The element with the highest third ionisation enthalpy is:

Step-by-step solution

The electronic configurations of the given elements and their divalent ions are: Mn (Z=25): [Ar] 3d^5 4s^2 Mn²⁺: [Ar] 3d^5 V (Z=23): [Ar] 3d^3 4s^2 V²⁺: [Ar] 3d^3 Fe (Z=26): [Ar] 3d^6 4s^2 Fe²⁺: [Ar] 3d^6 Cr (Z=24): [Ar] 3d^5 4s^1 Cr²⁺: [Ar] 3d^4 The third ionization enthalpy is the energy required to remove an electron from the +2 ion. Mn²⁺ has a stable half-filled 3d^5 configuration. Removing an electron from this highly stable configuration requires a very high amount of energy. Therefore, Manganese has the high