[Physics FAQ] - [Copyright]

Original by Dave Heisterberg 1994.





Relativity in Chemistry: The Color of Gold

Why is gold yellow? Metals exhibit their characteristic shininess as the delocalised electron sea in the metallic bonds are able to absorb and re-emit photons over a wide range of frequencies. Thus the reflectance spectra of most metals appears fairly flat and they appear silver in colour. A few metals, such as copper and gold, have a reflectance spectrum where the red end (400–700nm) dominates. Why is this so? I first thought that it may be something to do with the single unpaired electron in the outermost valence shell, but Silver also displays this but has a flatter reflectance curve. Can anybody shed some light (groan) on this?

Chemists often consider the first sub-shell of a given angular momentum to be anomalous. The 3d, filled in copper, is less shielded by the s and p subshells than you might otherwise expect. Silver, with a filled 4d behaves more like you think it should. Now when you get to gold (5d) relativistic effects become important. Compared to non-relativistic results the s and p subshells are more contracted (the so-called relativistic stabilization) while d and f are destabilized and more diffuse. So gold also behaves somewhat differently. If you were to do a solid state calculation on gold without including relativistic effects you would predict it to be silvery. Including relativistic effects you get reasonably good agreement with reality.