surprise when I tell you that under one-half of a per cent of chromium produced is chromium in its elemental form. So, to some extent, Vauquelin's prediction from two centuries ago about the limited usefulness of elemental chromium was spot on. On the other hand, the first picture in my mind for chromium (after gemstones, of course) is when it is in its metallic form, such as for the mirrored corrosion and wear-resistant "chrome" surfaces of ball bearings and the shiny silvery trim on car parts.

Meera Senthilingam

So it's shiny and colourful as well as corrosion and wear resistant. I don't think I would say chromium had limited uses, would you? That was Oxford University's Christopher Blanford with the complex and colourful chemistry of chromium. Next week, a planetary element.

Brian Clegg

We're so familiar with uranium and plutonium that it's easy to miss that they are named after the seventh and ninth planets of the solar system. (At least, Pluto was the ninth planet until it was stripped of its status in 2006.) Between those planets sits Neptune, and the gap between the two elements leaves a space for their relatively unsung cousin, neptunium - element number 93 in the periodic table. In June 1940, American physicists Edwin McMillan and Philip Abelson, working at the Berkeley Radiation Laboratory, wrote a paper describing a reaction of uranium that had been discovered when bombarding it with neutrons using a cyclotron particle accelerator. Remarkably, the openly published Berkeley paper would show the first step to overcoming one of the biggest obstacles to building an atomic bomb.

Meera Senthilingam

And Brian Clegg will reveal how this obstacle was overcome in next week's Chemistry in its Element. Until then, I'm Meera Senthilingam and thank you for listening.


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