1.5.8Physical Chemistry
Predicting Structures — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 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 5Electron Pair Repulsion Theory
Core idea
- Core idea Electron pairs around a central atom repel each other and arrange themselves as far apart as possible.
Lone pair effect
- Lone pair effect Lone pairs repel more than bonding pairs, so they squeeze bond angles slightly smaller.
Bonding regions
- Bonding regions Double bonds count as one region of electron density when predicting shape.
Shape names
- Shape names The molecule shape depends on how many bonding pairs and lone pairs surround the central atom.
Section 2 of 5Shapes and Bond Angles
Shapes and Bond Angles
Linear
- Linear Two bonding pairs and no lone pairs, as in CO₂, give an angle of 180°.
Trigonal planar
- Trigonal planar Three bonding pairs, as in BF₃, give 120° in a flat triangle.
Tetrahedral
- Tetrahedral Four bonding pairs, as in CH₄, give 109.5°.
Pyramidal and bent
- Pyramidal and bent NH₃ has three bonding pairs and one lone pair giving 107°;
- H₂O has two lone pairs giving 104.5°.
| Species | Bonding pairs | Lone pairs | Shape | Angle |
|---|---|---|---|---|
| CH₄ | 4 | 0 | Tetrahedral | 109.5° |
| NH₃ | 3 | 1 | Pyramidal | 107° |
| H₂O | 2 | 2 | Bent | 104.5° |
| PCl₅ | 5 | 0 | Trigonal bipyramidal | 90° and 120° |
| SF₆ | 6 | 0 | Octahedral | 90° |
Section 3 of 5Predicting Structure from Properties
1
If the substance has a low melting point and does not conduct, it is probably simple molecular.
2
If it has a high melting point, does not conduct as a solid but conducts when molten, it is ionic.
3
If it conducts as a solid and is malleable, it is metallic.
4
If it has a very high melting point, is hard and does not conduct, it is a giant covalent network.
5
Always support the choice by naming the particles, the forces or bonds, and what must be overcome on melting.
Section 4 of 5Polarity and Shape
Symmetrical molecules
- Symmetrical molecules CO₂ and CH₄ contain polar bonds, but the dipoles cancel, so the overall molecule is non-polar.
Unsymmetrical molecules
- Unsymmetrical molecules Water and ammonia have dipoles that do not cancel, making them polar overall.
Why it matters
- Why it matters Polar molecules attract each other and dissolve in polar solvents such as water.
Exam approach
- Exam approach Draw the shape, mark the dipoles on the bonds and check whether their effects cancel.
Section 5 of 5Exam Checklist & Tips
1.5.8 – Key Points for the Exam
- Electron pair repulsion Electron pairs around an atom arrange to minimise repulsion.
- Bond angle The angle between two bonds at a central atom.
- Lone pair repulsion Lone pairs repel more strongly than bonding pairs.
- Polar molecule A molecule with an overall dipole because bond dipoles do not cancel.
Common Mistakes
- Giving the shape of NH₃ as tetrahedral; the electron pairs are tetrahedral but the molecule is pyramidal.
- Stating 109.5° for water without allowing for lone pair repulsion.
- Saying a molecule with polar bonds is always polar; symmetry can cancel dipoles.
- Forgetting to count lone pairs on the central atom before choosing the shape.
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