Four successive members of the first row transition elements are listed below with their atomic numbers. Which one of them is expected to have the highest third ionisation enthalpy?
- Vanadium (Z = 23)
- Chromium (Z = 24)
- Manganese (Z = 25)
- Iron (Z = 26)
Answer
(3) Manganese (Z = 25)
Solution
The third ionisation removes an electron from the M2+ ion:
V2+: 3d3, Cr2+: 3d4, Mn2+: 3d5, Fe2+: 3d6
Mn2+ has the stable half-filled 3d5 configuration
removing an electron from it (d5 d4) needs the most energy.
As per NCERT, the high third ionisation enthalpy of Mn shows that the d5 d4 change is difficult.
Manganese has the highest third ionisation enthalpy.
Answer: (3)