Q.20
The ionization energy follows the order:
Ionization Energy Order of O2, O2+, O2−, O22− – Explained Using Molecular Orbital Theory
Understanding the ionization energy order of oxygen species is a common question in
chemical bonding and molecular orbital theory. Ionization energy depends on
charge, bond order, and stability.
Using these concepts, we can easily determine the correct order.
Basic Concept
What is Ionization Energy?
Ionization energy is the energy required to remove an electron from a molecule or ion.
Important Rules
- Positive charge → higher ionization energy
- Negative charge → lower ionization energy
- Higher bond order → higher stability → higher ionization energy
Molecular Orbital Theory of O2
Electronic configuration:
σ2s2 σ*2s2 σ2pz2
π2px2 π2py2
π*2px1 π*2py1
Bond order = (Bonding − Antibonding) / 2 = (10 − 6) / 2 = 2
Species-wise Explanation
1. O2+ (Cation)
- One electron removed
- Bond order = 2.5
- Most stable
- Electron removal is difficult
Result: Highest ionization energy
2. O2 (Neutral)
- Bond order = 2
- Stable molecule
Result: Second highest ionization energy
3. O2− (Superoxide)
- One extra electron in antibonding orbital
- Bond order = 1.5
- Less stable
Result: Lower ionization energy
4. O22− (Peroxide)
- Two extra electrons
- Bond order = 1
- Weakest bond and least stable
Result: Lowest ionization energy
Final Order
O22− < O2− < O2 < O2+
Correct Answer: (A) O2+ > O2 > O2− > O22−
Quick Comparison Table
| Species | Charge | Bond Order | Stability | Ionization Energy |
|---|---|---|---|---|
| O2+ | +1 | 2.5 | Highest | Highest |
| O2 | 0 | 2 | High | High |
| O2− | −1 | 1.5 | Medium | Low |
| O22− | −2 | 1 | Lowest | Lowest |
