Which of the following ion is expected to be colourless? — Atomic Structure Chemistry Question

💡 Solution & Explanation

Hydrogenic or transition-metal ions that lack any partially filled d-subshell (i.e., d^0 or d^10 configurations) cannot undergo d-d electronic transitions in the visible range and are therefore colourless. Let's examine the configurations: (a) Ni^2+ (Z = 28) = [Ar] 3d^8: partially filled d-subshell (coloured). (b) Mn^2+ (Z = 25) = [Ar] 3d^5: partially filled d-subshell (coloured). (c) Zn^2+ (Z = 30) = [Ar] 3d^10: completely filled d-subshell (colourless). (d) Cu^2+ (Z = 29) = [Ar] 3d^9: partially filled d-subshell (coloured). Thus, Zn^2+ is expected to be colourless.