i. Transition metal ions exhibit colour due to the presence of unpaired electrons in (n-1)d-orbitals which undergo d-d transition.
ii. The metal ions which do not have unpaired electrons i.e., (n-1)d0 or which have completely filled d-orbitals i.e., (n-1)d10 do not absorb radiations in visible region, since d-d transitions are not possible. Hence, they are colourless ions.
iii. Ti3+, V3+, Mn2+, Fe3+ and Co2+ contain 1, 2, 5, 5 and 3 unpaired electrons respectively in their (n-1) d-orbital. Hence, they are coloured in aqueous solution.
iv. Cu+ and Sc3+ do not contain unpaired electrons. Hence, they are colourless.