AQA A-Level Chemistry: Electron Pair Repulsion and Molecular Geometry — mark scheme explained

The question

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

Model answer

A full-mark response to the question above, worked through step by step.

⏱Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, apply VSEPR theory, and check your work.

1. Identify the central atom and count the number of bonding pairs and lone pairs.1 mark
   Central atom: Nitrogen (N)Number of bonding pairs: 3 (one with each hydrogen atom)Number of lone pairs: 1
2. Use VSEPR theory to predict the molecular shape.2 marks
   With 4 electron pairs (3 bonding, 1 lone), the shape is trigonal pyramidal.
3. Determine the bond angle.1 mark
   The presence of a lone pair compresses the bond angles slightly from the ideal tetrahedral angle of 109.5° to approximately 107°.

Final answer: Trigonal pyramidal, 107°

✓Work through every step correctly and you earn all 4 marks.

Common mistakes

• Forgetting to account for lone pairs when determining molecular shape.

  Why it happens: Students sometimes focus only on bonding pairs and overlook the effect of lone pairs, leading to incorrect predictions of molecular geometry.

  Fix: Always consider both bonding pairs and lone pairs when applying VSEPR theory to determine molecular shape.

• Incorrectly estimating bond angles due to the presence of lone pairs.

  Why it happens: Students may not fully understand how lone pairs compress bond angles, leading to inaccurate estimates.

  Fix: Practice predicting bond angles by considering the hierarchy of repulsions: lp-lp > lp-bp > bp-bp. Lone pairs cause more compression than bonding pairs.

• Confusing trigonal planar and trigonal pyramidal shapes.

  Why it happens: Students may mix up the terms, leading to incorrect answers in problems involving molecular geometry.

  Fix: Remember that a trigonal planar shape has three bonding pairs and no lone pairs, while a trigonal pyramidal shape has three bonding pairs and one lone pair.

• Misinterpreting the hierarchy of repulsions.

  Why it happens: Students may not fully grasp that lp-lp repulsion is greater than lp-bp, which is greater than bp-bp, leading to incorrect predictions of molecular shape and bond angles.

  Fix: Practice comparing the repulsion between different types of electron pairs and how this affects molecular geometry. Use examples like NH 3 and H 2 O to reinforce understanding.

• Failing to apply VSEPR theory correctly to molecules with more than four electron pairs.

  Why it happens: Students may struggle with the more complex geometries of trigonal bipyramidal and octahedral shapes, leading to incorrect predictions.

  Fix: Practice with a variety of examples, including molecules with five and six electron pairs. Use visual aids like molecular models or diagrams to help visualize the arrangements.

• Incorrectly identifying the central atom in a molecule.

  Why it happens: Students may choose the wrong atom as the central atom, leading to incorrect predictions of electron pair arrangement and molecular shape.

  Fix: Always identify the least electronegative atom (excluding hydrogen) as the central atom. Practice with different molecules to reinforce this concept.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.

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