Question

The element with the highest third ionisation enthalpy is:

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

Hint:

Whenever a question asks for a high third ionization energy among 3d transition metals, look for the element that achieves a stable $\text{d}^5$ configuration at its $+2$ oxidation state. Since $\text{Mn}^{2+}$ is exactly $\text{d}^5$, breaking that stable configuration is incredibly difficult, making its third ionization enthalpy the highest.

Answer 1

The Correct Option is C

Concept: The third ionization enthalpy ($\Delta_i H_3$) represents the energy required to remove an electron from a doubly charged gaseous cation ($\text{M}^{2+} \rightarrow \text{M}^{3+} + e^-$). Its magnitude depends heavily on the electronic configuration of the $\text{M}^{2+}$ ion. Removing an electron from a completely filled or half-filled subshell requires an exceptionally high amount of energy due to the extra exchange energy and symmetric stability of such configurations.

Step 1: Write out the electronic configurations for each $\text{M}^{2+}$ cation.
Let's look at the outer electronic configurations for the transition metal ions after losing two electrons (from their $\text{4s}$ subshells):
• $\text{V}^{2+} (Z=23) \rightarrow [\text{Ar}] 3\text{d}^3$
• $\text{Cr}^{2+} (Z=24) \rightarrow [\text{Ar}] 3\text{d}^4$
• $\text{Mn}^{2+} (Z=25) \rightarrow [\text{Ar}] 3\text{d}^5$
• $\text{Fe}^{2+} (Z=26) \rightarrow [\text{Ar}] 3\text{d}^6$

Step 2: Analyze the stability to identify the highest energy barrier.
The $\text{Mn}^{2+}$ ion has a $3\text{d}^5$ valence shell configuration. The $\text{d}$ subshell is exactly half-filled, which gives it extra thermodynamic stability and high exchange energy. Removing a third electron from $\text{Mn}^{2+}$ requires disrupting this stable, symmetric half-filled shell, creating an exceptionally large energy barrier. In contrast, removing an electron from $\text{Fe}^{2+}$ ($3\text{d}^6$) leaves a stable half-filled $3\text{d}^5$ configuration, which occurs much more readily. Therefore, Manganese possesses the highest third ionization enthalpy.