The number of antibonding electron pairs in `O_(2)^(-)`

Options

A. 2
B. 3
C. 1
D. 4

Answer

To determine the number of antibonding electron pairs in the ion \( O_2^- \), we will follow these steps:

### Step 1: Determine the total number of electrons in \( O_2^- \)
- Oxygen (O) has 8 electrons. Therefore, \( O_2 \) has \( 8 + 8 = 16 \) electrons.
- The \( O_2^- \) ion has gained an additional electron, bringing the total to \( 16 + 1 = 17 \) electrons.

### Step 2: Write the molecular orbital (MO) configuration for \( O_2^- \)
- According to Molecular Orbital Theory (MOT), the electron configuration for \( O_2 \) is:
- \( \sigma_{1s}^2 \)
- \( \sigma^*_{1s}^2 \)
- \( \sigma_{2s}^2 \)
- \( \sigma^*_{2s}^2 \)
- \( \sigma_{2p_z}^2 \)
- \( \pi_{2p_x}^2 \)
- \( \pi_{2p_y}^2 \)
- \( \pi^*_{2p_x}^1 \)
- \( \pi^*_{2p_y}^0 \)

- For \( O_2^- \), we add one more electron to the next available orbital:
- \( \pi^*_{2p_x}^2 \)
- \( \pi^*_{2p_y}^0 \)

### Step 3: Identify the bonding and antibonding orbitals
- Bonding orbitals:
- \( \sigma_{1s}, \sigma_{2s}, \sigma_{2p_z}, \pi_{2p_x}, \pi_{2p_y} \)
- Antibonding orbitals:
- \( \sigma^*_{1s}, \sigma^*_{2s}, \pi^*_{2p_x}, \pi^*_{2p_y} \)

### Step 4: Count the number of electrons in antibonding orbitals
- From the configuration of \( O_2^- \):
- \( \sigma^*_{1s}^2 \) contributes 2 electrons (1 pair)
- \( \sigma^*_{2s}^2 \) contributes 2 electrons (1 pair)
- \( \pi^*_{2p_x}^2 \) contributes 2 electrons (1 pair)
- \( \pi^*_{2p_y}^0 \) contributes 0 electrons

### Step 5: Calculate the number of antibonding electron pairs
- Total antibonding electrons: \( 2 + 2 + 2 + 0 = 6 \) electrons
- Number of pairs: \( \frac{6}{2} = 3 \) pairs

### Conclusion
The number of antibonding electron pairs in \( O_2^- \) is **3**.

---