1.4Structure of Matter
Molecular Geometry & Intermolecular Forces — EST 2 Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 4 of 5 1 quiz
Lesson summary
Molecular Geometry & Intermolecular Forces EST Chemistry notes: Predict molecular shapes and bond angles using VSEPR.Symmetric molecules with polar bonds can be nonpolar: CO2, BF3, CCl4.H2O is bent and polar; NH3 is trigonal pyramidal and polar.Stronger IMFs → higher boiling poin…
- Hybridization matches the number of domains
- Predict the shape, bond angle and polarity of PCl3
- Molecular Geometry & Intermolecular Forces EST Chemistry: what to do next
- VSEPR theory
- Electron domain
- Dipole moment
Section 1 of 8Molecular Geometry & Intermolecular Forces
1
VSEPR theory counts electron domains around the central atom. A double or triple bond counts as a single domain because its electrons point in the same direction.
2
The electron geometry includes lone pairs, but the molecular shape describes only the atoms. Methane, ammonia and water all have four domains (tetrahedral electron geometry) but shapes of tetrahedral, trigonal pyramidal and bent.
3
Lone pairs take up more space than bonding pairs, so the H–N–H angle (107°) and H–O–H angle (104.5°) are smaller than 109.5°.
4Hybridization matches the number of domains
2 = sp, 3 = sp2, 4 = sp3, 5 = sp3d, 6 = sp3d2.
5
A molecule is polar if it has polar bonds arranged so their dipoles do not cancel. CCl4 is nonpolar because the four C–Cl dipoles cancel in a tetrahedron, but CH2Cl2 is polar.
6
Intermolecular forces (IMFs) act between molecules and are much weaker than covalent bonds within molecules. Boiling a molecular liquid overcomes IMFs; it does not break covalent bonds.
7
London dispersion forces exist in every substance and increase with molar mass and surface area, which is why large nonpolar molecules like octane or I2 can be liquids or solids at room temperature.
8
Hydrogen bonding requires H directly bonded to N, O or F. It explains water's high boiling point, high specific heat, surface tension and the structure of DNA and proteins.
Section 2 of 8Process & sequence
1
Draw the Lewis structure.
2
Count electron domains on the central atom.
3
Determine the electron-domain geometry.
4
Name the molecular shape from the positions of atoms only.
5
Decide if bonds are polar (electronegativity difference).
6
Decide if dipoles cancel (symmetric) or not to find overall polarity.
7
Identify the intermolecular forces and predict physical properties.
From Lewis structure to propertieswrite these steps in this order; each step is a marking point.
1
Draw Lewis structure
2
Count electron domains
3
Assign shape and angle
4
Check dipole cancellation
5
Identify strongest IMF
6
Predict boiling point
Section 3 of 8Compare
| Domains | Lone pairs | Shape | Bond angle | Hybridization / example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | sp / CO2 |
| 3 | 0 | Trigonal planar | 120° | sp2 / BF3 |
| 4 | 0 | Tetrahedral | 109.5° | sp3 / CH4 |
| 4 | 1 | Trigonal pyramidal | ~107° | sp3 / NH3 |
| 4 | 2 | Bent | ~104.5° | sp3 / H2O |
| 6 | 0 | Octahedral | 90° | sp3d2 / SF6 |
Lone pairs repel more strongly than bonding pairs, squeezing bond angles.
| Force | Occurs in | Relative strength | Example |
|---|---|---|---|
| London dispersion | All molecules | Weakest (grows with size) | CH4, I2 |
| Dipole-dipole | Polar molecules | Moderate | HCl, CH3Cl |
| Hydrogen bonding | H–N, H–O, H–F molecules | Strongest IMF | H2O, NH3, HF |
| Ion-dipole | Ions in polar solvent | Strong | NaCl in water |
Section 4 of 8Worked examples
1Predict the shape, bond angle and polarity of PCl3.
- P has 5 valence electrons: three bond to Cl and two form a lone pair, so 4 domains.
- Electron geometry tetrahedral; molecular shape trigonal pyramidal, angle a little under 109.5°.
- The dipoles do not cancel, so it is polar.
2Hydrogen bonds
Explain why H2O (18 g/mol) boils at 100 °C but H2S (34 g/mol) boils at −60 °C.
- Both are bent and polar, and H2S even has stronger dispersion forces.
- But water molecules form hydrogen bonds , which require much more energy to overcome.
- (H bonded to highly electronegative O)
- S is not electronegative enough for hydrogen bonding.
- But water molecules form hydrogen bonds , which require much more energy to overcome.
Section 5 of 8Keep going
Molecular Geometry & Intermolecular Forces EST Chemistry: what to do next
These Molecular Geometry & Intermolecular Forces EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Molecular Geometry & Intermolecular Forces EST Chemistry understanding with the practice questions below, then reread any Molecular Geometry & Intermolecular Forces EST Chemistry point you were unsure about before moving on.
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Previous lesson: 1.3 Bonding
3
Next lesson: 1.5 Nuclear chemistry
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Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks
Section 6 of 8Key terms
Section 7 of 8Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
VSEPR theory · Electron domain · Molecular geometry · Hybridization · Dipole moment · London dispersion forces · Dipole-dipole forces · Hydrogen bonding · Polarizability
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Marked practiceQuizzes for this lesson
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