A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Electron Pair Repulsion and Molecular Geometry — mark scheme explained
The short answer
In A-Level Chemistry, understanding the shapes of molecules and ions is crucial for predicting their behavior in chemical reactions. This section focuses on the concept of electron pair repulsion, which explains how bonding pairs and lone (non-bonding) pairs of electrons arrange themselves around a central atom to minimize repulsion.
The question
Determine the molecular shape and bond angle of ammonia (NH 3 ). [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the central atom and count the number of bonding pairs and lone pairs.
- S2
Central atom: Nitrogen (N)
- S3
Number of bonding pairs: 3 (one with each hydrogen atom)
- S4
Number of lone pairs: 1
- S5
Step 2: Use VSEPR theory to predict the molecular shape.
- S6
With 4 electron pairs (3 bonding, 1 lone), the shape is trigonal pyramidal.
- S7
Step 3: Determine the bond angle.
- S8
The presence of a lone pair compresses the bond angles slightly from the ideal tetrahedral angle of 109.5° to approximately 107°.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the central atom and count the number of bonding pairs and lone pairs.
- S2
Central atom: Nitrogen (N)
- S3
Number of bonding pairs: 3 (one with each hydrogen atom)
- S4
Number of lone pairs: 1
- S5
Step 2: Use VSEPR theory to predict the molecular shape.
- S6
With 4 electron pairs (3 bonding, 1 lone), the shape is trigonal pyramidal.
- S7
Step 3: Determine the bond angle.
- S8
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°
Common mistakes
- Forgetting to account for lone pairs when determining molecular shape. — 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. — 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. — 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. — 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. — 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. — Always identify the least electronegative atom (excluding hydrogen) as the central atom. Practice with different molecules to reinforce this concept.
Where the marks go
- Full worked solution (all marking points)4 marks