Question: Why is copper (I) oxide coloured?

Christopher Blanford answered on 11 Mar 2016:

Hi Matt covus

I’m glad you followed up on this.

Based on your questions, I’m going to assume that you’re OK if I drop a load of science on you. I don’t know what level you’re at, so I’ll have a lot of detail and try to avoid the jargon.

I’m also going to assume that if I get details wrong, Harriet or Andy will correct me. I’d rather be corrected than ignorant.

So, you were right: Cu(II) compounds are usually coloured and Cu(I) ones are typically not. This is true for molecules. The Cu(II) molecules have a empty space where one of their electrons go when it absorbs visible light. The Cu(I) molecules have an extra electron, and so don’t have the gap, and so don’t absorb visible light.

When you’re in a solid, the game changes. The way the electrons are arranged doesn’t just depend on the organic parts around it, but also on where the other copper and oxygen ions are. All of the gaps between where electrons sit normally and the spaces where they can go merge together. In Cu2O, you end up with a gap in energies between where the electrons sit and where the empty space for them to go. In Cu2O, this gap has an energy of 2.1 electronvolts. Every particle of light, or photon, that has more energy than that will be absorbed because it has enough energy to cross that gap; those that are lower won’t be absorbed.

The energy of this gap works out to being the colour of yellowy orange light (580 nm). This means that all the colours except red will be absorbed by Cu2O, and the red and some orange come through. We see that colour come out.

Put something in the comments if you want me to explain more or point you toward more details.

Chris