Which one of the following molecules is paramagnetic ?

Options

A. `F_2`
B. `B_2`
C. `Li_2`
D. `N_2`

Answer

To determine which of the given molecules (F2, B2, Li2, N2) is paramagnetic, we need to analyze their molecular orbital configurations and check for unpaired electrons. Here's a step-by-step solution:

### Step 1: Understand Paramagnetism and Diamagnetism
- **Paramagnetic substances** have unpaired electrons and are attracted to magnetic fields.
- **Diamagnetic substances** have all electrons paired and are slightly repelled by magnetic fields.

### Step 2: Determine the Total Number of Electrons
- **F2**: Each fluorine atom has 9 electrons, so F2 has 18 electrons.
- **B2**: Each boron atom has 5 electrons, so B2 has 10 electrons.
- **Li2**: Each lithium atom has 3 electrons, so Li2 has 6 electrons.
- **N2**: Each nitrogen atom has 7 electrons, so N2 has 14 electrons.

### Step 3: Fill the Molecular Orbitals for Each Molecule
1. **Li2 (6 electrons)**:
- Molecular orbital configuration: σ1s² σ*1s² σ2s²
- All electrons are paired. **Li2 is diamagnetic.**

2. **B2 (10 electrons)**:
- Molecular orbital configuration: σ1s² σ*1s² σ2s² σ*2s² (2 electrons in bonding and 2 in antibonding)
- For the 2p orbitals: π2p (2 electrons) and π*2p (1 unpaired electron).
- There is one unpaired electron. **B2 is paramagnetic.**

3. **N2 (14 electrons)**:
- Molecular orbital configuration: σ1s² σ*1s² σ2s² σ*2s² σ2p² (2 electrons in bonding)
- For the 2p orbitals: π2p (4 electrons) and σ2p (2 electrons).
- All electrons are paired. **N2 is diamagnetic.**

4. **F2 (18 electrons)**:
- Molecular orbital configuration: σ1s² σ*1s² σ2s² σ*2s² σ2p² (2 electrons in bonding)
- For the 2p orbitals: π2p (4 electrons) and π*2p (2 electrons).
- All electrons are paired. **F2 is diamagnetic.**

### Step 4: Conclusion
From the analysis:
- **Li2**: Diamagnetic
- **B2**: Paramagnetic (has unpaired electrons)
- **N2**: Diamagnetic
- **F2**: Diamagnetic

Thus, the molecule that is paramagnetic is **B2**.