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Originally Posted by Moontanman
Modest, how does relativity explain the color of gold
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Without relativistic effects, gold would be silver (in color), just like silver (the element).
Silver mostly doesn’t absorb photons of color. It absorbs ultraviolet photons and reflects all visible photons more-or-less equally—leading to its silver color. The ultraviolet frequency coincides with an electron jumping from one orbital to another. In the case of silver it’s from the 4d orbital to 5s.
Atomic orbital - Wikipedia, the free encyclopedia
Gold also could be predicted to absorb ultraviolet frequency when its 5d orbital electron jumps to a 6s orbital. But, the 6s orbital is contracted in gold enough to make the jump from 5d to 6s coincide with a blue photon (less energy than ultraviolet). When blue-frequency light is absorbed, all the redder colors are reflected which makes a gold color. The reason the 6s orbital is contracted while 5d is not is due to the shape of the orbital (or the probability of where the electron is). The s subshells get close to the nucleus of the atom while d does not. In gold, the 6s orbital gets closer (or, the electron has a higher probability of being closer) to the nucleus than does the 5d orbital (or electron).
This gives an electron in the 6s orbital a significant velocity compared to the speed of light (this is explained to some degree in the quote below). The greater the electrostatic charge of the nucleus, the grater the effective velocity. This is why a gold atom shifts the frequency enough to put it in the visible color range while a silver atom does not. Gold has a greater atomic number and more electro-positive protons in the nucleus. In other words, 6s valence electrons in gold are greater-affected by relativity than the s subshell valence electrons in lighter elements.
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Q26. If the electron cannot be localized, can it be moving?
In its lowest state in the hydrogen atom (in which l=0) the electron has zero angular momentum, so electrons in s orbitals are not in motion. In orbitals for which l>0 the electron does have an effective angular momentum, and since the electron also has a definite rest mass  = 9.11E31 kg, it must possess an effective velocity. Its value can be estimated from the Uncertainty Principle; if the volume in which the electron is confined is about  , then the uncertainty in its momentum is at least  = 6.6E–24 kg m / s, which implies a velocity of around 107 m / s, or almost one-tenth the velocity of light.
The stronger the electrostatic force of attraction by the nucleus, the faster the effective electron velocity. In fact, the innermost electrons of the heavier elements have effective velocities so high that relativistic effects set in; that is, the effective mass of the electron significantly exceeds its rest mass. This has direct chemical effects; it is the cause, for example, of the low melting point of metallic mercury and of the color of gold.
Quantum Primer
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This site also offers a good explanation.
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Originally Posted by Moontanman
and does it explain the colors of other metals?
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For the most part—no. The only other element heavy-enough with a valence s-subshell electron for this effect is cesium. As far as I know, it is the only other element besides gold that has a color significantly affected by relativity, giving it a blue spectral line and a slight gold hue. Mercury has two 6s electrons and is one proton heavier than gold. It's melting point is lowered from this relativistic effect which is explained here:
http://www.cengage.com/chemistry/boo....Ch07.CI08.pdf
~modest
