B Colour of Compounds

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1. Mar 13, 2018

PumpkinCougar95

I am not able to understand why compounds have colour. I have read that it is due to excitation of electrons from one atomic/Molecular orbital to another because of which certain wavelengths are absorbed, And we can see the "complementary colour" of those wavelengths.

What I don't understand is that when an electron gets excited, it has to eventually de-excite and come back to its initial orbital. Now let's say that on average an electron remains excited for some time $t$. After $t$ time, shouldn't there be an equal number of electrons getting excited per sec to those getting de-excited?

If this were to happen, the same wavelength that is absorbed by one atom should be emitted by another atom? Wouldn't this mean that the compound is colourless?

I am confused, and any clarification would be greatly appreciated.

2. Mar 13, 2018

sophiecentaur

But not necessarily via the same single transition. What is initially taught about EM interaction with atoms is not enough for this.
The energy at the incident frequency can be lost for ever (absorbed). In the solid state, there are not simple line transitions (you get energy bands) and energy passes around within the solid. Even in a low density gas, the energy that's absorbed from one direction will re radiate in all directions, causing energy at certain values to be scattered - less of the original wavelength passes through with the rest of the beam.

3. Mar 14, 2018

PumpkinCougar95

Thanks ,that makes sense.