Which of the following acts as a strong reducing agent? (Atomic number: Ce = 58, Eu = 63, Gd = 64, Lu = 71)

Concept: Reducing agents in lanthanoids

A species acts as a reducing agent if it readily loses electrons (gets oxidised).

Analysis:

| Species | Config | Tendency | Role | |---------|--------|----------|------| | $\text{Lu}^{3+}$ | $4f^{14}$ | Fully filled — very stable | Neither | | $\text{Gd}^{3+}$ | $4f^7$ | Half-filled — very stable | Neither | | $\text{Eu}^{2+}$ | $4f^7$ | Wants to reach $4f^7$ (half-filled) by losing e⁻ → $\text{Eu}^{3+}$ | Strong reducing agent ✓ | | $\text{Ce}^{4+}$ | $4f^0$ | Wants to gain e⁻ → $\text{Ce}^{3+}$ ($4f^1$) | Strong oxidising agent |

$\text{Eu}^{2+}$ ($4f^7$) is a strong reducing agent because it readily loses one electron to give $\text{Eu}^{3+}$ ($4f^6$)...

Actually: $\text{Eu}^{2+}$ ($4f^7$, half-filled) is stable but acts as reducing agent by converting to $\text{Eu}^{3+}$. The driving force is that $\text{Eu}^{2+}$ has a tendency to be oxidised to the more common +3 state.

Answer: Option (3) — $\text{Eu}^{2+}$