The big idea (VSEPR): Valence Shell Electron Pair Repulsion theory says: the electron domains around a central atom repel each other and spread out as far apart as possible.
An electron domain is any group of electrons: a single, double or triple bond each counts as one domain, and so does each lone pair. The arrangement that keeps them furthest apart fixes the shape and the bond angle.
4 bonding pairs, 0 lone pairs → tetrahedral, 109.5°.
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Count domains, not bonds: A double or triple bond is still one domain (one direction). So count the number of atoms bonded to the centre plus the number of lone pairs — that total decides the geometry.
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Count the bonding domains and lone pairs on the central atom, then read off the shape and angle:
| Bonding domains | Lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | CO2, HCN |
| 3 | 0 | Trigonal planar | 120° | BF3 |
| 2 | 1 | Bent | ~118° | SO2 |
| 4 | 0 | Tetrahedral | 109.5° | CH4 |
| 3 | 1 | Trigonal pyramidal | 107° | NH3 |
| 2 | 2 | Bent | 104.5° | H2O |
The two-step method: (1) Draw the Lewis structure → count bonding atoms + lone pairs on the centre. (2) Match that count to the table to name the shape and state the angle.
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A lone pair repels more strongly than a bonding pair, so it pushes the bonds closer together — shrinking the bond angle. Compare three molecules that all have four domains around the central atom:
3 bonding pairs + 1 lone pair → trigonal pyramidal, 107°.
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2 bonding pairs + 2 lone pairs → bent, 104.5°.
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The 109.5 → 107 → 104.5 pattern: All three have four electron domains, but the angle falls as lone pairs increase:
- CH4 — 0 lone pairs → 109.5° - NH3 — 1 lone pair → 107° - H2O — 2 lone pairs → 104.5°
Each lone pair squeezes the angle by roughly 2–3°.
How this is tested: 'State and explain the shape / bond angle of …' is a routine Paper 2 question, and Paper 1A likes to ask you to order molecules by bond angle.
For full marks: give the shape name, the bond angle, and the reason (number of bonding domains and lone pairs, and lone-pair repulsion if relevant).
IB-style question — hydrogen cyanide (a)
(a) State and explain the molecular shape of hydrogen cyanide, HCN. [2]
How to score the marks
- From the Lewis structure (H–C≡N), the central carbon is bonded to 2 atoms and has 0 lone pairs.
- Two electron domains repel to 180° apart.
- So HCN is linear with a bond angle of 180°. (The triple bond counts as one domain.)
Final answer
Linear, 180° — carbon has two bonding domains and no lone pairs.
2 bonding domains around C (the triple bond counts as one) → linear, 180°.
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IB-style question — sulfur dioxide (b)
(b) Deduce the electron-domain geometry and the molecular shape of sulfur dioxide, SO2. [2]
How to score the marks
- From the Lewis structure, sulfur has 2 bonding domains (to the two O atoms) and 1 lone pair → 3 electron domains in total.
- Three domains give a trigonal planar electron-domain geometry.
- But one domain is a lone pair, so the molecular shape is bent, with an angle of about 119° (slightly less than 120° because the lone pair repels more).
Final answer
Electron-domain geometry trigonal planar; molecular shape bent (~119°).
2 bonding domains + 1 lone pair on S → bent, ~119°.
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