Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Shapes of Molecules Exam Questions
Free Edexcel International A Level Chemistry exam questions for Shapes of Molecules, with a mark scheme for every pack. Covers electron pair repulsion theory, bonding and lone pairs, molecular shapes, bond angles and the shapes of molecules and ions.
Shapes of Molecules exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Shapes of Molecules
Shapes of molecules is a Unit 1 topic in Edexcel International A Level Chemistry that uses electron pair repulsion theory to predict the shapes of molecules and ions. You count bonding and lone pairs around a central atom, then use the rule that lone pairs repel more than bonding pairs to predict bond angles. The topic builds directly on Covalent bonding and is needed for explaining polarity and Intermolecular forces in Unit 2, and for complex ions with transition metals in Unit 5. Examiners frequently give unfamiliar molecules or ions and ask for the shape, the bond angle and a clear explanation.
What you need to know
- Electron pair repulsion theory Electron pairs around a central atom repel each other and arrange themselves to be as far apart as possible. Lone pairs repel more strongly than bonding pairs, each reducing the bond angle by about 2.5 degrees.
- Shapes with no lone pairs Two pairs: linear, 180 degrees (BeCl2, CO2). Three: trigonal planar, 120 degrees (BF3). Four: tetrahedral, 109.5 degrees (CH4). Five: trigonal bipyramidal, 90 and 120 degrees (PCl5). Six: octahedral, 90 degrees (SF6).
- Shapes with lone pairs NH3 has three bonding and one lone pair, trigonal pyramidal, about 107 degrees. H2O has two bonding and two lone pairs, bent (V-shaped), about 104.5 degrees.
- Multiple bonds Treat a double or triple bond as one bonding region when predicting shape, so CO2 is linear and the carbon in ethene is trigonal planar.
- Ions Add an electron for each negative charge and remove one for each positive charge before counting pairs. NH4+ is tetrahedral (109.5 degrees) and NH2- is bent.
- Explaining a shape State the number of bonding pairs and lone pairs, say that pairs repel to be as far apart as possible, name the shape and give the angle, mentioning extra lone pair repulsion where relevant.
Common mistakes in Shapes of Molecules
- Forgetting to adjust electron count for ions, which gives the wrong number of pairs.
- Saying atoms or bonds repel. It is the electron pairs that repel.
- Giving H2O a bond angle of 107 degrees. It is about 104.5 degrees because it has two lone pairs.
- Calling NH3 tetrahedral. Its electron pairs are arranged tetrahedrally but the shape of the atoms is trigonal pyramidal.
- Counting each bond in a double bond as a separate pair when predicting shape.
Try three Shapes of Molecules exam-style questions
1. Predict the shape and bond angle of PH3 and explain your answer. (3 marks)
Show the mark scheme answer
Three bonding pairs and one lone pair; lone pair repels more than bonding pairs; trigonal pyramidal, about 107 degrees.
2. State the shape of SF6 and the F to S to F bond angle. (2 marks)
Show the mark scheme answer
Octahedral; 90 degrees.
3. Explain why the bond angle in water is smaller than in methane. (2 marks)
Show the mark scheme answer
Water has two lone pairs and two bonding pairs, methane has four bonding pairs; lone pairs repel more strongly than bonding pairs, so the bond angle decreases from 109.5 to about 104.5 degrees.
Shapes of Molecules FAQs
What does Edexcel International A Level Chemistry Shapes of molecules cover?
It covers electron pair repulsion theory and using numbers of bonding and lone pairs to predict the shapes and bond angles of molecules and ions.
Which unit is Shapes of molecules examined in?
It is part of Unit 1, and you use it again for polarity and intermolecular forces in Unit 2 and for complex ion shapes in Unit 5.
Which shapes and angles must I memorise?
Learn linear 180, trigonal planar 120, tetrahedral 109.5, trigonal pyramidal 107, bent 104.5, trigonal bipyramidal 90 and 120, and octahedral 90 degrees, each with an example.
