Why do transition elements form coloured compounds?
Ulex says I will give you an outline which you could follow up yourself if you continue to be interested. The phenomenon of colour is a fascinating one. The colour of compounds relies on the subtractive principle the coloured material absorbs some of the wavelengths from white (visible) light; what we see is what is left. Absorption of light occurs when an electron jumps from one energy level to higher one. The difference in energy between the energy levels has to correspond to the energy of the wavelength absorbed. Most electronic transitions involving s and p electrons fall outside the visible spectrum, so that many compounds are colourless in solution. Transition metal ions, however, have partially filled d-orbitals. In the isolated atom these all have the same energy. However, when metal ions are surrounded by ligands in complexes, the d-orbitals separate into two groups, with one group having a slightly higher energy than the other. As it happens, the difference in energy here cor
