Brightly coloured crystals and solutions often contain metal ions in different valence states. Light of a certain frequency is absorbed and this gives a colour to the crystal or solution. The absorption causes electron transfer and the metal ion changes its valency.
    In the Marcus model this frequency is n = l /h where l is the reorganization energy and h the Planck constant.


Amethyst and aquamarine get their colours from the presence of iron ions as 'impurities' in different valence states in the minerals quartz and beryll, respectively.


If yellow hexacyanoferrate(II)
is mixed with Fe(III) chloride
in aqueous solution, 'Prussian
blue' is formed.

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