Cr2+ is stronger reducing agent than Fe2+ because :

The correct answer is Option (2) → d³ configuration of Cr³⁺ is more stable than d⁵ configuration of Fe³⁺ in aqueous solution

A reducing agent is oxidised itself. So M²⁺ is a strong reducing agent when it easily loses one more electron to become M³⁺, that is, when the M³⁺ ion is especially stable in water.

• Cr: Cr (Z = 24) is [Ar] 3d⁵4s¹; \( \ce{Cr^{2+}} \) is 3d⁴ and \( \ce{Cr^{3+}} \) is 3d³. In aqueous (octahedral) complexes 3d³ is a half-filled t_2g level (t_2g³), which is very stable. So \( \ce{Cr^{2+}} \) readily changes to \( \ce{Cr^{3+}} \) (E° \( \ce{Cr^{3+}} \)/\( \ce{Cr^{2+}} \) = −0.41 V).

• Fe: Fe (Z = 26) is [Ar] 3d⁶4s²; \( \ce{Fe^{2+}} \) is 3d⁶ and \( \ce{Fe^{3+}} \) is 3d⁵. In water the \( \ce{Fe^{3+}} \) ion, though half-filled, is less favoured than \( \ce{Cr^{3+}} \) (E° \( \ce{Fe^{3+}} \)/\( \ce{Fe^{2+}} \) = +0.77 V), so \( \ce{Fe^{2+}} \) is a much weaker reducing agent.

    \( \displaystyle \ce{Cr^2+ -> Cr^3+ + e^-}\quad E^{\circ}_{\mathrm{ox}} = +0.41\ \mathrm{V} \)

    \( \displaystyle \ce{Fe^2+ -> Fe^3+ + e^-}\quad E^{\circ}_{\mathrm{ox}} = -0.77\ \mathrm{V} \)

Why the other options are incorrect

• Option (1) it has exactly full filled configuration — Incorrect. \( \ce{Cr^{2+}} \) is 3d⁴, neither full nor half-filled; that is why it tends to lose an electron.

• Option (3) oxidises easily to form Cr⁴⁺ — Incorrect. \( \ce{Cr^{2+}} \) is oxidised to \( \ce{Cr^{3+}} \) (one electron), not to \( \ce{Cr^{4+}} \); Cr(IV) is not a stable oxidation state in water.

• Option (4) Fe²⁺ is quite stable and does not oxidise — Incorrect. \( \ce{Fe^{2+}} \) does get oxidised to \( \ce{Fe^{3+}} \) (for example by air or by \( \ce{KMnO4} \)); it is only a weaker reducing agent than \( \ce{Cr^{2+}} \).

NCERT (2026-27 edition):
Answers to Some Intext Questions – “4.7 Cr²⁺ is stronger reducing agent than Fe²⁺
Reason: d⁴ → d³ occurs in case of Cr²⁺ to Cr³⁺
But d⁶ → d⁵ occurs in case of Fe²⁺ to Fe³⁺
In a medium (like water) d³ is more stable as compared to d⁵ (see CFSE)”