Objects have color because their electrons resonate the strongest with light at certain wavelengths, which absorb those wavelengths and reflect all other wavelengths back to our eyes. Most metal electrons resonate with UV light the most intense, so they reflect visible light equally at all wavelengths, which makes them look silver. But gold is different, it presents a noble gold color. In order to explain the difference between gold and other metals, we need to apply special theory of relativity .
According to Einstein 's special theory of relativity, the faster an object's speed, its mass will become larger and larger. In light atoms like hydrogen, which have only one proton and one electron, the electrostatic force is very weak, so the electrons run around the nucleus of and very slowly. But gold has 79 protons in its nucleus, so electrons are attracted by huge static electricity. To avoid collisions with the nucleus, the innermost electrons need to travel at half the speed of light. When the speed of an object becomes so fast, the relativistic effect becomes crucial: the electron mass increases by about 20%, which has a direct effect on the atomic radius of of the electron orbit: the orbital radius decreases.
So far, we have been using the Bohr model of atom , which assumes that electrons orbit the nucleus, just as a planet orbits the sun. But to get to the bottom of it, we need to use more accurate but more complex quantum models. This model replaces orbital electrons with a probability cloud, and probability cloud shows where the electrons are most likely to appear.
The electron closest to the nucleus is on a 1s orbit, and the next orbit is a 2s orbit. In short, it has six s orbitals, and they are all spherical. But not all electron orbits are spheres, other orbits: like p orbits look like two balloons, while d and f orbits look even weirder.
If you observe the s track carefully, you will find that the probability distribution of is not evenly distributed. Electrons may be closer to the nucleus because they prefer to be in a lower energy state. The region where electrons are most likely to be found is called the probability peak, and the probability peaks of all s-orbitals are very close to the nucleus. As we discussed earlier, approaching the nucleus means that electrons move at super-high speed, which means that all six s-orbitals of gold atoms are relativistically contracted.
But the probability peak of the d orbital is further away from the nucleus. Because they don't feel powerful attraction, they won't reach very high speeds, so they are not affected by this relativistic contraction. More importantly, as the electrons in the s-orbital bind to the nucleus more closely, they also play a role in electrostatic shielding of . The electrons in the further d orbital thus feel the weaker force from the nucleus and further expand the radius.
In this quantum model, the absorption of wavelength also occurs between orbits. Most metals have peak absorption wavelengths in UV spectrum , meaning they reflect all visible light. For gold, this absorption occurs between the 5d and 6s orbits. The electrons on the 5d orbit will absorb photons of a certain wavelength and jump to the 6s orbit. If relativity is not considered, the energy required to jump from a 5d orbit to a 6s orbit will correspond to the frequency in the UV spectrum, just like other metals. However, due to the relativistic effect, the 6s and 5d orbits are closer, and scientists measured it to the frequencies of blue light and purple light.
absorbs blue and purple light and reflects the remaining visible light, and gold appears noble yellow. Why this does not happen with other heavier metals such as mercury and lead is because their peak absorption wavelength is not in the Blonde zone.
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