AP Chemistry Bond Order

Counting shared pairs, averaging across resonance, and using the result to rank bond length and strength.

Bond order looks like a vocabulary word, but on the exam it is a tool: it turns a Lewis structure into a prediction about how long and how strong a bond is. The skill has two parts, finding the bond order, which takes a little care when resonance is involved, and then ranking bonds by it. Both are built on the structures drawn in the Lewis structures guide, so a correct drawing comes first.

What Bond Order Is and How It Controls Bond Length and Strength

Bond order is the number of shared electron pairs between two atoms: 1 for a single bond, 2 for a double bond, 3 for a triple bond. When resonance makes several equivalent structures, the real bond is an average of them, so the bond order can be a fraction.

• Bond length: a higher bond order means a shorter bond, because more shared pairs pull the nuclei closer.
• Bond strength: a higher bond order means a stronger bond, so more energy is needed to break it.

The chart shows the pattern for carbon–carbon bonds. Strength rises with bond order but not in direct proportion: the triple bond is about 2.4 times the enthalpy of the single bond, not 3 times. The bond enthalpies here are the kind you use to estimate ΔH in endothermic and exothermic problems.

Worked Example 1: Finding Bond Order From a Lewis Structure

Give the bond order of each bond in N₂, CO₂, and HCN.

N₂: N≡N, a triple bond. Bond order 3.
CO₂: O=C=O, two double bonds. Each C–O bond has bond order 2.
HCN: H–C≡N. The C–H bond is order 1 and the C–N bond is order 3.

Worked Example 2: Finding a Fractional Bond Order With Resonance

What is the C–O bond order in the carbonate ion, CO₃²⁻? What about each O–O bond in ozone, O₃?

Carbonate: each resonance structure has one C=O (2 bonds) and two C–O (1 bond each), a total of 4 bonds shared across 3 equivalent C–O positions. Bond order = 4 ÷ 3 = 1.33.
Ozone: each structure has one O=O and one O–O, a total of 3 bonds across 2 equivalent positions. Bond order = 3 ÷ 2 = 1.5.

The method is always total bonds divided by number of positions. It matches what is measured: in carbonate all three C–O bonds are the same length, between a single and a double bond. The resonance idea itself is covered in the Unit 2 review.

Worked Example 3: Ranking Carbon–Oxygen Bond Lengths by Bond Order

Rank the C–O bond lengths in methanol (CH₃OH), the carbonate ion, and carbon dioxide from longest to shortest.

Methanol: a single C–O bond, order 1.
Carbonate: order 4/3, about 1.33.
Carbon dioxide: double bonds, order 2.
Higher order means shorter, so the ranking is CH₃OH (longest) > CO₃²⁻ > CO₂ (shortest). Measured values follow it: roughly 143 pm, 129 pm, and 116 pm, though you do not need to memorize them.

Worked Example 4: Explaining Why Nitrogen Gas Is So Unreactive

Nitrogen makes up most of the atmosphere yet barely reacts under ordinary conditions. Use bond order to explain this.

The N≡N bond has bond order 3, so it is very short and very strong. Its bond enthalpy is about 945 kJ/mol, compared with about 498 kJ/mol for the double bond in O₂ and about 418 kJ/mol for an N=N double bond. Breaking those three shared pairs requires a large energy input, which is why N₂ is largely inert and why fixing nitrogen into compounds takes high energy or special conditions.

Five Bond Order Practice Questions With Answers

1. What is the bond order of the C–N bond in HCN?
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   3. The Lewis structure is H–C≡N, a triple bond.
2. Each N–O bond in the nitrate ion has what bond order?
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   4/3, about 1.33: one double and two single bonds spread over three equivalent N–O positions.
3. Which is longer, an N=N bond or an N≡N bond?
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   N=N. It has the lower bond order, so the nuclei are held less tightly together.
4. Which is stronger, C=C or C–C?
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   C=C. Its higher bond order means a larger bond enthalpy (about 614 vs 348 kJ/mol).
5. What is each O–O bond order in ozone?
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   1.5: 3 bonds shared across 2 equivalent O–O positions.

Common Bond Order Mistakes

• Counting the bonds in the whole molecule. Bond order belongs to one pair of atoms, so look at that pair only.
• Using one resonance structure for a fractional order. Count total bonds over the equivalent positions and divide.
• Assuming a triple bond is three times as strong. It is stronger and shorter, but not by a factor of three.
• Calling a longer bond stronger. For the same two atoms, longer goes with weaker.
• Using molecular orbital bond order. The (bonding − antibonding) ÷ 2 formula is not assessed; use the Lewis structure.

Bond order also helps decide between competing resonance forms alongside formal charge, and the length and polarity of a bond depend on electronegativity as well. To turn your practice results into an estimate, the AP Chem Score Calculator is a quick next step.

Related Resources

• AP Chem Score Calculator
• Unit 2 Review: Compound Structure and Properties
• AP Chemistry Lewis Structures
• AP Chemistry Formal Charge
• AP Chemistry Electronegativity
• AP Chemistry Endothermic vs Exothermic
• AP Chemistry Study Guide

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