[george (OP)]

What is the chemical reason why metallic substances are shiny whereas non-metal solids tend not to be?

George

[Bored chemist]

Strictly it isn't a chemical reason. No reaction takes place. Anyway If I remember this rightly from long ago when I studied this sort of thing, the electrons in a metal are relatively free to move about (which is why metals conduct heat and electricity well). That means they have a lot of possible energy states and can absorb incoming radiation unusually well. Paradoxically, this is why they reflect the light. The absorbtion is very strong and the wavefunction of the photon is forced to fall to zero very abruptly. This rapid change in wavefunction is not easy to produce and the only way for the system to cope is to absorb the photon, then re-emit it immediately as a reflection. This isn't a very clear explanation and I hope someone comes up with a better one but, in short, it's the loose electrons that do it.

[chris]

This is a super answer from BC (written 5 years ago!) that I found by chance; thanks!

As an extension of this answer, what is the reason, then, that some metals are "silver" whereas others are "gold" or "coppery"? That is, despite having lots of free electrons that behave as Bored chemist explains above, why should some metals look a different base colour, and how do they do that?

Chris

[RD]

... why should some metals look a different base colour, and how do they do that?

looks like a resonance effect (e.g. 0% reflectivity silver @ 320nm ) ...



https://en.wikipedia.org/wiki/Reflectivity

https://en.wikipedia.org/wiki/Surface_plasmon_resonance  ?

[lightarrow]

It's possible to answer the OP using classical electodynamics only:
since a metal is a very good conductor, the electric field at the surface of it is zero even in presence of an incoming electromagnetic wave on it. This because if an electric field would be generated on it, electrons would move in an extremely rapid way to canceal that field in an incredibly short time.

But, if the E field on the metal surface is exactly zero, it means the metal have to re-emit an electromagnetic wave which canceal exactly (on the surface) the one impinging on it.

For this reason they are shiny.

[CZARCAR]

does this relate to KIRCHOFF'S law?