The characteristic colours exhibited by trivalent lanthanide cations in aqueous solutions arise from — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Color and paramagnetism require unpaired $f$ -electrons. $La^{3+}$ has completely lost all valence electrons, leaving an empty $4f^0$ subshell. $Lu^{3+}$ has lost its outer electrons but perfectly retains a closed, fully saturated $4f^{14}$ subshell. Having strictly zero unpaired electrons, both are colourless and diamagnetic.