Why is copper typically considered a brownish colour on its own, but becomes bluish when in ionic form in water? What changes to make it absorb a different wavelength of light? Is there a rhyme or reason to the colour changes or is it just a case-by-case basis. If we knew the structure of a molecule, would we know exactly what to change to change its colour?
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This is a basic spectrometry question, and I don’t really work with that much as a chemical engineer. The basic explanation still holds as far as I remember.
At the atomic scale, energy is quantized. Electromagnetic radiation is a form of this quantized energy, and each chemical structure absorbs specific bands that complement its energy gaps. The light we detect is the remainder of the full spectrum, or the portion that wasn’t absorbed.
ELI5: Think of a copper as a vending machine that doesn’t take coins. This machine will only take $1 or $5 bills. Now think of light as a stack of bills of many denominations. If you go to buy something from this vending machine, you’ll only be able to use exact change for your purchase, anything else will be rejected.
Now, what if this machine doesn’t have prices displayed? You could try each denomination, until it takes one of them, the rest will be rejected back to you. Therefore, if you insert a $5 bill for your purchase and it gets rejected, then it must be $1 item. In this scenario, the only tangible base of information you have is the fact that machine rejects $5 bills. All you see is $5 bills back to you.
A copper atom will interact with light (stack of money) and it will keep some, and rejects the ones it doesn’t take (rejected $5 bills). The rejected light is the color will you see. In summary, Copper absorbs all visible light, except for the color that you see (it was rejected back to you).
Now why does it do that? The machine doesn’t take fractions of denominations, and copper doesn’t take fractions of energy. The energy in this instance corresponds to something called electronic levels energy. A machine will only sell you a $1 item or a $5 item, or a combination of both. Copper will only take energy that matches one field, or another, or both, but not fractions of levels. Because of that, a machine will only reject $1, $5, or nothing at all if you use exact change. Copper will only reflect one color, or another, or nothing at all.
This is an oversimplification of a complicated topic, it is a fascinating and fundamental part of our understanding of the atom in general. It also a unique identification tool, that allows us to analyze what chemical structures look like, based on the rejected light spectrums. Also theres more than just visible light that plays into this, but that beyond the scope of your question.
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