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What makes gold such a noble metal?

A glorious, glistening metal well known to jewelers, bankers, and thieves, gold enchants artisans while mystifying scientists. So malleable is the precious metal that a craftsman can hammer an ounce of gold into a 300-square-foot sheet.

But why, chemists wonder, does gold remain tarnishfree? How does it manage to resist corrosion, shrugging off even the most highly reactive gases?

In an effort to shed light on gold's distinctive qualities, Jens K. Norskov and B. Hammer, both physicists at the Technical University of Denmark, studied the effects of hydrogen gas on a gold surface and compared them to similar interactions on surfaces of copper, platinum, and nickel--gold's neighbors on the periodic table.

The physicists found that gold's surface structure provides little freedom for bonding. Molecules hold only loosely onto the metal and fail to form long-lasting molecular or electronic attachments. Consequently, even reactive molecules tend to slide away without bonding or affecting gold's chemically unreactive surface, the researchers report in the July 20 Nature.

"The unique role that gold plays in society is to a large extent related to the fact that it is the most noble [unreactive] of all metals," the scientists state. "It is the least reactive metal towards atoms or molecules at the interface with a gas or a liquid." Yet gold's inertness "does not reflect a general inability to form chemical bonds," since it does form stable alloys with other metals.

The physicists examined the way hydrogen molecules (H2) dissociate, or break loose, at gold's surface. They measured, for instance, the energy necessary for chemical adsorption and the height of activation barriers, which determine how much energy is needed to prompt a reaction.

In their experiments, they distinguished between gold atoms' ability to break and form bonds and the ease with which they form new compounds, such as gold oxides. The two qualities are related: To make a compound, gold atoms must bond with other atoms, yet they cannot do so until they have sundered their bonds with neighboring gold atoms.

As a result, the scientists find that gold's nonreactivity rests on the inability of molecules adequately to fill gaps in the orbits of a gold atom's electrons.

Since gold and similar metals play such a major role in catalysts and microelectronics, studying the surface bonds of these metals has enabled scientists to fabricate more efficient devices, says Donald R. Hamann, a physicist at AT&T Bell Laboratories in Murray Hill, N.J.

"There's a legitimate, long-range goal driving research on metal and semiconductor surfaces," he says.
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Title Annotation:research on reactivity
Author:Lipkin, Richard
Publication:Science News
Date:Jul 22, 1995
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