How do transition elements form coloured compounds?

2 Answers
Jul 4, 2017

Movement of electrons between shells


I believe that due to the d orbitals being inside the outer s orbital, electrons are able to move into that s orbital if they have the required energy. They then drop back down to the d one emitting a photons at specific frequencies giving them their colour.

Aug 15, 2017

Transition elements form coloured compounds because they have unfilled #"d"# orbitals.


Transition metal ions are not coloured on their own.

It is only when they form complexes with other ions or molecules that they become coloured.

In a transition metal, the #"d"# orbitals are degenerate — they all have the same energy.

However, when the metal ion is complexed with other ions or molecules, some of the #"d"# orbitals become higher in energy than the others.

One common pattern is shown in the diagram above.

The difference in #"d"# orbital energy levels often corresponds to the wavelength of visible light.

Thus, an electron in a lower #"d"# level may absorb a quantum of red light and be excited to the higher level.

The non-absorbed light is reflected back to our eyes, so we would probably see a blue or green colour.

Note: a transition metal ion that has zero or ten #"d"# electrons will be colourless.

Here are the colours of some transition metal ions in aqueous solution.