Which of the following has second ionisation potential less than expected

Options

A. Cr
B. Zn
C. V
D. Mo

Explanation

To determine which element has a second ionization potential (IP) that is less than expected, we will analyze the electronic configurations of the given elements and their ionization potentials.

### Step-by-Step Solution:

1. **Understand Ionization Potential**:
- The second ionization potential refers to the energy required to remove the second electron from a singly charged cation (M⁺) to form a doubly charged cation (M²⁺).

2. **Identify the Elements**:
- The question likely involves transition metals, specifically zinc (Zn) and copper (Cu). We will analyze these two elements.

3. **Determine Electron Configurations**:
- **Zinc (Zn)**: Atomic number 30. The electron configuration is [Ar] 3d¹⁰ 4s².
- **Copper (Cu)**: Atomic number 29. The electron configuration is [Ar] 3d¹⁰ 4s¹.

4. **Consider the First Ionization**:
- When zinc loses one electron, it becomes Zn⁺: [Ar] 3d¹⁰ 4s¹.
- When copper loses one electron, it becomes Cu⁺: [Ar] 3d¹⁰.

5. **Analyze Stability of Cations**:
- Cu⁺ has a completely filled 3d subshell, which is a stable electronic configuration.
- Zn⁺ has one electron in the 4s subshell, making it less stable compared to Cu⁺.

6. **Evaluate Second Ionization Potential**:
- For Zn⁺ to lose another electron (to form Zn²⁺), it removes the 4s electron, which is relatively easier due to its higher energy level and less stability.
- For Cu⁺ to lose an electron (to form Cu²⁺), it would involve removing an electron from a stable filled 3d subshell, which requires more energy.

7. **Conclusion**:
- Therefore, the second ionization potential of zinc (Zn) is less than expected compared to copper (Cu) because it is easier to remove the second electron from Zn⁺ than from Cu⁺.

### Final Answer:
- The element with a second ionization potential less than expected is **Zinc (Zn)**.