2.2.5Additional lessons
Shapes of Molecules — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 29 of 172
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 6The VSEPR Idea
The shape of a molecule is determined by repulsion between electron domains on the central atom.
Electron domain
- Electron domain A region of electron density: a bonding pair, a lone pair, or a whole multiple bond.
Repulsion
- Repulsion Domains repel each other and arrange themselves as far apart as possible.
Lone pairs
- Lone pairs They occupy more space than bonding pairs, so they squeeze the bond angles smaller.
Order of repulsion
- Order of repulsion Lone pair–lone pair repulsion is greater than lone pair–bonding pair, which is greater than bonding pair–bonding pair.
Section 2 of 6Shapes With No Lone Pairs
Shapes With No Lone Pairs
Two domains
- Two domains BeCl₂ and CO₂ are linear, with a bond angle of 180°.
Three domains
- Three domains BF₃ is trigonal planar, with bond angles of 120°.
Four domains
- Four domains CH₄ is tetrahedral, with bond angles of 109.5°.
Five and six
- Five and six PCl₅ is trigonal bipyramidal and SF₆ is octahedral, with 90° angles.
Section 3 of 6Shapes With Lone Pairs
Ammonia
- Ammonia NH₃ has four domains and one lone pair, so it is trigonal pyramidal with a bond angle near 107°.
Water
- Water H₂O has four domains and two lone pairs, giving a bent shape with an angle near 104.5°.
Sulfur dioxide
- Sulfur dioxide SO₂ has three domains and one lone pair, so it is bent with an angle slightly under 120°.
| Domains | Lone pairs | Shape |
|---|---|---|
| 4 | 0 | Tetrahedral |
| 4 | 1 | Trigonal pyramidal |
| 4 | 2 | Bent |
Section 4 of 6Higher Level Shapes
1
Five domains with one lone pair give a see-saw shape, as in SF₄.
2
Five domains with two lone pairs give a T-shape, as in ClF₃.
3
Five domains with three lone pairs give a linear molecule, as in XeF₂.
4
Six domains with one lone pair give square pyramidal BrF₅, and with two lone pairs give square planar XeF₄.
5
Lone pairs are placed in positions that minimise the strongest repulsions.
Section 5 of 6Worked Example
Question Predict the shape of SF₄.
- Valence electrons on S6; four F atoms each form one bond.
- Electrons6 + 4 = 10 around S, which is five pairs: four bonding and one lone pair.
- Five domains arrange as a trigonal bipyramid, and the lone pair takes one position.
Answer SF₄ is see-saw shaped.
Section 6 of 6Exam Checklist & Tips
2.2.5 – Key Points for the Exam
- VSEPR A model predicting molecular shape from repulsion between electron domains.
- Electron domain A bonding pair, lone pair or multiple bond region on the central atom.
- Bond angle The angle between two bonds at the central atom.
- Molecular geometry The arrangement of atoms in space, ignoring lone pairs.
Common Mistakes
- Counting a double bond as two domains instead of one.
- Naming the shape from all domains instead of from the atoms only.
- Forgetting that lone pairs reduce bond angles below the ideal value.
- Giving tetrahedral for NH₃; it has a lone pair and is trigonal pyramidal.
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