JEE Main · 2024 · 29 Jan 2024 · Shift 2 · d and f Block Elements

Among lanthanide ions, +3 is the common and thermodynamically favoured oxidation state, so an ion sitting at an unusual +2 or +4 state tends to move toward +3, and that tendency sets its redox behaviour.

$\mathrm{E}{\mathrm{u}}^{2+}$ ($[\mathrm{Xe}]4f^7$) readily loses an electron to reach the standard $\mathrm{E}{\mathrm{u}}^{3+}$ state, so it acts as a reducing agent:


By contrast, $\mathrm{C}{\mathrm{e}}^{4+}$ is a strong oxidising agent because it is readily reduced to the stable $\mathrm{C}{\mathrm{e}}^{3+}$ state, while $\mathrm{L}{\mathrm{u}}^{3+}$ ($f^{14}$) and $\mathrm{G}{\mathrm{d}}^{3+}$ ($f^7$) already sit at the stable +3 state and show no comparable redox tendency.

$\mathrm{E}{\mathrm{u}}^{2+}$ is the strong reducing agent.