Why is gold golden? It’s relative

Gold’s unique shine is due to an intriguing interplay between quantum mechanics and Einstein’s theory of relativity.

Gold’s distinctive appearance and value are not just due to its rarity and non-reactive nature. Its unique hue, appreciated for centuries, owes its existence to an intriguing interplay between quantum mechanics and Einstein’s theory of relativity.

As an element, gold (Au) is relatively unreactive, like its neighbor on the periodic table, silver (Ag). Both metals have a single electron in their outer shell, but their reactivity and color differ significantly.

Silver reflects all visible light, giving it a shiny, silvery appearance. Gold, however, absorbs blue light and reflects the rest, producing its characteristic golden-yellow color. This discrepancy puzzled scientists for years until a key idea emerged: relativistic effects.

The gold nucleus, with its 79 protons, exerts a strong positive charge that pulls its electrons closer to itself. The electrons in the innermost shells orbit the nucleus at speeds greater than half the speed of light to avoid colliding with it. At such relativistic speeds, these electrons gain effective mass and are drawn even closer to the nucleus, compressing the structure of the atom.

This compression affects the outermost electrons in gold, particularly the two outermost shells. The proximity between these layers reduces the energy required to move an electron to a higher energy state when exposed to photons of light.

In silver, this energy threshold is in the ultraviolet spectrum, meaning that visible light is not absorbed and the metal reflects a uniform silver tone. In gold, the threshold falls in the blue region of the visible spectrum, allowing blue light to be absorbed while reflecting reds and greens.. The combination of these reflected colors produces the warm glow characteristic of gold.

“The chemical difference between silver and gold has received a lot of attention throughout the history of chemistry”, explains a 1978 article on the subject, which points to this relativistic effect, citing .

The relativistic phenomenon not only explains the appearance of gold, but also its exceptional stability. The same contraction of the electron shells makes gold less reactive than silverwhich is why the material is used in currency, electronics and jewelry.