Bond order is a key idea in IB Chemistry Topic 4 (Bonding) and Topic 14 (HL Bonding). It tells us how strong a bond is, how short it is, and how stable a molecule will be. Although it seems simple at first, bond order becomes especially important when dealing with resonance structures and molecular orbital theory. Understanding bond order helps students explain differences in bond properties and predict molecular behavior with confidence.
What Is Bond Order?
Bond order is the number of electron pairs shared between two atoms in a chemical bond.
For simple covalent bonds:
- Single bond → bond order 1
- Double bond → bond order 2
- Triple bond → bond order 3
Higher bond order means:
- Stronger bond
- Shorter bond
- Higher bond energy
Bond order directly reflects how tightly atoms are held together.
Bond Order in Simple Molecules
These follow straightforward counting:
- H₂ → 1
- O₂ → 2
- N₂ → 3
- CO₂ → each C–O is 2
A triple bond like N₂ is extremely strong, explaining why nitrogen is unreactive under normal conditions.
Bond Order in Resonance Structures
In molecules with resonance, bond order is not a whole number. Instead, it is an average across all resonance structures.
Formula for resonance bond order:
Bond order = (number of bonding pairs) ÷ (number of bond positions)
Examples:
1. Ozone (O₃)
Three bonding pairs across two O–O positions →
Bond order = 3 ÷ 2 = 1.5
2. Nitrate ion (NO₃⁻)
Four bonding pairs across three N–O positions →
Bond order = 4 ÷ 3 ≈ 1.33
3. Benzene (C₆H₆)
Nine bonding pairs across six C–C positions →
Bond order = 9 ÷ 6 = 1.5
These fractional bond orders explain why all bond lengths in benzene are identical.
Bond Order in Molecular Orbital Theory (HL)
In HL IB Chemistry, bond order is also calculated using molecular orbitals:
Bond order = (bonding electrons − antibonding electrons) ÷ 2
Examples:
- O₂ → (10 − 6)/2 = 2
- O₂⁺ → (10 − 5)/2 = 2.5
- O₂⁻ → (10 − 7)/2 = 1.5
This method explains magnetic properties:
- O₂ is paramagnetic due to unpaired electrons.
How Bond Order Affects Bond Length
Higher bond order → Shorter bond
Bond lengths follow a consistent trend:
C–C single > C=C double > C≡C triple
For example:
- C–C: ~154 pm
- C=C: ~134 pm
- C≡C: ~120 pm
Shorter bonds are harder to break and have higher energy.
How Bond Order Affects Bond Strength
Higher bond order → Stronger bond
Triple bonds are much stronger than single bonds.
Examples:
- C–C single bond energy ≈ 350 kJ/mol
- C=C double bond ≈ 610 kJ/mol
- C≡C triple bond ≈ 830 kJ/mol
More shared electrons strengthen electrostatic attraction.
How Bond Order Affects Reactivity
Higher bond order means:
- Less reactive
- More stable
- Harder to break
This is why:
- N₂ is chemically inert (bond order 3)
- O₂ is more reactive (bond order 2)
- O₂⁻ is even more reactive (bond order 1.5)
Bond order gives insight into chemical stability.
Bond Order in Polyatomic Ions
Bond order helps explain:
- Equal bond lengths in carboxylates
- Partial double-bond character
- Stability of nitrate and sulfate
For example, in CO₃²⁻ (carbonate):
Bond order = 4 bonding pairs / 3 positions = 1.33
Thus, all C–O bonds are equal.
Common IB Misunderstandings
“Bond order must be a whole number.”
False—resonance often leads to fractional values.
“Bond order only applies to covalent bonds.”
Ionic and resonance structures also use bond order to describe effective bonding.
“Longer bonds are stronger.”
Incorrect—shorter bonds are stronger.
“Double bonds are twice as strong as single bonds.”
Not exactly—bond strength does not scale linearly.
FAQs
Why does resonance increase stability?
Delocalization spreads electrons over multiple atoms, lowering energy.
Can bond order predict magnetic properties?
Yes—Molecular Orbital Theory explains paramagnetism using bond order and unpaired electrons.
Is bond order the same as bond energy?
They are related—higher bond order usually means higher bond energy, but values differ.
Conclusion
Bond order measures the number of shared electron pairs between atoms and determines bond length, strength, and stability. Whether dealing with simple covalent bonds, resonance structures, or molecular orbital calculations, bond order helps IB Chemistry students predict molecular behavior and explain bonding trends confidently.
