[cuso4 (OP)]

Transition metal compounds are coloured. Is the colour produced by excited electrons falling to a lower energy level?

From the example Pat said in the other topic: copper sulphate is originally a pale blue colour, once excess ammonia is added it becomes a rich dark blue. What cause the colour to become deeper? And how is electron get excited in the first place?


AG

[Quantumcat]

An electron gets excited when a piece of energy hits it. We have only really covered photons at school but I guess any part of the electromagnetic spectrum that hits it will get it excited ... anyway the shorter the wavelength (I think ... ?) the more energy levels it jumps up(correct me if I'm wrong, I probably am), and then falls back down to the original level, liberating the energy it gained exactly the same as it came in.

[adianadiadi]

Copper(II) sulphate solution, for example, contains the blue hexaaquacopper(II) ion - [Cu(H₂O)₆]²⁺. It is symmetrical and the d-d transitions are symmetry forbidden. Hence pale blue color.

But the complex formed after adding excess ammonia is [Cu(NH₃)₄(H2O)₂]²⁺. It is not much symmetric as that of complete hydrate. This shows deep blue color as the transition is not forbidden

[chris]

What does "forbidden" mean, and why should this affect the colour?

Chris