Which of the following species is not paramagnetic ?

Options

A. CO
B. `O_2`
C. `B_2`
D. NO

Answer

To determine which of the given species is not paramagnetic, we will analyze the molecular orbital configurations of each species. A species is considered paramagnetic if it has unpaired electrons in its molecular orbitals.

### Step 1: Analyze Carbon Monoxide (CO)
1. **Count the total number of electrons**:
- Carbon (C) has 6 electrons, and Oxygen (O) has 8 electrons.
- Total = 6 + 8 = 14 electrons.

2. **Fill the molecular orbitals**:
- The molecular orbital energy levels for CO are:
- σ1s, σ*1s, σ2s, σ*2s, σ2p_z, π2p_x, π2p_y
- Filling the orbitals with 14 electrons:
- σ1s (2), σ*1s (0), σ2s (2), σ*2s (0), σ2p_z (2), π2p_x (2), π2p_y (2)
- The filling would look like this:
- σ1s: ↑↓
- σ*1s:
- σ2s: ↑↓
- σ*2s:
- σ2p_z: ↑↓
- π2p_x: ↑↓
- π2p_y: ↑↓
- All orbitals are filled with paired electrons, hence **CO is not paramagnetic**.

### Step 2: Analyze the Second Species (Assumed to be O2)
1. **Count the total number of electrons**:
- Oxygen (O) has 8 electrons, and for O2, total = 8 + 8 = 16 electrons.

2. **Fill the molecular orbitals**:
- The molecular orbital energy levels for O2 are:
- σ1s, σ*1s, σ2s, σ*2s, σ2p_z, π2p_x, π2p_y, π*2p_x, π*2p_y
- Filling the orbitals with 16 electrons:
- σ1s (2), σ*1s (0), σ2s (2), σ*2s (0), σ2p_z (2), π2p_x (2), π2p_y (2), π*2p_x (1), π*2p_y (1)
- The filling would look like this:
- σ1s: ↑↓
- σ*1s:
- σ2s: ↑↓
- σ*2s:
- σ2p_z: ↑↓
- π2p_x: ↑↓
- π2p_y: ↑↓
- π*2p_x: ↑
- π*2p_y: ↑
- There are unpaired electrons in π* orbitals, hence **O2 is paramagnetic**.

### Step 3: Analyze the Third Species (B2)
1. **Count the total number of electrons**:
- Boron (B) has 5 electrons, and for B2, total = 5 + 5 = 10 electrons.

2. **Fill the molecular orbitals**:
- The molecular orbital energy levels for B2 are:
- σ1s, σ*1s, σ2s, σ*2s, π2p_x, π2p_y
- Filling the orbitals with 10 electrons:
- σ1s (2), σ*1s (0), σ2s (2), σ*2s (0), π2p_x (2), π2p_y (2)
- The filling would look like this:
- σ1s: ↑↓
- σ*1s:
- σ2s: ↑↓
- σ*2s:
- π2p_x: ↑↓
- π2p_y: ↑
- There are unpaired electrons in π2p_y, hence **B2 is paramagnetic**.

### Conclusion:
The only species that is not paramagnetic among the given options is **Carbon Monoxide (CO)**.