[Physics FAQ] - [Copyright]

Original by Dave Heisterberg, 1994.

Relativity in Chemistry: The Color of Gold

A reader asks:
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.

Pekka Pyykko's Relativistic theory of atoms and molecules gives a nice overview of this and many other phenomena, as well as a huge bibliography of papers dealing with relativistic effects in chemistry.  Oh, that mercury is a liquid is another one.