2.2.3Additional lessons
Multiple Bonds — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 27 of 172
Part 1 of 5Double and Triple Bonds
Atoms can share more than one pair of electrons with the same neighbour.
Single bond
- One shared pair of electrons between two atoms, shown as a single line.
Double bond
- Two shared pairs, so four electrons are shared between the atoms.
Triple bond
- Three shared pairs, so six electrons are shared between the atoms.
Why they form
- Multiple bonds let atoms such as C, N and O complete their octets when single bonds are not enough.
Part 2 of 5Common Examples
Common Examples
Oxygen
- O₂ contains a double bond, O=O, with two lone pairs on each atom.
Nitrogen
- N₂ contains a triple bond, N≡N, with one lone pair on each atom.
Carbon dioxide
- CO₂ has two C=O double bonds, so O=C=O is linear.
Ethene and ethyne
- C₂H₄ has a C=C double bond and C₂H₂ has a C≡C triple bond.
Part 3 of 5Length and Strength Trends
Length
- For the same pair of atoms, bond length decreases from single to double to triple.
Strength
- Bond enthalpy increases from single to double to triple because more shared electrons attract the nuclei.
Not a simple multiple
- A double bond is stronger than a single bond but less than twice as strong.
Reactivity
- Multiple bonds often make molecules more reactive, as in alkenes, because the extra electrons are more exposed.
| Bond | Length / pm | Bond enthalpy / kJ mol⁻¹ |
|---|---|---|
| C–C | 154 | 346 |
| C=C | 134 | 614 |
| C≡C | 120 | 839 |
Part 4 of 5Placing Multiple Bonds
1
Draw the skeleton with single bonds and distribute lone pairs to the outer atoms.
2
Count the electrons on the central atom; if it has fewer than eight, it needs more sharing.
3
Convert a lone pair on a neighbouring atom into a second bonding pair to make a double bond.
4
Repeat if the central atom is still short, giving a triple bond.
5
Check that the total number of valence electrons has not changed.
Part 5 of 5Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Double bond Two shared pairs of electrons between two atoms.
- Triple bond Three shared pairs of electrons between two atoms.
- Bond order The number of shared pairs between two bonded atoms.
- Unsaturated Containing at least one carbon–carbon multiple bond.
Common Mistakes
- Saying that a triple bond is three times as strong as a single bond.
- Forgetting the lone pairs on atoms that carry multiple bonds.
- Writing CO₂ as a bent molecule; it is linear.
- Mixing up the trendmultiple bonds are shorter, not longer.
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