Which of the following set represents only coloured ions?
(1) \(Fe^{3+}\)     (2) \(Sc^{3+}\)    (3) \(Cu^{2+}\)     (4) \(Cr^{3+}\)

1, 3 and 4

The correct answer is (4), 1, 3 and 4.

The ions \(Fe^{3+}\), \(Cr^{3+}\) and \(Cu^{2+}\) have unpaired electrons in their d orbitals, which can absorb photons of light in the visible region. This causes the electrons to be excited to a higher energy d orbital, and the ion appears coloured.

The ion \(Sc^{3+}\) has a filled d orbital, so it does not have any unpaired electrons. Therefore, Sc3+ is colourless.