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CBSE Class 12 · 2003 · Delhi · d and f Block ElementsVery Short Answer

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Why is the third ionisation energy of manganese (Z = 25) unexpectedly high?

Answer: Mn²⁺ is 3d⁵ — exactly half-filled and extra stable; removing the third electron breaks this configuration.

Solution

Mn has the configuration [Ar]3d — an exactly half-filled d-subshell with maximum exchange-energy stability.
The third ionisation must disrupt this stable arrangement (3d → 3d), so IE of manganese is exceptionally high.

CBSE Class 12 Board · 2003 · Delhi · Subjective

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