$word_{count}$: 200

While +3 dominates lanthanoid chemistry, certain elements show anomalous +2 or +4 states driven by the special stability of $f^0$, $f^7$, and $f^{14}$ configurations.

$Ce^{4+}$ (4$f^0$): Empty 4f subshell. Strong oxidising agent (E degree = +1.61 V). CeO2 used in catalytic converters and glass polishing. The only common +4 lanthanoid.

$Eu^{2+}$ (4$f^7$): Half-filled 4f gives extra exchange energy. Reducing agent. Similar ionic radius to $Ba^{2+}$, so EuSO4 is insoluble like BaSO4. Used for Eu separation.

$Yb^{2+}$ (4$f^{14}$): Fully filled 4f. Less commonly encountered than $Ce^{4+}$ or $Eu^{2+}$.

$Tb^{4+}$ (4$f^7$): Another route to half-filled stability, but rare.

$Gd^{3+}$ (4$f^7$): Already achieves half-filled stability in the standard +3 state, so no anomalous oxidation states. Has 7 unpaired electrons and maximum paramagnetism, used in MRI contrast agents.

Actinoids show wider OS ranges (+3 to +7) because 5f, 6d, and 7s energies are comparable. U can be +3 to +6 (UF6 for enrichment). The actinoid contraction is greater than lanthanoid contraction due to even poorer 5f shielding. All actinoids are radioactive.