Gold (idea)

The 'goldy' colour of gold actually has an interesting origin...
The colour of anything depends on how the photons of light interact with the electrons in the material. Take silver it's 'colour' originates from an electron absorbing an ultra-violet photon; eveything else is reflected, giving the silvery white appearance. Now the electronic configuration of gold is similar to that of silver, and to a first approximation calculations show that gold too should have a 'silver' appearance.

The reason why it doesn't lies with relativity. The electrons which orbit the atomic nucleus must have enough energy to not fall into the nucleus, for hydrogen this corresponds to a speed of about 2 × 10⁶ metres per second. (Please read the electron orbitals node for a grounding on this topic. As you go up the periodic table, the attraction between the (ever larger) nucleus and the electron increases. The electron has to have more energy, or it will fall into the nucleus. By the time you've got to gold (atomic number 79, or 79 protons), the electron nearest the nucleus (1s) have a speed of 1.6 ⁸ metres per second. This is about half the speed of light, and now relativistic effects have to be taken into account....

Special relativity shows us that mass depends on speed (check the relativity node for the equations), and with gold this means it's electrons are 1.2 times heavier than they would be at rest. Now the distance the electrons orbit the nucleus depends on their mass; the greater the mass, the smaller the orbit. If you ignore relativity gold should be about 290 pico metres in diameter, but it is actually closer to the relativity predicted value of 244 pm. (Which is in fact smaller than the silver atom!) The electrons in the s and p orbitals actually spend most of their time near the nucleus, and so they are heavily influenced by this effect, the electrons in 'higher' orbitals d,f don't spend time near the nucleus and so are less affected. This in turn means that now the (negatively charged) s and p electrons are much closer to the (positive) nucleus, they increase the shielding of the higher orbitals from the attraction of the nucleus, which destabilises these higher orbitals, and they balloon outwards, losing energy.

Now the ultra-violet absorbtion in silver described above takes place between the 5s and 4d orbitals, and a similar transistion between 6s and 5d in gold gives it it's colour. In gold the relativistic effects have decreased the gap between these levels, and the frequency of light absorbed is similary decreased and is now in the visible range. Which is why gold has a colour, and it's yellow!

               Non-rel  Rel     Non-Rel  Rel
Orbital   -5_
energies     |  _____
(eV)         |       \_____      _____
             |   5s                   \_____
             |                   6s
          -10|
             |
             |                   5d    _____
             |   4d    _____     _____/
             |  ______/
          -15|
                Silver           Gold