Physics explains why gold stays pristine

Steel rusts over time; copper goes green. But gold seems impervious to the elements. Scientists have now discovered a new detail behind how gold stays so pristine.

Atoms on the surface of gold rearrange into a geometry that hinders oxidation, the process that causes many metals to tarnish. Without that rearrangement, gold would begin to oxidize in seconds, researchers report May 21 in Physical Review Letters.

For a metal to become oxidized, it must first split the surrounding air’s oxygen molecules, which each consist of two oxygen atoms. Then the oxygen atoms can form compounds that stick to the surface of the metal. So the researchers calculated how well gold’s surfaces could split oxygen.

Once a new surface of gold is exposed, for example by cutting it, atoms shift from their original arrangement within the lattice of atoms, a process called reconstruction. Different arrangements of atoms can occur on gold’s exterior, and the researchers studied two common ones, for which atoms are originally laid out in squares, but reconstruct into hexagons.

Based on quantum mechanical calculations, the researchers found that the square arrangement was much better at splitting oxygen than the hexagonal one. In order for the hexagonal structure to split oxygen, it would first need to distort back into the original square shape, a hurdle that forestalls oxidation. (A third common surface structure of gold, already known to be bad at oxidizing, is inherently hexagonal.)

Gold oxide itself is unstable, so even if the square arrangement could be maintained under certain conditions, the material would likely form only a thin layer of oxide, says chemical engineer Matthew Montemore, a coauthor of the study. But the findings could help scientists understand how to better design catalysts, materials that encourage chemical reactions to progress.

Just how much more reticent the reconstructed gold was to oxidize was “definitely a surprise,” says Montemore, of Tulane University in New Orleans. The tiny position shifts make a tremendous difference: “It’s something like a billion to a trillion times slower oxidation once you rearrange.”