New type of gold developed using synthesis

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Monday 5 June 2006.

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Golden month for most malleable of metals

The most newsworthy chemical element of the last month had to be gold. The amazing rescue of Brant Webb and Todd Russell from the Beaconsfield gold mine in Tasmania after 14 days trapped underground made headlines around the world. In addition, the price of gold reached $US23.31 per gram on May 12, the highest since the late 1970s, and this, too, proved a headline-grabber.

So it is appropriate that this month’s column be devoted to one of the most well known chemical elements on the Periodic Table.

Gold is one of the few chemical elements for which no date of discovery is recorded, although it is mentioned in Egyptian inscriptions dating from around 2600BCE. One of the reasons for gold’s early discovery and use is the fact it is a very unreactive element. This means that it exists in nature in its elemental state — in other words, as pure gold — and can literally be dug straight out of the ground. This contrasts with metals such as iron, chromium and nickel which must be obtained from chemical treatment of their ores. Its lack of reactivity, particularly towards oxygen, is one of the reasons for the value apportioned to gold since ancient times. While many metals form oxide coatings through reaction with oxygen in the air, thereby losing their customary metallic lustre, gold does not, and this makes it an ideal decorative material. It was also realised by the Lydians, the inhabitants of what is now western Turkey, that gold would be a good material from which to make coins, the first of which date from around 560 BCE.

Gold’s extremely low reactivity is manifested by the fact that it is impervious to attack by concentrated hydrochloric acid, sulphuric acid and nitric acid. In fact, a solution of aqua regia (literally “royal water”), 3 parts concentrated hydrochloric acid and 1 part concentrated nitric acid, is required to dissolve gold. Such a solution was used by the Hungarian chemist and future Nobel laureate George de Hevesy, working in Copenhagen at the time of the German invasion in 1940, to dissolve the gold Nobel medals of laureates Max von Laue and James Franck. He did this to save the medals from being confiscated, and the medals remained dissolved for the duration of the war in a bottle on a shelf in his laboratory. After the war, the gold was recovered, and von Laue and Franck were presented with new medals by the Nobel Foundation.

Gold is the most malleable of all the metals. This means that it can be hammered out into sheets as thin as 0.00009 millimetres, or to put it another way, 1g of gold can be beaten into a sheet measuring approximately 1 square metre! Such a sheet was instrumental in Rutherford’s famous “gold foil” experiment, in which he determined the structure of the atom.

Given the fact we have known about gold for a few thousand years, you might think we know all there is to know about it. We know, for instance, that metallic gold exists as individual gold atoms packed together in an infinite threedimensional array. However, last month a report came out on the synthesis of a new type of gold, in the form of “hollow golden cages” by chemists in the US. These molecules contain 16 to 18 gold atoms arranged in a cage-like structure, and the structure of the Au16 molecule is given in the figure. The cavity inside the Au16 molecule is sufficiently large to allow the encapsulation of a single atom of another metal, and doubtless the researchers will be attempting to prepare such a thing.

So, all in all, May was indeed quite a month for gold.