In the lanthanoids, the most common and stable oxidation state is +3. Ions that exhibit +2 and +4 states tend to revert to the +3 state to achieve stability. The formation of $Ce^{IV}$ is favoured by its stable noble gas configuration, but it acts as a strong oxidant (oxidising agent) because it readily accepts an electron to revert to the common +3 state. Conversely, $Eu^{2+}$ is formed by losing its two s electrons, leaving a highly stable half-filled $f^7$ configuration. However, it acts as a strong reducing agent (reductant) as it loses an electron to change back to the common +3 state . Thus, the pair with one strong oxidant and one strong reductant is Ce(IV) and Eu(II).