Gold is a transition metal that has been treasured throughout human history for its beautiful golden yellow color. But why is gold yellow in the first place? The yellow color of gold actually stems from relativistic effects, meaning effects described by Einstein’s theory of relativity.
In metals like gold, the color is determined by the ways the electrons in the atoms interact with light. The free electrons in the metal absorb certain frequencies of light, and reflect back other frequencies. The reflected light is what gives gold its yellowish color. So to understand why gold is yellow, we need to look at why the electrons in gold atoms interact with light in such a way to produce a yellow color.
The Properties of Gold
Gold is element number 79 on the periodic table. It has an atomic mass of 196.967 amu. Gold has properties of both a metal and a nonmetal. Like metals, gold has high thermal and electrical conductivity. But it is also shiny, non-reactive, and malleable like nonmetals.
Some key properties of gold include:
- Atomic number: 79
- Atomic mass: 196.967 amu
- Melting point: 1064 °C
- Boiling point: 2808 °C
- Density: 19.3 g/cm3
- Phase at room temperature: Solid
- Element category: Transition metal
Gold atoms have 79 electrons arranged across 7 electron shells. The electron configuration of gold is [Xe]4f145d106s1. The important thing to note is that gold has one 6s orbital electron. This outermost electron is key to understanding the yellow color of gold, as we’ll explore more later.
The Origin of Color in Metals
In metals, the valence electrons are not tightly bound to individual atoms. Instead, they float around freely, forming a “sea of electrons” that is spread out across the entire metal lattice. These free electrons are responsible for the metallic properties of high conductivity.
When light shines on a metal, these free electrons can interact with the oscillating electric field of the light. If the light frequency matches the natural frequency of oscillation of the electrons, they can absorb that light energy and jump to a higher oscillation state.
The reflected light that our eyes see is the light that was not absorbed by the electrons. The frequencies that were absorbed are missing from the reflected light, which alters its color.
So in summary:
- Metals contain freely moving electrons that can interact with light
- Electrons can absorb certain frequencies of light and move to excited states
- The reflected light is missing those absorbed frequencies, changing its color
This is the basics of how light absorption by free electrons gives metals their color. But it doesn’t yet explain why gold’s electrons specifically absorb frequencies that make it appear yellow. To understand that, we need to dig into quantum physics and relativity.
The Relativistic Origin of the Color
Gold’s yellow color has its origins in Albert Einstein’s theory of relativity. Let’s break this down step-by-step:
- Gold atoms have electrons that occupy d and s electron orbitals
- Higher atomic number elements experience stronger relativistic effects
- This affects s electrons more than d electrons in gold
- The s electrons screen charge differently, altering d orbital energies
- Absorption frequencies of electrons change, giving absorption at blue end of spectrum
- Reflected light is yellow, complementary color to absorbed blue
Let’s go through each of these points in more detail:
1. Gold Atoms Have d and s Electrons
Gold atoms have electrons occupying d and s electron orbitals. The d orbitals are the penultimate (next-to-highest) energy level, and the s orbital is the outermost orbital.
The d orbitals are filled with 10 electrons. The s orbital contains just 1 electron. This outer s electron is key for the yellow color.
2. Relativistic Effects Are Stronger in Heavy Atoms Like Gold
According to Einstein’s theory of relativity, as an object moves faster, its mass increases. This means electrons that are moving extremely fast will have significantly increased mass compared to electrons moving slower.
In heavy elements like gold, the inner electrons near the nucleus move extremely quickly, approaching the speed of light. This means the inner electrons like 1s, 2s, and 3s electrons take on a lot more mass from relativity.
The outer electrons don’t move as fast, so they aren’t affected as much by relativity. This difference in relativistic mass increase between inner and outer electrons leads to important impacts on the chemistry of heavy elements like gold.
3. Relativity Affects the s Electrons More Than the d Electrons
Specifically, the s electrons are affected by relativity much more strongly than the d or f electrons. This is because the s electrons have a higher probability of being found close to the nucleus, where electrons move fastest.
The d and f orbitals are more shielded from the charged nucleus, farther away on average. So the s electrons experience the strongest mass increase from relativistic effects.
4. The s Electrons Screen Charge Differently, Altering d Orbital Energies
Because the s electrons are moving faster and have higher mass, they act differently in terms of nuclear charge screening.
Usually, the inner electrons would screen some of the nuclear charge experienced by the outer electrons. But the heavier s electrons do not screen as well.
This means the d electrons are exposed to more nuclear charge than they would be without relativity. The d orbitals therefore get pulled in closer to the nucleus in energy.
5. The Absorption Frequencies of Electrons Change
The relativistic effect on the d orbital energies has consequences for light absorption.
The d electron energy levels are lowered, while the s level stays put. This increases the energy gap between the d and s levels.
The bigger energy gap requires photons with higher frequencies (bluer light) to excite electrons between these levels. So the absorption frequency changes to the blue end of the spectrum.
6. Reflected Light is Yellow, Complementary to Absorbed Blue
When white light shines on the gold atoms, the blue frequencies are preferentially absorbed by exciting d electrons.
The reflected light is therefore missing these blue components. What remains is predominantly yellow and red light, which gives gold its yellowish color.
This is a consequence of the d orbital shift caused by relativity affecting the s electrons. White light appears yellow because the absorbed complementary color is blue.
Why Other Metals Are Different Colors
If relativity causes gold’s yellow color, why aren’t other metals also yellow? The effect depends on the interplay between d and s electrons.
In silver, the 5s orbital is completely filled while the 4d is partially filled. With a full s level, the relativistic effect is less. The d electrons absorb at all visible wavelengths, causing white silver color.
In copper, the d orbitals absorb red light. In cesium, the absorption is in the infrared. Each element is different based on electron configurations.
But in gold, the relativistic shrinking of the d orbital combined with a partially filled s level leads specifically to absorption of blue and green light.
Other Evidence for Relativity Explaining Gold’s Color
There are other lines of evidence that support relativity dictating gold’s yellow color:
- Gold chloride compounds are not yellow, since the s electron is donated and relativity effect is lost.
- Removing the outer s electron also removes the yellow color.
- Calculations including relativity match measurements of electron movements and absorption frequencies.
- Silver iodide compounds can be made yellow by replacing some silver atoms with heavier gold atoms.
Experiments with gold nanoparticles also match with theoretical predictions of how plasmon frequencies and absorption should change with size due to relativistic effects.
So both theoretical calculations and experimental observations strongly confirm that relativity plays a central role in gold’s yellow color.
Conclusion
Gold’s famous golden yellow color has its origins in Einstein’s theory of relativity. The high speed of inner electrons in gold atoms causes them to increase in mass significantly.
This relativistic effect is strongest for the s electrons, causing them to screen the nuclear charge differently. The d electrons experience more nuclear attraction, shifting their orbitals inward.
The changed energy levels between the d and s orbitals lead to preferential absorption of blue light. The remaining reflected light appears yellow, making gold its prized yellow color.
So by digging into the quantum physics of electrons in gold, we find that the explanation for its color lies in the strange consequences of relativity theory. Gold’s yellow color is truly relativistic!
