Question

Predict which of the following will be coloured in aqueous solution? 

Ti³⁺, V³⁺, Cu⁺ , Sc³⁺, Mn²⁺, Fe³⁺ and Co²⁺. Give reasons for each.

Answer 1

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.