Q.Which is more stable? Fe³⁺ or Fe²⁺ - explain.

Step 1. Compare electronic configurations: Fe³⁺ is [Ar]3d⁵ (an exactly half-filled, extra-stable d sub-shell), while Fe²⁺ is [Ar]3d⁶ (one electron paired up, not as symmetric).

Step 2. Recall the standard reduction potential for the M³⁺/M²⁺ couple: Fe³⁺ + e⁻ → Fe²⁺, E° = +0.77 V. This is a comparatively LOW positive value (contrast Mn's much higher +1.51 V, or Co's +1.81 V), meaning the drive to reduce Fe³⁺ down to Fe²⁺ is only moderate -- not the strong, decisive drive seen for Mn³⁺→Mn²⁺.

Step 3. Because this reduction potential is low (though still positive), BOTH Fe³⁺ and Fe²⁺ can and do coexist readily under normal laboratory and environmental conditions, without one form overwhelmingly dominating the other -- but Fe³⁺'s extra half-filled-shell stability generally makes it the somewhat more thermodynamically favoured, more persistent form in many aqueous and mineral contexts (e.g. rust, Fe₂O₃, is an Fe³⁺ compound). …

Compare Fe³⁺ and Fe²⁺'s electronic configurations for extra-stability effects, then use the M³⁺/M²⁺ standard reduction potential to judge which state is thermodynamically …

• Assuming a positive E°(M³⁺/M²⁺) automatically means the +2 state is strongly favoured, without noticing that Fe's value (+0.77 V) is comparatively LOW next to Mn's (+1.51 V) or Co's (+1.81 V), …