Ce^4+ (4f^0): Achieves noble-gas-like empty 4f configuration. Ce^4+ is a strong oxidising agent (E° for Ce^4+/Ce^3+ = +1.61V). CeO2 (ceria) used in self-cleaning ovens and automotive catalytic converters. Eu^2+ (4f^7): Achieves half-filled 4f stability. Eu^2+ is a reducing agent, similar to Ba^2+ in properties (similar ionic radius). EuSO4 is insoluble like BaSO4. Yb^2+ (4f^14): Achieves fully-filled 4f stability. Less common than Ce^4+ and Eu^2+. Tb^4+ (4f^7): Another half-filled configuration, but rare. These anomalous states arise from the extra stability of f^0, f^7, and f^14 configurations.