1.5.1Physical Chemistry
Shapes of Simple Molecules and Ions — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 27 of 102
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.
Part 1 of 4Electron Pair Repulsion
Central principle
- Central principle Pairs of electrons in the outer shell of the central atom repel each other and so arrange themselves as far apart as possible.
Bonding pairs
- Bonding pairs Shared pairs between atoms are held between two nuclei, and each counts as a region of repulsion.
Lone pairs
- Lone pairs Non-bonding pairs are closer to the central nucleus, so they repel more strongly than bonding pairs.
Repulsion order
- Repulsion order Lone pair–lone pair repulsion is greater than lone pair–bonding pair, which is greater than bonding pair–bonding pair.
Multiple bonds
- Multiple bonds A double or triple bond counts as one region of electron density when working out the shape.
Part 2 of 4The Shapes
The Shapes
Linear
- Linear Two bonding pairs and no lone pairs give an angle of 180°, as in BeCl₂ and CO₂.
Trigonal planar
- Trigonal planar Three bonding pairs give 120° in BF₃, and the molecule is flat.
Tetrahedral
- Tetrahedral Four bonding pairs give 109.5°, as in CH₄ and NH₄⁺.
Pyramidal
- Pyramidal Three bonding pairs and one lone pair give about 107°, as in NH₃.
Non-linear
- Non-linear Two bonding pairs and two lone pairs give about 104.5°, as in H₂O.
| Bonding pairs | Lone pairs | Shape | Angle |
|---|---|---|---|
| 2 | 0 | Linear | 180° |
| 3 | 0 | Trigonal planar | 120° |
| 4 | 0 | Tetrahedral | 109.5° |
| 3 | 1 | Pyramidal | 107° |
| 2 | 2 | Non-linear | 104.5° |
| 5 | 0 | Trigonal bipyramidal | 90° and 120° |
| 6 | 0 | Octahedral | 90° |
Part 3 of 4Method for Any Species
1
Find the number of outer electrons on the central atom and adjust for the charge if the species is an ion.
2
Subtract the electrons used in bonds to find how many remain as lone pairs on the central atom.
3
Count the total regions of electron density to choose the basic arrangement of pairs.
4
Name the shape by looking only at the positions of the atoms, not the lone pairs.
5
State the bond angle and explain any reduction from the ideal angle by the extra repulsion from lone pairs.
Part 4 of 4Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- VSEPR Valence shell electron pair repulsion theory, used to predict molecular shapes.
- Bond angle The angle between two covalent bonds at the same atom.
- Lone pair A pair of outer electrons not used in bonding.
- Regions of electron density Bonding pairs and lone pairs (a multiple bond counts as one) around a central atom.
Common Mistakes
- Forgetting that lone pairs repel more than bonding pairs, so the angle is smaller than the ideal.
- Naming the shape from the arrangement of electron pairs instead of the arrangement of atoms.
- Treating a double bond as two regions of electron density when deciding the shape.
- Giving 109.5° for ammonia or water instead of the reduced angles.
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