Concept: Molecular Orbital Theory (MOT)

Molecular Orbital Theory describes the bonding in molecules by combining atomic orbitals to form molecular orbitals. For diatomic molecules, the order of filling molecular orbitals depends on the total number of electrons.

Why (A) is correct:

First, let's determine the total number of electrons in O₂⁺. An oxygen atom has 8 electrons, so O₂ has 16 electrons. O₂⁺ is formed by removing one electron from O₂, so it has 15 electrons.

The molecular orbital configuration for molecules with more than 14 electrons (like O₂⁺) follows the order:

σ1s, σ*1s, σ2s, σ*2s, σ2pᵂ, (π2pₓ = π2pᵢ), (π*2pₓ = π*2pᵢ), σ*2pᵂ

Filling 15 electrons according to this order:

• σ1s²
• σ*1s²
• σ2s²
• σ*2s²
• σ2pᵂ²
• π2pₓ²
• π2pᵢ²
• π*2pₓ¹

The last electron in O₂⁺ (the 15th electron) is in one of the π*2p orbitals, specifically π*2pₓ or π*2pᵢ. Therefore, if an electron is removed from O₂ to form O₂⁺, it is removed from the highest occupied molecular orbital (HOMO) of O₂. The HOMO of O₂ (16 electrons) is π*2pₓ or π*2pᵢ.

Thus, the last electron is removed from the π*2pₓ orbital (or π*2pᵢ, as they are degenerate).

Option Analysis:

• A) π*2pₓ: This is the correct orbital from which the last electron is removed to form O₂⁺, as it is the highest energy occupied molecular orbital in O₂.
• B) σ*2s: This is a lower energy antibonding orbital and is fully occupied in O₂.
• C) σ*2pᵂ: This is a higher energy antibonding orbital and is unoccupied in O₂.
• D) σ2pᵂ: This is a lower energy bonding orbital and is fully occupied in O₂.

Correct Answer: (A)