Question

The chloride of a copper metal is colourless. Why ?

Answer 1

The copper atom is in the +1 oxidation state in which it has a full (totally occupied) set of 'd' orbitals. There can be no transitions between the non-degenerate orbitals and so there can be no frequencies of light absorbed due to d-d transitions. That is true for CuCl. $Cu:\left[Ar\right]3d^{10}4s^1$$C{u}^{+}:\left[Ar\right]3d^{10}$. Thus, copper is +1.

Therefore, it is $C{u}_{2}C{l}_2$ or $CuCl$ : Copper (I) chloride is colourless.