Transition metal ion colours. The coloured solutions are of transition metal salts, with one colourless solution of a non-transition metal salt (sodium sulphate) for comparison. The electronic structure of transition metals absorb light at particular wavelengths, giving rise to strongly coloured ions and complexes. From left to right, the solutions and their chemical formulae are: iron (II) sulphate (FeSO4), sodium sulphate (Na2SO4), copper (II) sulphate (CuSO4), nickel (II) sulphate (NiSO4), cobalt (II) chloride (CoCl2), and iron (III) chloride (FeCl3).