I read in my Chemistry AP class that the colour of the transition metal complexes is due to the wavelength of the visible light that is left unabsorbed, when the d subshell is split due to the repulsion between the ligand electrons and d subshell electron in the transition metal ion. When white light is passed through the complexes, the wavelength that corresponds to the energy ($\Delta E$) required to promote electrons in the split d subshell is absorbed. The part of the visible light wavelength that is left unabsorbed becomes the colour of the transition metal complex.

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I was intrigued by a concept of absorption of light, relating it to absorption and emittion spectra. My question is: If we keep that complex isolated in the dark, where no light could reach it would the colour of the complex still be blue? I understand if that is done it wouldn't be visible to the naked human eye. I wonder if there is a way to find that out. Secondly, I was wondering if the colour elements obtain their colour in a similar way?

  • $\begingroup$ The concept of "color" would be meaningless in a pitch black room. Color depends on the wavelength (or group of) of light that reaches the eye. If there is no light to be detected, how can you decide on the color? $\endgroup$ – Pritt says Reinstate Monica Mar 30 '18 at 14:33
  • $\begingroup$ Pritt Balagopal, I did write that concern down. I was hoping it might be possible. I without percieving it through a naked eye, and rather through some technology. $\endgroup$ – Shah Saud Mar 30 '18 at 15:45

Most colour in chemistry comes from selective absorption of light by electronic transitions of the appropriate energy

Colour is not limited to transition metal complexes. They have a particularly rich range of colours as the electronic transitions often match the energy range of visible light but also because they are easily altered by the nature of the ligands surrounding the metal (for example, Cobalt complexes can be almost any colour of the spectrum depending on the ligand).

But plenty of other compounds are coloured. Most dyes are purely organic and the colour comes from the absorption of light in transitions between molecular orbitals made from unsaturated chains in the molecules. Many elements are coloured because they, too, have some electronic transmission that matches the energy in some part of visible light. The mechanism for colour is the same in all these cases.

There are other ways to generate colour but they are less common. Some colours come from interference not selective absorption of light: some physical structures that contain features close to the wavelength of light can reflect light but in a way that causes selective interference that cuts out some colours. This is why oil films on water produce rainbow colours but is also the source of unusual colours in some fruits, butterflies and birds.

Are things coloured if the light isn't present? That is a philosophical question like "does a tree falling the the forest make a sound if nobody hears it". From a chemical point of view the feature that creates colour is still present whether or not there is light. But in the absence of light nobody is going to notice it.

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  • $\begingroup$ I was hoping to maybe learn if there is some sort of advance technology present to make the deduction of colour without light hitting them. I find it intriguing $\endgroup$ – Shah Saud Mar 30 '18 at 15:48
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    $\begingroup$ @ShahSaud The chemical structures (and their electronic transitions) exist whether or not there is light present. They are a function of the molecular structure (which can be observed without light by a variety of techniques). The electronic property that creates light is there: but asking whether there is colour when there is no light is straying into philosophy not chemistry. $\endgroup$ – matt_black Mar 30 '18 at 17:28
  • $\begingroup$ A bit further than than the tree in forest. At least in that case something happens, which is not the case for something standing in dark. Plus 1 $\endgroup$ – Alchimista Mar 31 '18 at 9:23

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