Four successive members of the first row transition elements are listed below with their atomic number. Which one of them is expected to have the highest third ionisation enthalpy ?

Options

A. Vanadium (Z = 23)
B. Chromium (Z = 24)
C. Manganese (Z = 25)
D. Iron (Z = 26)

Answer

To determine which of the four successive members of the first row transition elements has the highest third ionization enthalpy, we need to analyze the electronic configurations of each element and understand the stability of their electron configurations after the removal of electrons.

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers**:
- Vanadium (V) - Atomic Number: 23
- Chromium (Cr) - Atomic Number: 24
- Manganese (Mn) - Atomic Number: 25
- Iron (Fe) - Atomic Number: 26

2. **Write the Electronic Configurations**:
- Vanadium (V): \( \text{[Ar]} 3d^3 4s^2 \)
- Chromium (Cr): \( \text{[Ar]} 3d^5 4s^1 \)
- Manganese (Mn): \( \text{[Ar]} 3d^5 4s^2 \)
- Iron (Fe): \( \text{[Ar]} 3d^6 4s^2 \)

3. **Understand the Concept of Ionization Enthalpy**:
- Ionization enthalpy is the energy required to remove an electron from an atom in the gas phase.
- The third ionization enthalpy refers to the energy required to remove the third electron from the atom.

4. **Determine Which Electron is Removed**:
- For the third ionization enthalpy, we focus on the removal of the third electron from the 3d subshell.
- The stability of the remaining electron configuration after the removal of the third electron is crucial.

5. **Analyze the Stability of the Remaining Configuration**:
- For Manganese (Mn), after removing three electrons, the configuration will be \( 3d^4 \) (from \( 3d^5 \)).
- For Chromium (Cr), after removing three electrons, the configuration will be \( 3d^2 \) (from \( 3d^5 \)).
- For Iron (Fe), after removing three electrons, the configuration will be \( 3d^5 \) (from \( 3d^6 \)).
- For Vanadium (V), after removing three electrons, the configuration will be \( 3d^2 \) (from \( 3d^3 \)).

6. **Identify the Most Stable Configuration**:
- The half-filled \( 3d^5 \) configuration (in Manganese and Chromium) is particularly stable due to exchange energy and symmetry.
- Manganese (Mn) has a half-filled \( 3d^5 \) configuration before the removal of the third electron, making it more stable than the others after ionization.

7. **Conclusion**:
- Since Manganese retains a half-filled configuration after the removal of the third electron, it is expected to have the highest third ionization enthalpy among the given elements.

### Final Answer:
**Manganese (Mn)** is expected to have the highest third ionization enthalpy.