Question

The number of antibonding electron pairs in $O_2^{2-}$ molecular ion on the basis of molecular orbital theory is (at. no. of $O$ is $18$)

• A. 5
• B. 2
• C. 4
• D. 6

Answer 1

The Correct Option is C

To determine the number of antibonding electron pairs in the O_2^{2-} molecular ion using Molecular Orbital (MO) theory, we will follow these steps:

1. Write the electronic configuration for an O_2^{2-} ion.
2. Identify the antibonding molecular orbitals (MOs).
3. Count the number of electrons in these antibonding MOs and determine the number of antibonding electron pairs.

Step 1: Determine the Total Number of Electrons

Oxygen has an atomic number of 8. Therefore, a neutral O_2 molecule has 16 electrons. The ion O_2^{2-} has gained 2 additional electrons, resulting in a total of:

16 + 2 = 18 electrons

Step 2: MO Configuration

The molecular orbital filling order for diatomic molecules like oxygen is:

\sigma_{1s}, \sigma^*_{1s}, \sigma_{2s}, \sigma^*_{2s}, \sigma_{2p_z}, \pi_{2p_x} = \pi_{2p_y}, \pi^*_{2p_x} = \pi^*_{2p_y}, \sigma^*_{2p_z}

The electronic configuration of 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

This totals to 18 electrons in the molecular orbitals.

Step 3: Determine the Number of Antibonding Electrons

Antibonding orbitals are denoted with an asterisk (*):

• \sigma^*_{1s}^2: 2 electrons
• \sigma^*_{2s}^2: 2 electrons
• \pi^*_{2p_x}^2 = \pi^*_{2p_y}^2: 4 electrons

Total antibonding electrons: 2 + 2 + 4 = 8

Since each pair consists of 2 electrons, the number of antibonding electron pairs is:

\frac{8}{2} = 4 pairs

Conclusion: The correct answer is 4 antibonding electron pairs in O_2^{2-}.