Statement I: The ionisation enthalpy of Mn+ is higher than that of Cr…

The correct answer is Option (2) → Only Statement II is correct.

Exchange energy stabilises a set of electrons with parallel spins in degenerate orbitals, and it is largest for the half-filled d⁵ set. Removing an electron from a stable d⁵ set costs more energy, while removing one from d⁶ causes no loss of exchange energy and is easier.

Statement I: The ionisation enthalpy of Mn⁺ is higher than that of Cr⁺. — False. Cr⁺ is d⁵, so its next electron must come out of the stable half-filled set. Mn⁺ is 3d⁵4s¹ and loses the 4s electron, leaving the d⁵ set intact. So the ionisation enthalpy of Mn⁺ is lower than that of Cr⁺.

Statement II: The third ionisation enthalpy of iron is lower than that of manganese. — True. The third ionisation removes an electron from M²⁺. Fe²⁺ is d⁶ and becomes the stable d⁵ Fe³⁺; Mn²⁺ is d⁵ and would have to lose that stable arrangement. So Fe²⁺ is ionised more easily.

NCERT (2026-27 edition):
4.3.3 Ionisation Enthalpies – “…Exchange energy is responsible for the stabilisation of energy state.” … “There is no loss of exchange energy at d⁶ configuration. Mn⁺ has 3d⁵4s¹ configuration and configuration of Cr⁺ is d⁵, therefore, ionisation enthalpy of Mn⁺ is lower than Cr⁺. In the same way, Fe²⁺ has d⁶ configuration and Mn²⁺ has 3d⁵ configuration. Hence, ionisation enthalpy of Fe²⁺ is lower than the Mn²⁺. In other words, we can say that the third ionisation enthalpy of Fe is lower than that of Mn.”