Gold, while deemed valuable by humans, isn’t that exciting as an element, being fairly unreactive with most of the other elements available to play around with.
Humans love the rare metal anyway, partly due to its rarity and unreactive nature making it an ideal element to use as currency, and its unique shiny color making it attractive when fashioned into jewelry. But what gives it that unique shine, we magpies humans love so much? It turns out that in order to answer that question, you need a little of quantum mechanics and a sprinkling of Einstein’s relativity.
Relativistic effects are also present in mercury.
With a heavy nucleus (79 protons) and a lone electron in its outermost electron shell, gold should have fairly similar properties to silver, with its 47 protons and electrons, and a lone electron in the outermost or “valence” shell. Yet silver (Ag) is more reactive than gold, and for reasons that took a long while to figure out.
“The chemical difference between silver and gold has received a great deal of attention during the history of chemistry,” a 1978 paper on the topic explained. “It seems to be mainly a relativistic effect.”
Due to the large number of positively charged protons within a gold nucleus, negatively charged electrons on the innermost shells are pulled in closer to the nucleus, more than they are in silver. This close to the atom, in order to not fall into it, they must zip around the nucleus at over half the speed of light in contracted innermost shells.
While they zip around at relativistic speeds, gaining effective relativistic mass, the two outermost electron shells (also drawn in towards the heavy nucleus) are slightly closer to each other as a result of this relativistic contraction. This means it takes slightly less energy, when hit by photons of light, to kick an electron to transition to the higher energy state.
In silver, the energy needed for this kick is in the ultraviolet frequency, meaning visible light is not absorbed during the transition, and is reflected, giving it its silvery appearance. In gold, however, the energy required to kick an electron to its higher energy state is lower, in visible blue light. Blue light is absorbed, but the rest of the visible range is reflected back, with the reds and greens combining to make the yellow/golden color we love to gawp at so much.
Source Link: Gold Looks The Way It Does Because Of A Relativistic Effect