Question
Cu forms coloured salts while Zn forms colourless salts.

Answer

- Electronic configuration of ${ }_{29} CU$ is $[ Ar ] 3 d^{10} 4 s^1$ while that of Zn is $[ Ar ] 3 d^{10} 4 s^2$.
- Electronic configuration of Cu in its +1 oxidation state is $[ Ar ] 3 d^{10}$ while that in +2 oxidation state is $[ Ar ] 3 d^9$.
- Therefore, Cu contains partially filled d orbitals in +2 oxidation state and thus, $Cu ^{2+}$ salts are coloured.
- However, Zn has completely filled d orbital which is highly stable and hence, it does not form coloured ions.
Hence, Cu forms coloured salts while Zn forms colourless salts.

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