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1.3.5Physical Chemistry

Shapes of Simple Molecules and Ions — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 63

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Electron Pair Repulsion

1.3.5 Shapes of Simple Molecules and Ions
Principle
  • Pairs of electrons around a central atom repel each other and arrange themselves as far apart as possible.
Bonding pairs
  • Include all pairs in single, double or triple bonds and count each multiple bond as one region of electrons.
Lone pairs
  • Lone pairs are closer to the central atom and repel more strongly than bonding pairs.
Order of repulsion
  • Lone pair – lone pair repulsion is greater than lone pair – bonding pair, which is greater than bonding pair – bonding pair.

Part 2 of 5Shapes with No Lone Pairs

Linear
  • Two bonding pairs, bond angle 180°, as in BeCl₂ and CO₂.
Trigonal planar
  • Three bonding pairs, 120°, as in BF₃.
Tetrahedral
  • Four bonding pairs, 109.5°, as in CH₄ and NH₄⁺.
Trigonal bipyramidal
  • Five bonding pairs, 90° and 120°, as in PCl₅.
Octahedral
  • Six bonding pairs, 90°, as in SF₆.

Part 3 of 5Shapes with Lone Pairs

Shapes with Lone Pairs
Pyramidal
  • Three bonding pairs and one lone pair, about 107°, as in NH₃.
Bent (V-shaped)
  • Two bonding pairs and two lone pairs, about 104.5°, as in H₂O.
Angle reduction
  • Each lone pair compresses the bond angle by about 2.5° compared with the tetrahedral angle.
Ions
  • For ions, adjust the electron count for the charge, for example H₃O⁺ is pyramidal like NH₃.

Part 4 of 5Method

1
Work out the number of outer electrons on the central atom from its group.
2
Add one electron for each bond formed by other atoms, adjusting for the ion charge.
3
Divide by two to get the total number of electron pairs, and subtract the bonding pairs to find the lone pairs.
4
Arrange the pairs to minimise repulsion and name the shape from the positions of the atoms only.
5
State the bond angle, explaining any distortion by lone pair repulsion.

Part 5 of 5Exam Checklist & Tips

1.3.5 – Key Points for the Exam

  • Bonding pair A pair of electrons shared between two atoms.
  • Lone pair A pair of outer electrons on an atom that is not used in bonding.
  • Bond angle The angle between two bonds at the same central atom.
  • Electron pair repulsion The theory that pairs of electrons around an atom repel to be as far apart as possible.

Common Mistakes

  • Omitting the lone pairs and giving NH₃ the bond angle of 109.5°.
  • Naming a shape by the electron pair arrangement rather than the atoms' positions.
  • Counting a double bond as two separate regions of repulsion.
  • Forgetting to state that lone pairs repel more strongly than bonding pairs.

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