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AS · Practice questions · Shapes of molecules

Predict the shape.

Six original Cambridge-style questions on VSEPR: counting electron pairs, shapes and bond angles of molecules and ions, and explaining the effect of lone pairs.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Count pairs, then name the shape from the atoms.

01
Explain
[3 marks]

State the shape of an ammonia molecule and its bond angle, and explain your answer.

  • Nitrogen has three bonding pairs and one lone pair. ✓
  • The pairs repel to be as far apart as possible, and the lone pair repels more strongly than the bonding pairs. ✓
  • Shape: pyramidal; bond angle about 107°. ✓
02
Analysis
[2 marks]

Predict the shape and bond angle of the PH4+ ion.

  • P has 5 outer electrons, + 4 from H, − 1 for the charge = 8, so four bonding pairs and no lone pairs. ✓
  • Tetrahedral, 109.5°. ✓
03
Analysis
[2 marks]

Phosphorus pentachloride, PCl5, has five bonding pairs around phosphorus. Name its shape and state its two bond angles.

  • Trigonal bipyramidal. ✓
  • 120° between the three equatorial bonds, 90° between axial and equatorial bonds. ✓
04
Explain
[3 marks]

The bond angles in CH4, NH3 and H2O are 109.5°, 107° and 104.5°. Explain this trend.

  • Each central atom has four electron pairs, arranged roughly tetrahedrally. ✓
  • CH4 has no lone pairs, NH3 one and H2O two. ✓
  • Lone pairs repel more strongly than bonding pairs, so each extra lone pair pushes the bonds closer and reduces the angle. ✓
05
Analysis
[2 marks]

Predict the shape of the sulfur dioxide molecule, which has two S=O double bonds and one lone pair on sulfur, and estimate the O–S–O angle.

  • Three regions of electron density (two double bonds and one lone pair) arrange trigonal planar. ✓
  • With one lone pair, the molecule is non-linear (bent), with an angle slightly less than 120° (about 119°). ✓
06
Analysis
[2 marks]

State the shape of (a) beryllium chloride, BeCl2, and (b) the tetrachloroaluminate ion, AlCl4−, with bond angles.

  • (a) Two bonding pairs, no lone pairs: linear, 180°. ✓
  • (b) Al has 3 outer electrons, + 4 from Cl, + 1 for the charge = 8, four bonding pairs: tetrahedral, 109.5°. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.