Color of atoms (2014)
fnal.gov
fnal.gov
> atoms do not have colors
Then why do group of atoms (eg. gold) have a color, but not an individual atom?
> If the electrons give out exactly the same light as they absorb, the substance is "colorless"
Shouldn't the answer be "white" rather than "colorless"? Colorless sounds like transparent or clear to me.
He uses absorb, reflect, and emit inconsistently. Surely, there has to be a clearer explanation somewhere on the Internet?
Color is not an inherent property of particles (I'm not talking about "color charge" here of course, which is something else entirely and has nothing to do with daily life colors). It is meaningless to ask the color of a bare electron. Color is the way humans perceive the wavelength/frequency of electromagnetic radiation. There are many different mechanisms that lead to emission of electromagnetic radiation. Then the question becomes: which radiation are you specifically talking about?
Electrons in matter do absorb and emit radiation at particular resonant frequencies, because why live in a particular potential determined by their surroundings. However, except for very simple scenarios (such as an electron bound to a nucleus with screened potential, or a weakly interacting gas of such atoms), the wavelength of the absorbed and emitted frequency depends on the collective behavior of electrons due to electron-electron, electron-hole and electron-lattice interactions. Thus color is an emergent property of matter, which does not exist at fundamental level.
> Shouldn't the answer be "white" rather than "colorless"? Colorless sounds like transparent or clear to me.
No, he is trying to say that color is not an inherent property of the atom. In a simple scenario, when you shine a "cocktail" of light involving many different colors, electrons, which "like" certain photons with certain energies, "eat" those photons and gain a quantum of energy (the value is determined by their internal state and surroundings) and after a while they return to their ground states by releasing a photon with the same energy/wavelength. Depending on the system, they can return to their ground state via some intermediate states, releasing more than one photon, whose wavelengths are different (larger) from the original one they "ate".
That's not true. All molecules absorb radiation at certain frequencies because the energy of an electronic transition of an electron excitation into a higher orbital matches the frequency of the radiation - and if that wavelength is in the visible range (especially organic molecules with pi-bond networks) it's color. The molecule will have this "color" property in a non-emergent fashion, even if it's a single molecule suspended in vacuum.
If I'm not mistaken, a single atom of gold does this as well.
An electron in a gold atom is identical to an electron in hydrogen atom and has no intrinsic property of color. Be it an electron in a populated conduction band, Cooper pair in a superconductor or an isolated electron, the wavelength of the radiation is still a collective behavior of the total system determined by the effective Hamiltonian particles see, not its constituent parts.
An isolated atom is, albeit small, a composite system (with many energy levels corresponding to different internal degrees of freedom, meaning it can emit photons at many many different wavelengths which means electrons in atoms have an associated energy spectrum, not some particular color), and if you're looking at its physics are high energies sufficient enough to excite the nucleus too, a quite complicated one too.
I should also add that as much as it is tempting to think that once you understand how a single isolated gold atom works you can understand the behavior of electrons in bulk gold, that's just not how it works, and is a subject of condensed matter physics.
...And is identical to an electron in a colored molecule, and has no intrinsic property of color.
I think the point is the question implicitly assumes you can take an atom, blow it up to the size of a softball, and then observe what color the softball is. It's correct to reply that atoms are actually a smaller-scale phenomenon than color, so the color of the softball is "not applicable". (With the correction pointing out that sometimes individual atoms do exhibit "colored" phenomena, although it's not the same as most colors, and doesn't apply to most atoms.)
"If the atoms don't have color, why does the substance" is a great philosophical question, similar to the question of why words have meaning, but not letters. By analogy I'd probably start with something like "atoms make color, but that doesn't mean they have it".
I don't think the answer given is a great one, but I like that they sent a correction.