Ch 5 · Shapes of molecules · AS & A Level Chemistry 9701
Topic 5 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
The shape of a molecule follows directly from the number and type of electron pairs around its central atom. Cambridge questions ask you to deduce a shape and bond angle from a formula or dot-and-cross diagram, and to explain any deviation from the "ideal" angle using the greater repulsion of lone pairs — both skills built entirely on Valence Shell Electron Pair Repulsion (VSEPR) theory.
Key ideas — VSEPR theory
VSEPR theory states that the electron pairs (bonding pairs and lone pairs) around a central atom repel each other and arrange themselves as far apart as possible in space, to minimise repulsion. This arrangement of electron pairs determines the shape of the molecule and the bond angles between the atoms bonded to the central atom. The number of electron pairs, not the number of atoms, is what fixes the basic geometry.
🧑🏫 Teacher's explanation
Imagine you are sitting at a round dinner table with several friends. If everyone wants as much personal space as possible, you will naturally spread out evenly around the table. In a molecule, the 'people' are the electron pairs (both bonding pairs and lone pairs) surrounding the central atom. Because electrons are negatively charged, they repel each other strongly.
📖 Definitions
🔍 Worked example — Easy
Predict the shape and bond angle of CH_4.
🔍 Worked example — Standard
Predict the shape and bond angle of NH_3.
🔍 Worked example — Exam-style
Predict the shape and bond angle of SF_6.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Shapes with only bonding pairs
🧑🏫 Teacher's explanation
Imagine you are trying to hold several large, inflated balloons together at a single point. Because the balloons are filled with air, they naturally push against each other to get as much space as possible. In a molecule, the 'balloons' are the electron pairs (bonding pairs) surrounding the central atom. Since electrons are negatively charged, they repel one another.
📖 Definitions
🔍 Worked example — Easy
Predict the shape and bond angle of CO_2.
🔍 Worked example — Standard
Predict the shape and bond angle of CH_4.
🔍 Worked example — Exam-style
A molecule has a central atom with 5 bonding pairs and no lone pairs. Predict its shape and the two distinct bond angles present.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Shapes with lone pairs present
When lone pairs are present, the basic electron-pair geometry is still used to fix the arrangement, but the molecular shape is named from the positions of the atoms only. Ammonia, NH_3, has four electron pairs (three bonding, one lone) arranged tetrahedrally, giving a pyramidal shape with a bond angle of 107°. Water, H_2O, has four electron pairs (two bonding, two lone) also arranged tetrahedrally, giving a bent (non-linear) shape with a bond angle of 104.5°.
🧑🏫 Teacher's explanation
Imagine you are holding four balloons tied together at a single point. If you let them go, they naturally push away from each other to occupy as much space as possible, forming a tetrahedron. This is the 'electron-pair geometry'—the arrangement of all electron pairs (both bonding and lone pairs) around the central atom.
📖 Definitions
🔍 Worked example — Easy
Predict the shape and bond angle of CH_4 (methane).
🔍 Worked example — Standard
Predict the shape and bond angle of NH_3 (ammonia).
🔍 Worked example — Exam-style
Predict the shape and bond angle of H_2O (water).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Effect of lone pairs on bond angle
Lone pairs occupy more space around the central atom than bonding pairs, because a lone pair is held by only one nucleus and spreads out more, while a bonding pair is drawn between two nuclei and is more confined. The order of repulsion strength is therefore lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. Each lone pair present compresses the bond angles between the remaining bonding pairs by about 2.5° per lone pair relative to the ideal tetrahedral angle: methane (no lone pairs) is 109.5°, ammonia (one lone pair) is 107°, and water (two lone pairs) is 104.5°.
🧑🏫 Teacher's explanation
Imagine a group of people standing in a circle, all holding hands. If everyone is holding hands (bonding pairs), they spread out as much as possible to stay comfortable. However, if one person decides to use their hands to hold a large, bulky balloon (a lone pair) instead of holding hands, that balloon takes up much more space. To make room for the balloon, the people holding hands have to pull closer together, shrinking the angle between them.
📖 Definitions
🔍 Worked example — Easy
Predict the shape and bond angle of CH_4 using VSEPR theory.
🔍 Worked example — Standard
Explain why the bond angle in NH_3 is 107^° instead of 109.5^°.
🔍 Worked example — Exam-style
A molecule has a central atom with 3 bonding pairs and 2 lone pairs. Predict its shape and bond angle.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Predicting shapes of related molecules and ions
The same method applies to any species: count the electron pairs (bonding pairs plus lone pairs) around the central atom from its dot-and-cross structure, including any negative or positive charge, then assign the basic geometry and adjust the name for any lone pairs present. For example, NH_4^+ has four bonding pairs and no lone pairs, so it is tetrahedral (109.5°), like CH_4; H_3O^+ has three bonding pairs and one lone pair, so it is pyramidal, like NH_3.
🧑🏫 Teacher's explanation
Imagine you are holding several inflated balloons tied together at a single point. The balloons naturally push away from each other to find the most comfortable, spacious arrangement possible. In a molecule, the 'balloons' are the electron pairs (either bonding pairs between atoms or lone pairs sitting on an atom). Because electrons are negatively charged, they repel each other. This repulsion is the driving force behind molecular shape.
📖 Definitions
🔍 Worked example — Easy
Predict the shape and bond angle of CO_2.
🔍 Worked example — Standard
Predict the shape and bond angle of NH_3.
🔍 Worked example — Exam-style
Predict the shape and bond angle of SF_6.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Predict the molecular shape of PF_5. [1] (b) Predict the molecular shape of BF_4^-. [1] (c) Explain why the bond angles in PF_5 are not all equal. [2] [4 marks]
Worked example
Question. Deduce the shape and bond angle of a phosphorus trichloride molecule, PCl_3, and explain your reasoning. [3]
🔍 Worked example — Easy
Deduce the shape and bond angle of CO_2.
🔍 Worked example — Standard
Predict the shape and bond angle of CH_4.
🔍 Worked example — Exam-style
Deduce the shape and bond angle of PCl_3 and explain your reasoning. [3]
⚠️ Common errors
Common mistakes
Practice
Answers