Which of the following is paramagnetic?

Options

A. `O_(2)^(-)`
B. `CN^(-)`
C. `CO`
D. `NO^(+)`.

Answer

To determine which of the given molecules is paramagnetic, we need to analyze the molecular orbital configurations of each molecule and check for the presence of unpaired electrons. Paramagnetic molecules have unpaired electrons, while diamagnetic molecules have all paired electrons.

### Step-by-Step Solution:

1. **Identify the Molecules**: The molecules given are O2^-, CN^-, CO, and NO^+.

2. **Count the Total Electrons**:
- **O2^-**: Each oxygen atom has 8 electrons. For 2 oxygen atoms, that’s 16 electrons. The negative charge indicates one extra electron, so total = 17 electrons.
- **CN^-**: Carbon has 6 electrons and nitrogen has 7 electrons. Total = 6 + 7 = 13 electrons. The negative charge adds one more electron, so total = 14 electrons.
- **CO**: Carbon has 6 electrons and oxygen has 8 electrons. Total = 6 + 8 = 14 electrons.
- **NO^+**: Nitrogen has 7 electrons and oxygen has 8 electrons. Total = 7 + 8 = 15 electrons. The positive charge means one electron is removed, so total = 14 electrons.

3. **Determine the Molecular Orbital Configuration**:
- For molecules with 14 electrons or less, the order of filling is:
- σ1s < σ*1s < σ2s < σ*2s < (π2px = π2py) < σ2pz
- For molecules with more than 14 electrons, the order is:
- σ1s < σ*1s < σ2s < σ*2s < σ2pz < (π2px = π2py) < (π*2px = π*2py) < σ*2pz

4. **Write the Molecular Orbital Configurations**:
- **O2^-** (17 electrons):
- σ1s² σ*1s² σ2s² σ*2s² σ2pz² (π2px² = π2py²) π*2px¹
- **Unpaired Electrons**: 1 (π*2px¹)
- **CN^-** (14 electrons):
- σ1s² σ*1s² σ2s² σ*2s² (π2px² = π2py²) σ2pz²
- **Unpaired Electrons**: 0
- **CO** (14 electrons):
- σ1s² σ*1s² σ2s² σ*2s² (π2px² = π2py²) σ2pz²
- **Unpaired Electrons**: 0
- **NO^+** (14 electrons):
- σ1s² σ*1s² σ2s² σ*2s² (π2px² = π2py²) σ2pz²
- **Unpaired Electrons**: 0

5. **Identify Paramagnetic Molecule**:
- **O2^-** has 1 unpaired electron, making it paramagnetic.
- **CN^-**, **CO**, and **NO^+** have no unpaired electrons, making them diamagnetic.

### Conclusion:
The paramagnetic molecule among the given options is **O2^-**.