1.4.8Physical Chemistry
Shapes of Covalent Compounds — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 17 of 42
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Electron-Pair Repulsion
The shape depends on the number of bonding pairs and lone pairs around the central atom.
Principle
- Principle Pairs of electrons around a central atom repel each other and move as far apart as possible, which sets the shape.
Repulsion order
- Repulsion order Lone pair–lone pair repulsion is greatest, then lone pair–bonding pair, and bonding pair–bonding pair is least.
Lone pairs
- Lone pairs Each lone pair squeezes the bond angle by about 2.5° because it repels more strongly than a bonding pair.
Multiple bonds
- Multiple bonds A double or triple bond counts as one region of electron density when predicting shape.
Steps
- Steps State the number of bonding pairs and lone pairs, name the electron-pair geometry, and then give the shape and bond angle with a reason.
Section 2 of 4Shapes with No Lone Pairs
Shapes with No Lone Pairs
Linear
- Linear Two bonding pairs, bond angle 180°, as in BeCl₂ and CO₂.
Trigonal planar
- Trigonal planar Three bonding pairs, bond angle 120°, as in BF₃.
Tetrahedral
- Tetrahedral Four bonding pairs, bond angle 109.5°, as in CH₄ and NH₄⁺.
Trigonal bipyramidal and octahedral
- Trigonal bipyramidal and octahedral Five pairs give angles of 90° and 120° in PCl₅, and six pairs give 90° in SF₆.
Ions
- Ions The same method works for ions, so the sulfate ion SO₄²⁻ and the ammonium ion NH₄⁺ are both tetrahedral with 109.5° angles.
| Shape | Bond angle | Example |
|---|---|---|
| Linear | 180° | CO₂ |
| Trigonal planar | 120° | BF₃ |
| Tetrahedral | 109.5° | CH₄ |
| Trigonal bipyramidal | 90°, 120° | PCl₅ |
| Octahedral | 90° | SF₆ |
Section 3 of 4Shapes with Lone Pairs
1
Count the bonding pairs and lone pairs around the central atom to find the total number of electron pairs.
2
Place all the pairs in the geometry that holds them apart, such as tetrahedral for four pairs.
3
Name the shape by the arrangement of atoms only, ignoring the lone pairs.
4
Ammonia has three bonding pairs and one lone pair, giving a trigonal pyramidal shape with a bond angle of about 107°.
5
Water has two bonding pairs and two lone pairs, giving a bent (V-shaped) molecule with a bond angle of about 104.5°.
Section 4 of 4Exam Checklist & Tips
1.4.8 – Key Points for the Exam
- Bond angle The angle between two covalent bonds at a central atom.
- Lone pair repulsion Greater repulsion exerted by lone pairs, which reduces bond angles.
- Tetrahedral A shape with four bonds at 109.5°.
- Electron-pair repulsion theory The idea that electron pairs arrange themselves to minimise repulsion.
Common Mistakes
- Naming the shape from the arrangement of all pairs, including lone pairs, instead of just the atoms.
- Using 109.5° for water and ammonia without allowing for lone pair repulsion.
- Counting a double bond as two separate regions of electron density.
- Forgetting that the bond angle in NH₄⁺ is 109.5° because it has no lone pair.
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