Does $C u^{2 +}$ ion show colour?

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Solution

$C u^{2 +}$ ion is blue in colour. Its configuration is $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{9}$ .
In order to absorb visible light, an electron would have to move to a higher energy orbital. The difference in energy of the orbitals would have to equal the energy of a visible wavelength photon. It has one space in $3 d$ orbital making it a transition metal. For d orbital splitting, the energy difference is right for the visible range and therefore, $C u^{2 +}$ is blue in colour.

(In a normal metal atom, the only possible energy jumps are too great for visible light to be affected, and so the solutions appear colourless. However, transition metals are different. When there are ligands present, the 3d level splits up into two levels of energy, with two orbitals at a slightly higher level and three at a lower level. The energy jump when an electron is transferred between two of these levels corresponds to the energy of visible light. This is why the solutions of the transition ions are coloured.)

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