Among the transition metals with the following atomic numbers, which one has the lowest third ionisation enthalpy?

Step-by-step solution

The elements corresponding to the given atomic numbers are Mn ( Z=25 ), Fe ( Z=26 ), Co ( Z=27 ), and Ni ( Z=28 ). The third ionisation enthalpy corresponds to the energy required to remove an electron from the M ²⁺ ion. The electronic configurations of the +2 ions are: Mn ²⁺ ( Z=25 ): [ Ar ] 3 d ⁵ Fe ²⁺ ( Z=26 ): [ Ar ] 3 d ⁶ Co ²⁺ ( Z=27 ): [ Ar ] 3 d ⁷ Ni ²⁺ ( Z=28 ): [ Ar ] 3 d ⁸ Removing an electron from Fe ²⁺ ( 3 d ⁶ ) yields Fe ³⁺ ( 3 d ⁵ ), which has a highly stable half-filled d-subshell. Because this proc