Question

Discuss the stability of lanthanoids in aqueous solutions.

Answer 1

1. The most stable oxidation state of lanthanoids in aqueous solutions is +3. Other states like +2 and +4 are less common and less stable in solution.
2. The +3 state is more stable because the combined ionisation and lattice energies of tripositive ions are more negative, making them energetically favored.
3. +2 states occur only in some lanthanoids (Sm, Eu, Yb, etc.) and are strong reducing agents but not very stable; +4 states are rare and limited to a few elements like Ce and Pr, often existing as unstable oxides.
4. In solution, all lanthanoids mostly exist as Ln³⁺ ions, which form various salts and complexes, showing good stability.
5. The greater stability of +3 ions also stems from electronic configurations and the energy favorability of losing three electrons.

Thus, lanthanoids mainly exist as stable +3 ions in aqueous solutions, with +2 and +4 states being exceptions and less stable overall.