MO diagram of N₂⁻
Configuration
(σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (σ2p)² (π2pₓ)² (π2pᵧ)² (π*2pₓ)¹
Bond order = ½ × (10 − 5) = 2.5.
With more than 14 electrons, σ2p sits below π2p.
What this means
The extra electron goes into an antibonding π* orbital, so the bond order drops to 2.5. It's less stable than N₂⁺, which has the same bond order but fewer antibonding electrons.
Species with the same number of electrons (15), and so the same bond order: O₂⁺, NO.
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 N₂⁻?
2.5. N₂⁻ has 10 bonding and 5 antibonding electrons, so ½ × (10 − 5) = 2.5.
Is N₂⁻ paramagnetic or diamagnetic?
N₂⁻ is paramagnetic, with 1 unpaired electron.
What is the electronic configuration of N₂⁻?
(σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (σ2p)² (π2pₓ)² (π2pᵧ)² (π*2pₓ)¹.
Does N₂⁻ exist?
Yes. A bond order of 2.5 means there's a net bond holding it together.