Cerium ($\mathrm{Ce}$)

Cerium is a lanthanide chemical element with symbol $\mathrm{Ce}$ and atomic number $58$. Its accessible $+3$ and $+4$ oxidation states distinguish much of its chemistry from rare earths dominated by only $+3$.^[ref]

Changing the cerium oxidation state exchanges an electron with the surroundings and can couple to oxygen defects in cerium-containing oxides. Such redox flexibility helps explain why composition, defect population and chemical environment must be specified when describing a cerium material.