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Why silver looks like silver, and gold looks like gold

by Bruno Van de Casteele

June 2, 2013

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Donate I used to give introductory trainings to astronomy. One nice way to start those was asking people to look at their rings or any other gold object. The point was that this gold, though mined on Earth, was really created in the violent explosion during a supernova, the end of a gigantic star. Gold, together with all other elements created during those short, cataclysmic events, got dispersed into clouds, eventually ending up in the cloud that formed our Sun and our planets. I later learned (I swear!) that this was also used by Carl Sagan in Cosmos.

But there is something else very interesting and science-related to gold. Do you actually know why gold looks gold, and doesn't look like silver? The correct answer is provided by Einstein, or more precisely, the theory of special relativity working on the individual atoms. At first that seems odd, because the obvious examples are spaceships and big heavy objects to explain this theory.

Gold and silver (and copper) are actually quite similar. They can be found in the same column in the table of Mendeljev, indicating they have similar electron configurations. One can predict that gold, without relativistic effects, would also look quite silvery.

Basically, when a photon of the right energy hits an electron in a (full) orbital or subshell of electrons, it jumps up to a higher orbital, which are not "full". The excited electron doesn't stay very long there, and jumps back to its own orbital emitting a photon. In silver, where the distance between these two orbitals is quite large, electrons get excited by photons in the energy range of ultraviolet light while other wavelengths just bounce off. The result is that we see silver without any absorption in visible light, so it has all colours and the result is white.

However with gold there is something trickier going on. The gold core is quite heavy, and so the innermost electrons move at a speed of about 58% of the speed of light, fast enough that it has real effects on the electrons. Higher speed means more mass, and as a result some orbitals become more contracted or closer to one another, and more specifically those that take part in the excitation process described above. Since they are closer together, a photon of lesser energy is needed to jump over. In the case of gold these are photons with the energy of blue light. So blue light gets absorbed and the remaining visible light is red and yellow, giving us the colour gold.

I couldn't really find why copper is then copper coloured (I have no access to this article), but I could find that apparently, the orbital in question is less "shielded" by other orbitals, so it seems that it is easier for electrons to jump.

For gold however, I find this simply amazing. Not only, in the case of a ring, you have the remnants of a giant explosion of a dying superstar on your fingertips, but just by watching the colour, you get to see the special theory of relativity at work. Science is certainly not boring.

by Bruno Van de Casteele

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