Is there a relationship between the loss of electrons in a cation and the amount of electromagnetic spectrum, a compound of this cation would subsequently reflect or transmit?

For example, any compound with $\ce{Fe^3+}$ ions is dark orange. This must mean compounds with $\ce{Fe^3+}$ reflect the longer wavelengths of the visible light spectrum more strongly, so the range of $\approx\pu{635–590 nm}$.

While a compound with a $\ce{Fe^2+}$ cations is light green. This, I think means that compounds that house $\ce{Fe^2+}$ ions reflect wavelengths of the range $\approx\pu{560–520 nm}$ more than others.

Now, I am not aware at all of any link between atomic electron loss influencing the transmission/reflection of wavelengths at all, so it could very well be something else entirely that may instead be influenced by the gain/loss of electron. I do not know.

So is there a relationship between the two? If so, what is it?

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    $\begingroup$ The is a direct link, but the topic is very complex, covered by UV-VIS molecular spectroscopy and quantum chemistry. $\endgroup$
    – Poutnik
    Commented Jun 3, 2019 at 12:11

1 Answer 1


Actually, I think you have it backwards. The color you see is due to the colors of light which are removed from white light when traveling past the iron. That is, if you go and find an absorption spectrum of iron cations in different oxidation states, you will find they primarily absorb the complementary colors of the color you actually see.

As far as the relationship between charge and the color, the ions with a smaller charge will likely have a larger HOMO-LUMO gap which means you will see colors of shorter wavelength, which is what you report. This property will depend very sensitively in the presence of ligands and other environmental changes, so it is not always possible to guess the color of a compound based only on charge.

This is typically how colors work for systems that absorb light. It is, however, possible for the we see to be due to reflection. Pure reflection, though, is independent of wavelength, so the phenomenon you describe is actually scattering, which does depend on wavelength. This type of phenomenon is responsible for the color of the sky.

Finally, you sometimes see the color which is emitted after absorption, as is the case in fluorescence and phosphorescence.

Hopefully that clears up your confusion

  • $\begingroup$ I thought that x coloured objects would reflect x wavelength of light. for example red objects would reflect red light, are you suggesting that the light is absorbed as opposed to reflected? $\endgroup$
    – Hisham
    Commented Jun 1, 2019 at 22:12
  • $\begingroup$ @Ubaid Hassan I am saying that it depends on the material. For instance, a wall that is painted red will have some material in it which primarily absorbs blue light. Thus, the light that is reflected back is going to be a mixture of all the visible wavelengths other than blue. Therefore, you will tend to see red light. $\endgroup$
    – jheindel
    Commented Jun 1, 2019 at 23:08
  • $\begingroup$ See, for instance, this website, especially the picture that shows the absorption spectrum of chlorophyll. Chlorophyll is what makes leaves appear green, but this is because chlorophyll absorbs blue and red light, not because it reflects green light. You are not wrong to say a particular color is reflected. That is a necessary but not sufficient condition for an object to appear a certain color. $\endgroup$
    – jheindel
    Commented Jun 1, 2019 at 23:09

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