The Lanthanoid ion which would form coloured compounds is -------------. Atomic numbers: Yb =70, Lu =71, Pr =59, La =57

Step-by-step solution

Lanthanoid ions exhibit colour due to f-f electronic transitions. Ions with partially filled f -orbitals ( f^1 to f¹³ ) are coloured, while ions with f^0 or f¹⁴ configurations are colourless. Electronic configurations of the given ions are: 1. La ³⁺ ( Z=57 ): [ Xe ] 4f^0 . Since it has no f -electrons, it is colourless. 2. Lu ³⁺ ( Z=71 ): [ Xe ] 4f¹⁴ . Since the f -subshell is completely filled, it is colourless. 3. Yb ²⁺ ( Z=70 ): [ Xe ] 4f¹⁴ . Since the f -subshell is completely filled, it is colourless. 4. Pr ³⁺