Transition elements
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During the reaction between copper(II) sulphate and potassium iodide a white precipitate is formed of copper(I)iodide. Why is the copper(I) iodide white, while copper(II) compounds are coloured?
Colour in transition metal ions often arises when the 3d energy levels are not full and when the ligands around the metal ion are such that the five energy levels split into two groups with slightly different energies. This means that electrons can jump from a lower energy level to a higher level absorbing energy corresponding to a photon in the visible part of the spectrum. If all the 3d energy levels are full, then electronic transitions of this kind are not possible.
The electron configuration of the copper(II) ion is [Ar]3d9, while in the copper(I) ion the configuration is [Ar]3d10. So electronic transitions are not possible in the d energy levels of the copper(I) ion because they are full.
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updated: 21 April 2004
