Among the following species, the diamagnetic molecule is :

Options

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

Explanation

To determine which of the given species is diamagnetic, we need to analyze the electron configuration of each molecule based on molecular orbital theory (MOT). A diamagnetic molecule is one that has all its electrons paired, meaning there are no unpaired electrons.

### Step-by-Step Solution:

1. **Identify the Species**: The options given are:
- Option 1: NO (Nitric Oxide)
- Option 2: O2 (Oxygen)
- Option 3: CO (Carbon Monoxide)
- Option 4: B2 (Boron Dimer)

2. **Determine Total Electrons**:
- **NO**: Nitrogen (7 electrons) + Oxygen (8 electrons) = 15 electrons.
- **O2**: Oxygen (8 electrons) + Oxygen (8 electrons) = 16 electrons.
- **CO**: Carbon (6 electrons) + Oxygen (8 electrons) = 14 electrons.
- **B2**: Boron (5 electrons) + Boron (5 electrons) = 10 electrons.

3. **Apply Molecular Orbital Theory**:
- **NO (15 electrons)**:
- Filling order: σ1s (2), σ1s* (2), σ2s (2), σ2s* (2), σ2p (2), π2p (4), π*2p (1).
- Configuration: σ1s² σ1s*² σ2s² σ2s*² σ2p² π2p² π*2p¹
- **Unpaired Electrons**: 1 (in π*2p) → **Paramagnetic**.

- **O2 (16 electrons)**:
- Filling order: σ1s (2), σ1s* (2), σ2s (2), σ2s* (2), σ2p (2), π2p (4), π*2p (2).
- Configuration: σ1s² σ1s*² σ2s² σ2s*² σ2p² π2p² π*2p²
- **Unpaired Electrons**: 2 (in π*2p) → **Paramagnetic**.

- **CO (14 electrons)**:
- Filling order: σ1s (2), σ1s* (2), σ2s (2), σ2s* (2), σ2p (2), π2p (4).
- Configuration: σ1s² σ1s*² σ2s² σ2s*² σ2p² π2p²
- **Unpaired Electrons**: 0 → **Diamagnetic**.

- **B2 (10 electrons)**:
- Filling order: σ1s (2), σ1s* (2), σ2s (2), σ2s* (2), π2p (4).
- Configuration: σ1s² σ1s*² σ2s² σ2s*² π2p²
- **Unpaired Electrons**: 2 (in π2p) → **Paramagnetic**.

4. **Conclusion**:
- Among the given species, only **CO** has all electrons paired, making it the **diamagnetic molecule**.

### Final Answer:
The diamagnetic molecule among the given options is **Option 3: CO (Carbon Monoxide)**.

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