Energy Diagram of Oxygen, Superoxide and Peroxide (O2, O2−, O22−)
Oxygen, one of the most essential elements for life, exists in various molecular forms — dioxygen (O₂), superoxide ion (O₂⁻), and peroxide ion (O₂²⁻). Each of these species exhibits a distinct molecular orbital configuration and bond order, which directly influence their magnetic and chemical properties. Let’s understand this through their energy diagrams and the concept of molecular orbital theory (MOT).
🌿 Molecular Orbital Concept in Oxygen Molecules
The Molecular Orbital Theory (MOT) explains bonding by combining atomic orbitals of two atoms to form molecular orbitals that can be either bonding or antibonding. The general energy order for oxygen and other elements from O₂ to F₂ is:
Total number of electrons in O₂ = 16 Filling these orbitals in order of increasing energy gives the configuration:
σ(1s)² σ*(1s)² σ(2s)² σ*(2s)² σ(2pz)² π(2px)² π(2py)² π*(2px)¹ π*(2py)¹
🔹 Bond Order and Magnetic Property
Bond order (B.O.) = ½ × (Nb − Na) where Nb = number of electrons in bonding orbitals and Na = number in antibonding orbitals.
| Species | Total Electrons | Configuration (2p level) | Bond Order | Magnetic Nature |
|---|---|---|---|---|
| O₂ | 16 | π*(2px,y)¹¹ | 2 | Paramagnetic (2 unpaired e⁻) |
| O₂⁻ (Superoxide) | 17 | π*(2px,y)³ | 1.5 | Paramagnetic (1 unpaired e⁻) |
| O₂²⁻ (Peroxide) | 18 | π*(2px,y)⁴ | 1 | Diamagnetic (no unpaired e⁻) |
⚡ Energy Level Diagram Explanation
As we move from O₂ → O₂⁻ → O₂²⁻, the extra electrons are added to the antibonding π* orbitals. This causes the bond order to decrease from 2 → 1.5 → 1, meaning the bond becomes weaker and the bond length increases. The magnetic property also changes from paramagnetic (unpaired electrons) to diamagnetic (all paired).
🧩 Graphical Representation
The following conceptual energy diagram summarizes the trend:
O₂⁻: Bond Order = 1.5
O₂²⁻: Bond Order = 1
Energy increases upward → Additional antibonding electrons lower stability.
🌍 Real-world Importance
- O₂ — supports combustion and respiration; essential for aerobic life.
- O₂⁻ — found in biological systems; acts as a reactive oxygen species (ROS).
- O₂²⁻ — found in compounds like H₂O₂ (hydrogen peroxide); strong oxidizing agent.
📘 Summary
- Oxygen’s energy diagram explains its magnetic and bonding properties.
- Extra electrons in antibonding orbitals reduce bond strength.
- Bond order trend: O₂ (2) > O₂⁻ (1.5) > O₂²⁻ (1)
- Magnetic trend: Paramagnetic → Diamagnetic.
