# Which of the following salts are colourless?

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

Answer: (2) and (4)
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.
answered Jan 29, 2014
edited Aug 4, 2014