Worked example 1
Rank the carbon-carbon bonds in ethane, ethene and ethyne by length and explain the order.
Try it first: Write the bond order for each molecule.
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BondingSigma and Pi bondsContent level: Challenge 18 min
Bond Order, Length, Strength and Rotation
What you'll be able to do: Use bond order to compare bond length and strength, and explain why pi bonds block rotation.
Best after: Sigma and Pi Bonds
Introduction
Bond order is a single number that predicts length, strength and even whether a molecule can twist.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Calculate bond order including fractional values from resonance
- Rank bonds by length and strength using bond order
- Explain free rotation about sigma bonds
- Explain restricted rotation about double bonds and cis-trans isomerism
- Relate delocalization to intermediate bond lengths
Lesson
Bond order and its consequences
Bond order is the number of bonding pairs shared between two atoms. C-C is 1, C=C is 2 and a carbon-carbon triple bond is 3. Length falls and strength rises as bond order rises: roughly 154, 134 and 120 picometres respectively for those three. More shared electron density between the nuclei means a stronger electrostatic pull holding them closer together.
Shorter almost always means stronger for bonds between the same pair of elements.
Fractional bond order from resonance
When resonance structures spread bonding over several positions, the real bond order is the average. Ozone has one single and one double bond in each resonance form, so each O-O bond has order 1.5 and both bonds are the same length, between a typical single and double bond. The carbonate ion is similar with three equivalent bonds of order 4/3.
Rotation and isomerism
A sigma bond is symmetric about the internuclear axis, so the two ends can spin freely, which is why ethane has many conformations. A pi bond requires the p orbitals to stay parallel, so twisting the ends would break the overlap. That energy cost locks a double bond in place and makes cis-trans isomers of alkenes distinct compounds that cannot interconvert at room temperature.
Restricted rotation is a property of pi bonds, not of bond strength in general.
Key ideas
Definition
Bond order
The number of shared bonding pairs between two atoms, averaged over resonance structures if necessary.
Rule
Length and strength trend
As bond order rises, bond length falls and bond dissociation energy rises.
Rule
Rotation
Sigma bonds rotate freely; any pi bond blocks rotation.
Key concept
Ozone
Bond order 1.5 gives two identical O-O bonds intermediate in length between single and double.
Equation
Resonance bond order
bond order = total bonds between the pair / number of resonance structures
Worked examples
Worked example 2
Why does 2-butene have cis and trans isomers while butane does not have separable isomers of the same kind?
Try it first: Identify which bond would need to rotate.
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Common mistakes
Assuming ozone has one short and one long O-O bond.
Why it's wrong: Resonance delocalizes the pi electrons, so both bonds are identical with order 1.5.
Check instead: Average the bond orders across all resonance structures.
Saying a double bond is simply twice as strong as a single bond.
Why it's wrong: The pi component is weaker than the sigma component, so the total is less than double.
Check instead: Compare measured bond enthalpies rather than multiplying.
Thinking single bonds cannot rotate.
Why it's wrong: Sigma overlap is unchanged by rotation about the axis.
Check instead: Ask whether a pi bond is present before claiming restricted rotation.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Bond order counts the number of bonds between two atoms, including resonance-averaged fractional values. Higher bond order means shorter and stronger bonds. Sigma bonds are cylindrically symmetric so rotation is free, but a pi bond must break for rotation, which locks double bonds and creates cis and trans isomers.
- Bond order counts shared bonding pairs and can be fractional
- Higher bond order means shorter and stronger bonds
- Resonance averages bond orders and equalizes bond lengths
- Sigma bonds rotate freely because they are symmetric about the axis
- Pi bonds lock geometry and create cis-trans isomers
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.