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2.2.3Additional lessons

Multiple Bonds — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 24 of 98

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Part 1 of 6Bond Order

2.2.3 Multiple Bonds

Atoms sometimes need to share more than one pair to complete their octets.

Single bond
  • One shared pair of electrons, as in H–H or the carbon-carbon bond in ethane.
Double bond
  • Two shared pairs of electrons, as in O=O and in the C=C of ethene.
Triple bond
  • Three shared pairs of electrons, as in N≡N and in ethyne HC≡CH.
Bond order
  • The number of shared pairs between two atoms is called the bond order, which is 1, 2 or 3.

Part 2 of 6Common Examples

Common Examples
Oxygen, O₂
  • The two oxygen atoms share two pairs, and each atom keeps two lone pairs.
Nitrogen, N₂
  • The two nitrogen atoms share three pairs, and each atom keeps one lone pair;
    • the strong triple bond makes N₂ unreactive.
Carbon dioxide, CO₂
  • Carbon forms a double bond to each oxygen, so the molecule is O=C=O with two lone pairs on each oxygen.
Ethene, C₂H₄
  • The two carbon atoms are joined by a double bond and each carbon also has two bonds to hydrogen.

Part 3 of 6Effect on Length and Strength

1
Increasing the number of shared pairs increases the electron density between the two nuclei.
2
The greater electron density attracts the nuclei more strongly towards each other.
3
The nuclei are pulled closer, so the bond length decreases as bond order increases.
4
Breaking the bond requires more energy, so bond enthalpy increases as bond order increases.
5
However, the triple bond is less than three times as strong as the single bond, so the relationship is not exactly proportional.
BondLength / pmEnthalpy / kJ mol⁻¹
C–C154346
C=C134614
C≡C120839

Part 4 of 6Reactivity

Unsaturation
  • Compounds with C=C or C≡C are called unsaturated because they can add extra atoms.
Alkene reactions
  • The double bond in ethene is the reactive site that allows addition reactions to occur.
Stable molecules
  • Molecules with very strong multiple bonds, such as N₂, need a large energy input before they react.
Drawing check
  • Each carbon in an organic molecule must have exactly four bonds counted as shared pairs.

Part 5 of 6Worked Example

Question Draw the Lewis formula of carbon dioxide.

  1. Valence electronsC gives 4 and each O gives 6, so 4 + 12 = 16 electrons, which is 8 pairs.
  2. Place C in the centre with a single bond to each O (2 pairs used) and add three lone pairs on each O (6 pairs used).
  3. Carbon has only four electrons, so move one lone pair from each oxygen to form a second bond to carbon.
  4. Now each atom has eight electronsC has four shared pairs and each O has two shared pairs and two lone pairs.

Answer O=C=O with two lone pairs on each oxygen atom.

Part 6 of 6Exam Checklist & Tips

2.2.3 – Key Points for the Exam

  • Double bond A bond consisting of two shared pairs of electrons.
  • Triple bond A bond consisting of three shared pairs of electrons.
  • Bond order The number of shared electron pairs between two bonded atoms.
  • Unsaturated Containing at least one carbon-carbon double or triple bond.

Common Mistakes

  • Saying that a double bond is twice as strong as a single bond; it is stronger but not exactly double.
  • Forgetting that bond length decreases as the number of bonds increases.
  • Leaving off lone pairs on oxygen or nitrogen in Lewis formulas.
  • Drawing a carbon atom with five bonds.

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