Which one of the following molecules is/are expected to exhibit diamagnetic behaviour?

Options

A. `N_2`
B. `O_2`
C. `S_2`
D. `He^+`

Answer

To determine which of the given molecules exhibit diamagnetic behavior, we need to analyze the electronic configurations of each molecule and check for the presence of unpaired electrons. A molecule is diamagnetic if all its electrons are paired.

### Step-by-Step Solution:

1. **Identify the Molecules**: The question does not specify which molecules to analyze, but we will consider the common examples: N₂ (Nitrogen), O₂ (Oxygen), S₂ (Sulfur), and He⁺ (Helium ion).

2. **Determine the Total Number of Electrons**:
- **N₂**: Each nitrogen atom has 7 electrons, so N₂ has a total of 14 electrons.
- **O₂**: Each oxygen atom has 8 electrons, so O₂ has a total of 16 electrons.
- **S₂**: Each sulfur atom has 16 electrons, so S₂ has a total of 32 electrons.
- **He⁺**: Helium has 2 electrons, but He⁺ has lost one electron, so it has 1 electron.

3. **Write the Electronic Configuration Using Molecular Orbital Theory (MOT)**:
- **N₂**:
- Configuration: σ1s² σ*1s² σ2s² σ*2s² (2 electrons each) → σ2p_z² (2 electrons) → π2p_x² π2p_y² (4 electrons).
- Total: 14 electrons, all paired. **N₂ is diamagnetic.**
- **O₂**:
- Configuration: σ1s² σ*1s² σ2s² σ*2s² (2 electrons each) → σ2p_z² (2 electrons) → π2p_x¹ π2p_y¹ (2 unpaired electrons).
- Total: 16 electrons, with unpaired electrons. **O₂ is paramagnetic.**
- **S₂**:
- Configuration: Similar to O₂ but with more electrons. It will have unpaired electrons in the π* orbitals.
- Total: 32 electrons, with unpaired electrons. **S₂ is paramagnetic.**
- **He⁺**:
- Configuration: 1 electron (from He), which is unpaired.
- Total: 1 electron. **He⁺ is paramagnetic.**

4. **Conclusion**: From the analysis:
- **Diamagnetic Molecule**: N₂
- **Paramagnetic Molecules**: O₂, S₂, He⁺

### Final Answer:
**The molecule that exhibits diamagnetic behavior is N₂.**