Which of the following metal is expected to have the highest third ionsation enthalpy?

Options

A. Cr(Z=24)
B. V(Z=23)
C. Mn(Z=25)
D. Fe(Z=26)

Answer

To determine which of the given metals has the highest third ionization enthalpy, we need to analyze the electronic configurations and the stability of the resulting ions after successive ionizations. The metals in question are chromium (Cr), vanadium (V), manganese (Mn), and iron (Fe).

### Step-by-Step Solution:

1. **Identify the Electronic Configurations**:
- Chromium (Cr): [Ar] 3d⁵ 4s¹
- Vanadium (V): [Ar] 3d³ 4s²
- Manganese (Mn): [Ar] 3d⁵ 4s²
- Iron (Fe): [Ar] 3d⁶ 4s²

2. **Understand Ionization Enthalpy**:
- Ionization enthalpy is the energy required to remove an electron from an atom in the gaseous state. The third ionization enthalpy (IE3) refers to the energy required to remove the third electron.

3. **Analyze the Third Ionization Process**:
- For Chromium:
- IE1: Cr → Cr⁺ + e⁻ (removing 1 electron)
- IE2: Cr⁺ → Cr²⁺ + e⁻ (removing 2nd electron)
- IE3: Cr²⁺ → Cr³⁺ + e⁻ (removing 3rd electron)
- Configuration after IE3: 3d³ (less stable due to incomplete subshell)

- For Vanadium:
- IE1: V → V⁺ + e⁻ (removing 1 electron)
- IE2: V⁺ → V²⁺ + e⁻ (removing 2nd electron)
- IE3: V²⁺ → V³⁺ + e⁻ (removing 3rd electron)
- Configuration after IE3: 3d² (less stable)

- For Manganese:
- IE1: Mn → Mn⁺ + e⁻ (removing 1 electron)
- IE2: Mn⁺ → Mn²⁺ + e⁻ (removing 2nd electron)
- IE3: Mn²⁺ → Mn³⁺ + e⁻ (removing 3rd electron)
- Configuration after IE3: 3d⁴ (half-filled stability is lost, but still relatively stable)

- For Iron:
- IE1: Fe → Fe⁺ + e⁻ (removing 1 electron)
- IE2: Fe⁺ → Fe²⁺ + e⁻ (removing 2nd electron)
- IE3: Fe²⁺ → Fe³⁺ + e⁻ (removing 3rd electron)
- Configuration after IE3: 3d⁵ (stable due to half-filled subshell)

4. **Compare the Stability of the Resulting Ions**:
- Manganese (Mn) has a half-filled 3d subshell (3d⁵) after losing two electrons, which provides extra stability. Thus, removing a third electron from Mn²⁺ requires significantly more energy due to this stability.
- Iron (Fe) also has a stable half-filled configuration after losing two electrons, but it is less stable than Mn²⁺.
- Chromium and Vanadium have less stable configurations after their third ionization.

5. **Conclusion**:
- Manganese (Mn) is expected to have the highest third ionization enthalpy due to the stability of its 3d⁴ configuration and the energy required to disrupt this stability.

### Final Answer:
**Manganese (Mn)** has the highest third ionization enthalpy among the given metals.