`Cu^(+)` salts are colourless but `Cu^(2+)` salts are coloured. Give reasons.

Explanation

`Cu^(+)([Ar]3d^(10)4s^(0))` ion has completely filled 3d - orbital. Hence d - d transition is not possible by absorption of visible light. But `Cu^(2+)([Ar]3d^(9)4s^(0))` ion has incompletely filled 3d - subshell. The d - d transition of electrons takes place by absorbing wavelengths from visible light. Hence, `Cu^(+)` salts are colourless, whereas `Cu^(2+)` salts are coloured.