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 anion \( O_2^- \), we can follow these steps:

### Step 1: Determine the total number of electrons in \( O_2^- \)
- The atomic number of oxygen (O) is 8. Since there are two oxygen atoms in \( O_2 \), the total number of electrons from the oxygen atoms is \( 8 \times 2 = 16 \).
- Since \( O_2^- \) is an anion, we add one additional electron, bringing the total to \( 16 + 1 = 17 \) electrons.

### Step 2: Write the molecular orbital configuration for \( O_2^- \)
- The molecular orbital configuration for \( O_2 \) is:
\[ 1s \sigma^2 1s \sigma^{*2} 2s \sigma^2 2s \sigma^{*2} 2p \pi^2 2p \pi^{*2} 2p \sigma^{*1} \]
- For \( O_2^- \), we add one more electron to the antibonding orbitals. The configuration becomes:
\[ 1s \sigma^2 1s \sigma^{*2} 2s \sigma^2 2s \sigma^{*2} 2p \pi^2 2p \pi^{*3} 2p \sigma^{*1} \]

### Step 3: Identify the antibonding orbitals
- The antibonding orbitals are indicated by the asterisk (*). In our configuration, the antibonding orbitals are:
- \( 1s \sigma^{*2} \)
- \( 2s \sigma^{*2} \)
- \( 2p \pi^{*3} \)
- \( 2p \sigma^{*1} \)

### Step 4: Count the number of antibonding electron pairs
- Each antibonding orbital can hold a maximum of 2 electrons. We need to count only the pairs:
- \( 1s \sigma^{*} \): 2 electrons (1 pair)
- \( 2s \sigma^{*} \): 2 electrons (1 pair)
- \( 2p \pi^{*} \): 3 electrons (1 pair + 1 unpaired)
- \( 2p \sigma^{*} \): 1 electron (unpaired)

- The total number of pairs of antibonding electrons is:
- From \( 1s \sigma^{*} \): 1 pair
- From \( 2s \sigma^{*} \): 1 pair
- From \( 2p \pi^{*} \): 1 pair (only counting the paired electrons)

Thus, the total number of antibonding electron pairs is \( 1 + 1 + 1 = 3 \).

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

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