Cerium ($Ce$) is a Lanthanide with configuration $[Xe]4f^{1}5d^{1}6s^2$.

• Most lanthanides typically show $+3$ states. However, Cerium can also show a $+4$ state.
• Reason: By losing $4$ electrons ($1$ from $4f$, $1$ from $5d$, $2$ from $6s$), $C{e}^{4+}$ attains the electronic configuration of the noble gas Xenon ($4f^0$). This 'noble gas' empty $f$-shell state provides extraordinary stability.
• $C{e}^{4+}$ is a widely used analytical reagent because it rapidly reverts to the common $+3$ state by acting as an oxidizing agent.