CBSE Class 12 · 2006 · All India · d and f Block ElementsShort Answer

Why is the +2 oxidation state of manganese quite stable, while the same is not true for iron? [Z: Mn = 25, Fe = 26]

Answer: Mn²⁺ is 3d⁵ — exactly half-filled and hard to oxidise; Fe²⁺ is 3d⁶ and oxidises readily to Fe³⁺ (3d⁵, half-filled).

Solution

^{2 +}

: [Ar]3d

^{5}

— the exactly half-filled d-subshell has maximum exchange stability, so Mn(+2) resists both oxidation and reduction — very stable.
Fe

^{2 +}

: [Ar]3d

^{6}

— one electron beyond the half-filled shell; losing it gives the stable 3d

^{5}

of Fe

^{3 +}

, so Fe(+2) is easily oxidised (even air oxidises ferrous to ferric). Hence +2 is far less stable for iron than for manganese.

CBSE Class 12 Board · 2006 · All India · Subjective