Why do the following transition metal ions appear colourless ? `Cu^(+), Ag^(+), Zn^(2+), Hg^(2+) and Cd^(2+)`.

Explanation

From the electronic configuration of the metal atoms, `Cu(3d^(10)4s^(1)), Ag(4d^(10)5s^(1)),Zn(3d^(10)4s^(2)), Hg(5d^(10)6s^(2))` and `Cd(4d^(10)5s^(-1)),` we can see that the given metal ions do not have unpaired electrons in their `(n-1)d` orbital and the orbitals are completely fielld `(d^(10))` with electrons : `Cu^(+)(3d^(10)), Ag^(+)(4d^(10)), Zn^(2+)(3d^(10)), Hg^(2+)(5d^(10)) and Cd^(2+)(4d^(10))`.
Hence, these electrons do not undergo `d-d` transition by absorbing light from visible spectrum and therefore the given metal ions appear colourless.