2.2.5Additional lessons
Shapes of Molecules — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 26 of 98
Part 1 of 6The VSEPR Model
The shape of a molecule depends on how many domains surround the central atom.
Electron domain
- A region of electron density around the central atom: a bonding pair, a lone pair, or a multiple bond counted as one domain.
Repulsion
- Electron domains repel one another because they are all negatively charged.
Arrangement
- Domains therefore take positions as far apart as possible around the central atom.
Lone pair strength
- Lone pairs are held closer to the nucleus and repel more strongly than bonding pairs, so they squeeze the angles between bonds.
Part 2 of 6Shapes with Two to Four Domains
Shapes with Two to Four Domains
Linear
- Two domains and no lone pairs, for example CO₂ and BeCl₂, give a bond angle of 180°.
Trigonal planar
- Three bonding domains and no lone pairs, as in BF₃, form a flat shape with angles of 120°.
Tetrahedral
- Four bonding domains, as in CH₄, give a three-dimensional shape with angles of 109.5°.
Pyramidal and bent
- Four domains with one or two lone pairs give pyramidal NH₃ (about 107°) and bent H₂O (about 104.5°).
| Species | Domains | Lone pairs | Shape | Angle |
|---|---|---|---|---|
| CO₂ | 2 | 0 | linear | 180° |
| BF₃ | 3 | 0 | trigonal planar | 120° |
| CH₄ | 4 | 0 | tetrahedral | 109.5° |
| NH₃ | 4 | 1 | trigonal pyramidal | 107° |
| H₂O | 4 | 2 | bent | 104.5° |
Part 3 of 6Predicting a Shape
1
Draw the Lewis formula and identify the central atom.
2
Count the electron domains on the central atom, treating a double or triple bond as a single domain.
3
Decide the electron domain geometry: two is linear, three trigonal planar and four tetrahedral.
4
Subtract lone pairs to name the molecular shape, which describes only the positions of the atoms.
5
Estimate the bond angle, reducing it by a few degrees for each lone pair present.
Part 4 of 6Examples with Multiple Bonds and Ions
CO₂
- Two double bonds count as two domains, so the molecule is linear with angle 180°.
NH₄⁺
- Four bonding domains and no lone pair make the ion tetrahedral with all angles 109.5°.
H₃O⁺
- Three bonds and one lone pair give a trigonal pyramidal ion, like NH₃.
SO₂
- Two domains for bonds and one lone pair on sulfur give a bent molecule with an angle near 120°.
Part 5 of 6Worked Example
Question Predict the shape of the ammonia molecule and explain why its bond angle is less than 109.5°.
- The central nitrogen atom has three bonding pairs and one lone pair, so four electron domains.
- Four domains arrange themselves tetrahedrally, but one is a lone pair, so the atoms form a trigonal pyramid.
- The lone pair repels the bonding pairs more strongly than they repel each other, closing the H–N–H angle to about 107°.
Answer Trigonal pyramidal, with a bond angle of about 107° because lone pair repulsion exceeds bonding pair repulsion.
Part 6 of 6Exam Checklist & Tips
2.2.5 – Key Points for the Exam
- Electron domain A region of electron density around an atom: a bonding pair, lone pair or multiple bond.
- VSEPR Valence shell electron pair repulsion, the model used to predict shapes.
- Bond angle The angle between two bonds at a central atom.
- Tetrahedral A shape with four groups around a central atom at 109.5°.
Common Mistakes
- Counting a double bond as two domains instead of one.
- Giving the electron domain shape instead of the molecular shape when lone pairs are present.
- Quoting 109.5° for water or ammonia instead of the smaller observed angles.
- Forgetting to include lone pairs when counting electron domains.
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