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9701 · 3.5

Shapes of molecules — practice questions

Practice and worked examples for 9701 Shapes of molecules. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Predict the shape and bond angle of a phosphorus trichloride (PCl₃) molecule. Explain your reasoning.

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  1. Identify central atom and valence electrons: The central atom is Phosphorus (P), which is in Group 15. It has 5 valence electrons.
  2. Count bonding pairs: P forms three single covalent bonds with three Chlorine (Cl) atoms. This means there are 3 bonding pairs.
  3. Count lone pairs: P uses 3 of its 5 valence electrons for bonding. The remaining 5−3=25 - 3 = 2 electrons form 1 lone pair.
  4. Total electron pairs: There are 3 bonding pairs + 1 lone pair = 4 electron pairs in total around the central P atom.
  5. Determine geometry: The arrangement of 4 electron pairs is tetrahedral.
  6. Determine shape and bond angle: With 3 bonding pairs and 1 lone pair, the molecular shape is trigonal pyramidal. The ideal tetrahedral angle is 109.5°. However, the lone pair-bonding pair (LP-BP) repulsion is greater than the bonding pair-bonding pair (BP-BP) repulsion. This extra repulsion from the lone pair pushes the P-Cl bonds closer together, reducing the Cl-P-Cl bond angle to approximately 107°.

Worked example 2

The hexafluorosilicate(IV) ion, SiF₆²⁻, has an octahedral shape. Predict the F-Si-F bond angle. Explain why the shape of sulfur hexafluoride, SF₆, is also octahedral with the same bond angle.

Show solution outline
  1. Analyse SiF₆²⁻: The central atom is Silicon (Si), in Group 14, with 4 valence electrons. It is bonded to 6 Fluorine (F) atoms. The ion has a 2- charge, meaning 2 extra electrons are present. Total valence electrons = 4 (from Si) + 6×7 (from F) + 2 (from charge) = 4 + 42 + 2 = 48. This is complex. A simpler method: Central atom Si has 4 valence electrons. Add 2 for the charge = 6 electrons. It forms 6 bonds with F. This means there are 6 bonding pairs and (6-6)/2 = 0 lone pairs.
  2. Geometry of SiF₆²⁻: With 6 bonding pairs and 0 lone pairs, the electron pairs arrange themselves in an octahedral geometry to minimise repulsion. The F-Si-F bond angle in an octahedral structure is 90°.
  3. Analyse SF₆: The central atom is Sulfur (S), in Group 16, with 6 valence electrons. It forms 6 single bonds with 6 Fluorine (F) atoms.
  4. Electron pairs in SF₆: There are 6 bonding pairs and (6-6)/2 = 0 lone pairs around the central S atom.
  5. Conclusion: Since both SiF₆²⁻ and SF₆ have 6 bonding pairs and 0 lone pairs on the central atom, they both adopt a perfect octahedral shape with bond angles of 90°.
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