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# Which of the following salts are colourless?

$\begin {array} {1 1} (1)\; Cu^{2+} \quad (2)\; Zn^{2+} \quad (3)\; Ni^{2+} \quad (4) \;Sc^{3+} \end {array}$

Both $Sc^{3+}$ and $Zn^{2+}$ have completely filled d-orbitals, ($3d^0$ and $3d^{10}$ respectively), while the other two have incompletely filled d-orbitals. Hence they are colourless.
edited Aug 4, 2014