JEE Main · 2023 · 06 Apr 2023 · Shift 1 · d and f Block Elements

The strong reducing agent must gain stability on being oxidised, and the strong oxidiser must gain stability on being reduced. ${\mathrm{Eu}}^{2+}$ readily loses one electron to attain the half-filled $4f^7$ configuration of ${\mathrm{Eu}}^{3+}$, so it is a powerful reducing agent. ${\mathrm{Ce}}^{4+}$ readily accepts one electron to attain the stable $4f^1$ (near $f^0$ noble-gas) configuration of ${\mathrm{Ce}}^{3+}$, so it is a powerful oxidising agent. Taken in the order asked, the strong reducing and oxidising agents are ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Ce}}^{4+}$.