MO diagram of He₂
σ*2p
π*2p
σ2p
π2p
σ*2s
σ2s
σ*1s↑↓
σ1s↑↓
Configuration
(σ1s)² (σ*1s)²
Bond order = ½ × (2 − 2) = 0.
With 14 or fewer electrons, π2p fills before σ2p.
What this means
Two electrons in bonding σ1s and two in antibonding σ*1s cancel out. Bond order 0 means He₂ isn't a stable molecule, which is why helium is a single-atom gas.
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 He₂?
0. He₂ has 2 bonding and 2 antibonding electrons, so ½ × (2 − 2) = 0.
Is He₂ paramagnetic or diamagnetic?
He₂ is diamagnetic, with all electrons paired.
What is the electronic configuration of He₂?
(σ1s)² (σ*1s)².
Does He₂ exist?
No. A bond order of 0 means there's no net bond, so it isn't stable.