MO diagram of O₂⁻
σ*2p
π*2p↑↓↑
π2p↑↓↑↓
σ2p↑↓
σ*2s↑↓
σ2s↑↓
σ*1s↑↓
σ1s↑↓
Configuration
(σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (σ2p)² (π2pₓ)² (π2pᵧ)² (π*2pₓ)² (π*2pᵧ)¹
Bond order = ½ × (10 − 7) = 1.5.
With more than 14 electrons, σ2p sits below π2p.
What this means
This is the superoxide ion, found in KO₂. The extra electron goes into π*, so the bond order is 1.5 and one electron is unpaired.
Bond order, length and stability
A higher bond order means a shorter, stronger bond. The oxygen family is the most asked example:
- Bond order: O₂⁺ (2.5) > O₂ (2) > O₂⁻ (1.5) > O₂²⁻ (1).
- Bond length: O₂⁺ < O₂ < O₂⁻ < O₂²⁻, the reverse order.
- Bond order 0 (He₂, Be₂, Ne₂) means the molecule doesn't exist as a stable species.
Bond order of common species
| Species | Electrons | Bond order | Unpaired | Magnetic |
|---|---|---|---|---|
| H₂ | 2 | 1 | 0 | Diamagnetic |
| H₂⁺ | 1 | 0.5 | 1 | Paramagnetic |
| He₂ | 4 | 0 | 0 | Diamagnetic |
| He₂⁺ | 3 | 0.5 | 1 | Paramagnetic |
| Li₂ | 6 | 1 | 0 | Diamagnetic |
| Be₂ | 8 | 0 | 0 | Diamagnetic |
| B₂ | 10 | 1 | 2 | Paramagnetic |
| C₂ | 12 | 2 | 0 | Diamagnetic |
| N₂ | 14 | 3 | 0 | Diamagnetic |
| N₂⁺ | 13 | 2.5 | 1 | Paramagnetic |
| N₂⁻ | 15 | 2.5 | 1 | Paramagnetic |
| O₂ | 16 | 2 | 2 | Paramagnetic |
| O₂⁺ | 15 | 2.5 | 1 | Paramagnetic |
| O₂⁻ | 17 | 1.5 | 1 | Paramagnetic |
| O₂²⁻ | 18 | 1 | 0 | Diamagnetic |
| F₂ | 18 | 1 | 0 | Diamagnetic |
| Ne₂ | 20 | 0 | 0 | Diamagnetic |
| NO | 15 | 2.5 | 1 | Paramagnetic |
| NO⁺ | 14 | 3 | 0 | Diamagnetic |
| CO | 14 | 3 | 0 | Diamagnetic |
| CN⁻ | 14 | 3 | 0 | Diamagnetic |
Questions students ask
What is the bond order of O₂⁻?
1.5. O₂⁻ has 10 bonding and 7 antibonding electrons, so ½ × (10 − 7) = 1.5.
Is O₂⁻ paramagnetic or diamagnetic?
O₂⁻ is paramagnetic, with 1 unpaired electron.
What is the electronic configuration of O₂⁻?
(σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (σ2p)² (π2pₓ)² (π2pᵧ)² (π*2pₓ)² (π*2pᵧ)¹.
Does O₂⁻ exist?
Yes. A bond order of 1.5 means there's a net bond holding it together.