Which of the following ions will exhibit colour in aqueous solution ?

Options

A. `Sc^(3+) (Z = 21)`
B. `La^(3+) (Z = 57)`
C. `Ti^(3+) (Z = 22)`
D. `Lu^(3+) (Z = 71)`

Answer

C

Explanation

Most of the compounds of transition metals are coloured in the solid or in solution states. The color of transition metal ions arises form the excitation of electrons form the d-orbitals of lower energy to the d-orbitals of higher energy. The energy required for d-d electron ecitations is available in the visible range. It is for this reason that transition metal ions have the property to absorb certain radiations form the visible region and exhibit the complementary color. The transition metals ions which have completely empty d-orbitals `(d^(0))` or which have completely filled d-orbitals `(d^(10))` are colorless as the excitation is not possible form one d-orbital to another. Similar inner transition metal ions colored on account of f-f transition which is absent for `f^(0)` or `f^(14)` configuration.
`La^(3+) (Z = 57) implies [Xe] 4f^(0) 5d^(0) 6s^(0)`
`Ti^(3+) (Z = 22) implies [Ar] 3d^(1) 4s^(0)`
`Lu^(3+) (Z = 71) implies [Xe] 4f^(14) 5d^(0) 6s^(0)`
`Sc^(3+) (Z = 21) implies [Ar] 3d^(0) 4s^(0)`
Thus, only `Ti^(3+)` with one unpaired electron in the subshell exhibits colour.