What is it about the atomic of molecular sturcture of atoms and molecules that gives it its colour?

Question

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?

In: 2

Answer 1

The way a material interacts with visible light – and therefore its colour – is determined by its electrons. Due to quantum mechanics, these electrons can’t have any amount of energy they want but are instead restricted to certain energy levels. They can jump between these energy levels by absorbing or releasing “little bits of light” (photons) of the right energy, which correspond to light of a certain colour. Other colours of light which cannot move an electron cleanly from one energy level to another cannot be absorbed or emitted.

>Why is copper typically considered a brownish colour on its own, but becomes bluish when in ionic form in water?

In copper metal, all of the copper atoms share their outer electrons and they become an “electron sea” throughout the entire piece of metal. This is very very different to the electrons in a copper ion, whose energy levels are determined by the single copper atom they are tightly bound to and the surrounding solution. The electrons are in completely different scenarios, so they have completely different energy levels, so they interact with different light and the result is a different colour.

>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?

All of this follows from the energy levels of the electrons which change in predictable ways governed by quantum mechanics. We can approximate the energy levels of atoms and molecules and therefore predict their colours. You also have to take into account how they interact with their local enviroment, as this can shift the energy levels.