Which of the following is diamagnetic ?

Question

Accepted Answer

To determine which of the given options is diamagnetic, we need to understand the concept of diamagnetism and how it relates to molecular orbital theory.

**Diamagnetism** is a property of substances that have all their electrons paired. Such substances are not attracted to a magnetic field and may even be slightly repelled. In contrast, paramagnetic substances have unpaired electrons and are attracted to magnetic fields.

Let's analyze each option:

1. **$O_2$ (Oxygen molecule):** 
   - The molecular orbital configuration for $O_2$ is: $\sigma_{1s}^2 \sigma_{1s}^2 \sigma_{2s}^2 \sigma_{2s}^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^1 \pi_{2p_y}^1$.
   - $O_2$ has two unpaired electrons in the $\pi^*$ antibonding orbitals, making it paramagnetic.

2. **$O_2^+$ (Oxygen cation):**
   - The molecular orbital configuration for $O_2^+$ is: $\sigma_{1s}^2 \sigma_{1s}^2 \sigma_{2s}^2 \sigma_{2s}^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^1$.
   - $O_2^+$ has one unpaired electron, making it paramagnetic.

3. **$O_2^-$ (Oxygen anion):**
   - The molecular orbital configuration for $O_2^-$ is: $\sigma_{1s}^2 \sigma_{1s}^2 \sigma_{2s}^2 \sigma_{2s}^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^2 \pi_{2p_y}^1$.
   - $O_2^-$ has one unpaired electron, making it paramagnetic.

4. **$O_2^{2-}$ (Peroxide ion):**
   - The molecular orbital configuration for $O_2^{2-}$ is: $\sigma_{1s}^2 \sigma_{1s}^2 \sigma_{2s}^2 \sigma_{2s}^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^2 \pi_{2p_y}^2$.
   - All electrons in $O_2^{2-}$ are paired, making it diamagnetic.

**Conclusion:**
- **Option 4 ($O_2^{2-}$)** is the correct answer because it is the only species with all electrons paired, thus exhibiting diamagnetism.
- Options 1, 2, and 3 are incorrect because they each have unpaired electrons, making them paramagnetic rather than diamagnetic.