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. `Fe(Z=26)`
B. `Mn(Z=25)`
C. `Cr(Z=24)`
D. `Co(Z=27)`

Answer

To determine which of the four successive first-row transition elements has the highest third ionization enthalpy, we will analyze the electronic configurations and the stability of the resulting ions after the removal of electrons.

### Step-by-Step Solution:

1. **Identify the Elements and Their Electronic Configurations:**
- Iron (Fe), Atomic Number 26: Electronic configuration is [Ar] 3d^6 4s^2.
- Manganese (Mn), Atomic Number 25: Electronic configuration is [Ar] 3d^5 4s^2.
- Chromium (Cr), Atomic Number 24: Electronic configuration is [Ar] 3d^5 4s^1.
- Cobalt (Co), Atomic Number 27: Electronic configuration is [Ar] 3d^7 4s^2.

2. **Remove Two Electrons for the Third Ionization:**
- For Iron (Fe): After removing two electrons, the configuration becomes [Ar] 3d^6 (removing 4s electrons).
- For Manganese (Mn): After removing two electrons, the configuration becomes [Ar] 3d^5 (removing 4s electrons).
- For Chromium (Cr): After removing two electrons, the configuration becomes [Ar] 3d^4 (removing 4s and one 3d electron).
- For Cobalt (Co): After removing two electrons, the configuration becomes [Ar] 3d^7 (removing 4s electrons).

3. **Analyze the Stability of the Resulting Ions:**
- Iron (Fe) will lose a third electron from the 3d^6 configuration. The resulting ion, Fe^3+, will have the configuration [Ar] 3d^5, which is half-filled and stable.
- Manganese (Mn) has a half-filled 3d^5 configuration. Removing an electron from this stable configuration (Mn^2+) will require a significant amount of energy.
- Chromium (Cr) will lose a third electron from the 3d^4 configuration, resulting in Cr^3+ with a configuration of [Ar] 3d^3, which is less stable than half-filled.
- Cobalt (Co) will lose a third electron from the 3d^7 configuration, resulting in Co^3+ with a configuration of [Ar] 3d^6, which is also relatively stable.

4. **Determine the Highest Third Ionization Enthalpy:**
- The third ionization enthalpy is highest for the element whose resulting ion configuration is the most stable. Since Mn^2+ has a half-filled 3d^5 configuration, it is very stable, making it difficult to remove another electron.
- Therefore, the highest third ionization enthalpy will be for Manganese (Mn) because it requires a large amount of energy to remove an electron from a stable half-filled configuration.

### Conclusion:
The element expected to have the highest third ionization enthalpy is **Manganese (Mn)**.