Which of the following is not diamagnetic ? .

Options

A. `O_(2)^(2-)`
B. `Li_(2)`
C. `N_(2)^(o+)`
D. `C_(2)`

Explanation

To determine which of the given options is not diamagnetic, we need to analyze the molecular orbital configurations of each compound and check for unpaired electrons. A substance is considered diamagnetic if all its electrons are paired, while it is paramagnetic if it has one or more unpaired electrons.

### Step-by-Step Solution:

1. **Option A: O2 2-**
- Oxygen has 8 electrons, so 2 oxygen atoms have \(2 \times 8 = 16\) electrons. With 2 additional electrons due to the 2- charge, we have a total of 18 electrons.
- Molecular orbital configuration:
- \( \sigma 1s^2 \)
- \( \sigma^* 1s^2 \)
- \( \sigma 2s^2 \)
- \( \sigma^* 2s^2 \)
- \( \sigma 2p_z^2 \)
- \( \pi 2p_x^2 = \pi 2p_y^2 \)
- \( \pi^* 2p_x^0 = \pi^* 2p_y^0 \)
- Total: 18 electrons, all paired.
- **Conclusion**: O2 2- is diamagnetic.

2. **Option B: Li2**
- Lithium has 3 electrons, so 2 lithium atoms have \(2 \times 3 = 6\) electrons.
- Molecular orbital configuration:
- \( \sigma 1s^2 \)
- \( \sigma^* 1s^2 \)
- \( \sigma 2s^2 \)
- \( \sigma^* 2s^0 \)
- Total: 6 electrons, all paired.
- **Conclusion**: Li2 is diamagnetic.

3. **Option C: N2+**
- Nitrogen has 7 electrons, so 2 nitrogen atoms have \(2 \times 7 = 14\) electrons. With a positive charge, we subtract 1 electron, giving us 13 electrons.
- Molecular orbital configuration:
- \( \sigma 1s^2 \)
- \( \sigma^* 1s^2 \)
- \( \sigma 2s^2 \)
- \( \sigma^* 2s^2 \)
- \( \pi 2p_x^2 = \pi 2p_y^2 \)
- \( \sigma 2p_z^1 \) (the 13th electron)
- Total: 13 electrons, with 1 unpaired electron in \( \sigma 2p_z \).
- **Conclusion**: N2+ is paramagnetic.

4. **Option D: C2**
- Carbon has 6 electrons, so 2 carbon atoms have \(2 \times 6 = 12\) electrons.
- Molecular orbital configuration:
- \( \sigma 1s^2 \)
- \( \sigma^* 1s^2 \)
- \( \sigma 2s^2 \)
- \( \sigma^* 2s^2 \)
- \( \pi 2p_x^2 = \pi 2p_y^2 \)
- Total: 12 electrons, all paired.
- **Conclusion**: C2 is diamagnetic.

### Final Conclusion:
Among the options, **N2+** is the only species that is not diamagnetic; it is paramagnetic due to the presence of an unpaired electron.

### Answer:
**N2+** is not diamagnetic.