Bond order calculator
Pick a diatomic molecule or ion and the calculator fills its molecular orbitals, counts bonding and antibonding electrons, and gives the bond order, the number of unpaired electrons and whether it is paramagnetic. Or enter the electron counts yourself.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose “Pick from the list” and select the molecule or ion, or choose “Electron counts”.
- For counts, enter how many electrons sit in bonding and in antibonding orbitals.
- Read the bond order; a higher order means a shorter, stronger bond.
How it works
The calculator counts all electrons (both atoms’ electrons minus the charge) and fills the molecular orbitals from the bottom:
- up to 14 electrons: σ1s, σ1s, σ2s, σ2s, π2p (two orbitals), σ2p, π2p, σ2p;
- above 14: σ1s, σ1s, σ2s, σ2s, σ2p, π2p, π2p, σ2p.
Degenerate π orbitals take one electron each before pairing. Bond order = (bonding − antibonding) ÷ 2. Counting only valence electrons gives the same order, because the inner σ1s and σ*1s cancel.
Worked example
N₂ has 14 electrons: σ1s² σ1s² σ2s² σ2s² π2p⁴ σ2p². Bonding 2 + 2 + 4 + 2 = 10, antibonding 2 + 2 = 4, bond order (10 − 4) ÷ 2 = 3, all paired, diamagnetic.
O₂⁻ (superoxide) has 17: one more electron in π*2p than O₂, so the order drops to (10 − 7) ÷ 2 = 1.5 with one unpaired electron.
Tips
- Bond order vs bond length. N₂ (3) < O₂ (2) < F₂ (1) in bond length; higher order, shorter bond.
- Removing an electron can strengthen a bond. O₂⁺ (2.5) is stronger than O₂ because the electron leaves an antibonding orbital.
- Isoelectronic species like N₂, CO, NO⁺ and CN⁻ all have 14 electrons and bond order 3.
Frequently asked questions
How do you calculate bond order?
Bond order = (electrons in bonding orbitals − electrons in antibonding orbitals) ÷ 2. For O₂, 10 bonding and 6 antibonding electrons give (10 − 6) ÷ 2 = 2, a double bond.
Why is O₂ paramagnetic?
Its last two electrons go into the two degenerate π*2p orbitals one each, with parallel spins (Hund's rule). Two unpaired electrons make oxygen paramagnetic; liquid oxygen is attracted to a magnet. A simple Lewis structure with a double bond does not show this.
Why does the orbital order change between N₂ and O₂?
In B₂, C₂ and N₂ the 2s and 2p orbitals mix, pushing σ2p above π2p. From O₂ on the mixing is weak and σ2p lies below π2p. The calculator uses the first order up to 14 electrons and the second above. The bond order is the same either way; the magnetism of B₂ and C₂ depends on it.
What does a bond order of 0 mean?
As many antibonding as bonding electrons, so there is no net bond. He₂ and Ne₂ have order 0 and do not form stable molecules. A fractional order such as 0.5 in H₂⁺ still means a weak bond.