Einstein & Gold

[Serendipity]

Why is gold yellow? Some unexpected physics is responsible for this precious metal’s familiar gleam.

https://www.sciencefocus.com/science/why-is-gold-yellow/

[Fastnick]

Thanks for posting - it makes for an interesting read.

"Relativity comes into play because, due to the size of gold atoms, its electrons are travelling at over half the speed of light. Einstein’s theory tells us that at these speeds the mass of the electrons increases, which in turn means the energy needed to kick them up to another orbital is reduced."

I'm probably missing something quite obvious to others, but I would have expected more energy would have been required when the mass of the electrons increases, rather than less.

Can someone help me out here please and explain why this is so?

[KDave]

Also interesting I did not know that the speed of an electron increases it's mass.

[augur]

@Fastnick the article is leaving out a few bits to make sense out of it: the electrons with the increased mass are in the lowest orbital while the ones absorbing the blue wavelength are situated in higher orbitals. 

The high speed high mass electrons are orbiting on a lower radius which leads to a destabilisation (approximation) of the other orbitals. In specific 6d and 5s orbital are closer than usual and it therefore takes less energy for the 6d electron to jump to the 5s orbit.

[Fastnick]

@augur Thank you!

(I knew that it would be something blindingly obvious )

[augur]

43 minutes ago, Fastnick said:

(I knew that it would be something blindingly obvious )

Quantum physics is not particularly intuitive; I was wondering myself, so had to look that up.

[Murph]

It also says, "So lower-energy blue photons are absorbed, and don’t get reflected by the gold. And if blue is removed, we see yellow."

Which makes it sound a bit like they are saying blue photons are less energetic than yellow, which they obviously can't be.

They are probably referring to the previously mentioned uv photons but it's not very clear.

Also why would we only see yellow and not green, orange or red?

[Serendipity]

7 minutes ago, Murph said:

It also says, "So lower-energy blue photons are absorbed, and don’t get reflected by the gold. And if blue is removed, we see yellow."

Which makes it sound a bit like they are saying blue photons are less energetic than yellow, which they obviously can't be.

They are probably referring to the previously mentioned uv photons but it's not very clear.

Also why would we only see yellow and not green, orange or red?

Gold appears yellow because it absorbs blue light more than it absorbs other visible wavelengths of light; the reflected light reaching the eye is therefore lacking in blue compared to the incident light. Since yellow is complementary to blue, this makes a piece of gold under white light appear yellow to human eyes.

[Serendipity]

Special relativity causes length contractions and time dilations in objects that travel at speeds approaching the speed of light. The valence electrons of large atoms such as gold have such high energies that their speeds actually approach the speed of light—and the relativistic effects on those electrons can become quite large.

Special relativity changes the energy levels of the 5d orbital in a gold atom so that the energy difference between 5d and 6s orbitals equals the energy of a ‘blue’ photon. Gold thus absorbs blue light when electrons are elevated from the 5d to the 6s orbitals, while other metals do not. These special relativistic changes to the energy levels of atomic orbitals are slightly different for each element.

Relativistic contractions on gold’s valence electrons (the 6s subshell) pull the 6s electron very close to the nucleus. Being closer to the nucleus makes the 6s electron less accessible to any potential reactants. Special relativity is not only the reason for gold’s yellow colour but also for its very low reactivity!

[Murph]

Since when is yellow complimentary to blue and what has that to do with it?

Orange is the compliment of blue afaik and purple of yellow.

I'd have thought they would emit photons in the range from yellow and longer wavelengths but more efficiently as the wavelength increased from blue/uv?

[Serendipity]

12 minutes ago, Murph said:

Since when is yellow complimentary to blue and what has that to do with it?

Orange is the compliment of blue afaik and purple of yellow.

I'd have thought they would emit photons in the range from yellow and longer wavelengths but more efficiently as the wavelength increased from blue/uv?

The human eye sees electromagnetic radiation with a wavelength near 600 nm as yellow. Gold appears yellow because it absorbs blue light more than it absorbs other visible wavelengths of light; the reflected light reaching the eye is therefore lacking in blue compared to the incident light. Since yellow is complementary to blue, this makes a piece of gold under white light appear yellow to human eyes.

The electronic transition from the 5d orbital to the 6s orbital is responsible for this absorption. An analogous transition occurs in silver, but the relativistic effects are smaller than in gold. While silver's 4d orbital experiences some relativistic expansion and the 5s orbital some contraction, the 4d-5s distance in silver is still much greater than the 5d-6s distance in gold. The relativistic effects increase the 5d orbital's distance from the atom's nucleus and decrease the 6s orbital's distance.

[Murph]

1 hour ago, Serendipity said:

Special relativity causes length contractions and time dilations in objects that travel at speeds approaching the speed of light. The valence electrons of large atoms such as gold have such high energies that their speeds actually approach the speed of light—and the relativistic effects on those electrons can become quite large.

Special relativity changes the energy levels of the 5d orbital in a gold atom so that the energy difference between 5d and 6s orbitals equals the energy of a ‘blue’ photon. Gold thus absorbs blue light when electrons are elevated from the 5d to the 6s orbitals, while other metals do not. 

5d and 6s?  Is this correct?  Your confusing me now as I have a limited chemistry education and it's been a long, long time since I even thought about such things (probably longer than most here have been alive!) but I thought orbitals went from s to the next levels d as they increased?  (Not a good choice of words I know)

It's OK I recall it now I have had to think about it with s being filled first, then p then d and then f.  It just takes my old brain a while to get into gear.

[Serendipity]

2 hours ago, Murph said:

5d and 6s?  Is this correct?  Your confusing me now as I have a limited chemistry education and it's been a long, long time since I even thought about such things (probably longer than most here have been alive!) but I thought orbitals went from s to the next levels d as they increased?  (Not a good choice of words I know)

Yes, you are absolutely right for orbitals that travel at speeds well below the speed of light. Relativistic effects, however,  increase the 5d orbital's distance from the atom's nucleus and decrease the 6s orbital's distance. The valence electrons of large atoms such as gold have such high energies that their speeds actually approach the speed of light - and the relativistic effects on those electrons can become quite large. Special relativity causes length contractions and time dilations in objects that travel at speeds approaching the speed of light.

Pyykkö, Pekka; Desclaux, Jean Paul (1979). "Relativity and the periodic system of elements". Accounts of Chemical Research. 12(8): 276.

http://web.uvic.ca/~mcindoe/relativity.pdf