Gold (Au, Z = 79) and silver (Ag, Z = 47) are the two classic noble metals, immediate neighbors in column 11 of the periodic table. They share the same valence configuration: (n−1)d¹⁰ ns¹. They have the same crystal structure, the same malleability, very similar chemistry. Yet one is silvery grey and the other golden yellow. The color difference hides fascinating physics.
Metal color: what our eyes see
When a beam of white light hits a metal, it interacts with the free surface electrons. Almost all the light is reflected — that's why a polished metal is a mirror. But reflection isn't perfectly uniform across wavelengths: if part of the spectrum is absorbed, the metal appears colored in the complementary hue.
For most metals (iron, aluminium, silver…), absorption sits in the ultraviolet — invisible to the eye. Visible light is therefore reflected uniformly, and the metal looks grey to silvery white. That's silver's case.
Gold is different. Gold absorbs blue (around 400-520 nm) and mostly reflects red and yellow. Its color comes from this selective absorption.
The electronic transition involved
For gold, the absorption near 520 nm corresponds to a 5d → 6s transition. An electron from the filled 5d subshell is excited to 6s, which is partially available (recall: 5d¹⁰ 6s¹). This transition demands a photon energy equal to the energy gap between the two levels.
For silver, the equivalent transition is 4d → 5s. But the energy gap is wider in silver than in gold: absorption falls in the ultraviolet (~310 nm), invisible. That's why silver looks grey.
So the question becomes: why is the 5d-6s gap in gold smaller than the 4d-5s gap in silver? Logically, higher shells (n = 5 and 6) should be closer in energy than lower shells (n = 4 and 5)... yes, but the effect is too pronounced to be explained by non-relativistic quantum mechanics alone.
Relativity again
In gold, core electrons reach ~58 % of the speed of light. The relativistic correction that follows:
1. Contracts 6s (and stabilizes it) — as in mercury's case. 2. Destabilizes 5d — by inverted screening: 5d electrons, indirectly "shielded" by the contracted 6s, see a less attractive effective potential and their energy rises.
The double effect — 6s coming down, 5d going up — shrinks the 5d-6s gap. In silver (Z = 47), inner electrons barely exceed 30 % of c, and these effects are ten times weaker. The 4d-5s gap stays wide, absorption stays UV, silver stays grey.
Order of magnitude
Without relativistic correction, ab initio calculation gives gold an absorption around 320 nm (UV) — "non-relativistic" gold would be grey like silver. With relativity, you recover the observed ~520 nm. The color difference between the two elements is, by itself, a spectroscopic measurement of special relativity at work in atomic matter.
Cousins from the same clan
Other anomalies in the same group (Cu, Au, Hg) share the same origin:
- Copper is red: not exactly relativity (Z too small), but a d-s absorption from the 3d¹⁰ 4s¹ structure.
- Mercury is liquid: 6s² locked by relativistic contraction (see our dedicated article).
- Lead prefers +2 over +4 ("6s inert pair"): 6s contraction makes the pair less available for covalent bonding.
Yellow gold is thus more than an aesthetic detail: it's the visible signature of fundamental physics in an everyday object that humans have handled since the bronze age. When you look at a gold ring, you literally see Einstein's relativity at work.