Account for the following:
Cu+2 salts are coloured, while Zn+2 salts are white.
Zn2+ salts are white while Cu2+ salts are coloured. Why ?
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Solution 1
Zn2+ salts have a completely filled set of d-orbitals (3d10), while Cu2+ has an incompletely filled set of d-orbitals (3d9), and therefore, d–d transition is possible in Cu2+, leading to blue colour.
Solution 2
| Element/Ion | Electronic Configuration |
| Zn | 3d10 4s2 |
| Zn2+ | 3d10 |
| Cu | 3d10 4s1 |
| Cu2+ | 3d9 |
The colours of salts of transition metals are due to the d-d transition that depend on the presence of incompletely filled d-orbitals. Zn2+ has completely filled d-orbitals (3d10), while Cu2+ has incompletely filled d-orbitals (3d9); therefore, d-d transition is possible in Cu2+, which imparts colour to copper(II) salts.
Concept: General Properties of the Transition Elements (D-block)
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