Is a water molecule straight or bent?
Water’s Lewis structure is often drawn as a straight H–O–H, but experiments show the molecule is bent: the two O–H bonds meet at 104.5°. CO₂ also has three atoms, yet it is perfectly straight, at 180°. As lesson 2.4.4 warned, a Lewis structure shows how atoms are connected, not the molecule’s shape. To predict shape, we need another model.
VSEPR theory
All the electron pairs around a central atom, both bonding pairs and lone pairs, are negatively charged. They repel each other and spread as far apart as possible to minimize that repulsion. This is valence-shell electron-pair repulsion (VSEPR) theory价层电子对互斥模型.
To use it, count the electron domains价层电子对 on the central atom: each bond to another atom counts as one domain, whether it is single, double, or triple (all the electrons in a multiple bond point the same way), and each lone pair counts as one domain. The easiest way is to draw the Lewis structure first and count around the central atom.
Chinese textbooks use a formula instead: the number of lone pairs on the central atom is ½(a − xb), where a is the central atom’s valence electrons (minus the charge for a cation, plus it for an anion), x is the number of atoms bonded to it, and b is the most electrons each bonded atom can accept (1 for H; 8 minus the valence electrons for other atoms, such as 2 for O and 1 for Cl). For water: a = 6, x = 2, b = 1, so there are ½ × (6 − 2) = 2 lone pairs; with 2 bonds, that makes 4 domains. Both methods always agree.
From electron domains to molecular shape
The number of domains fixes how the electron pairs are arranged in space, the electron geometryVSEPR 模型. Ignore the lone pairs and look only at where the atoms are, and you have the molecular geometry分子的空间结构:
| Domains | Lone pairs | Electron geometry | Molecular geometry | Angle (example) |
|---|---|---|---|---|
| 2 | 0 | linear | linear | 180° (CO₂) |
| 3 | 0 | trigonal planar | trigonal planar | 120° (BF₃) |
| 3 | 1 | trigonal planar | bent | below 120° (O₃, 116.8°) |
| 4 | 0 | tetrahedral | tetrahedral | 109.5° (CH₄) |
| 4 | 1 | tetrahedral | trigonal pyramidal | 107° (NH₃) |
| 4 | 2 | tetrahedral | bent | 104.5° (H₂O) |
CH₄, NH₃, and H₂O all have 4 domains, yet their bond angles shrink. A lone pair is held by only one nucleus, so it spreads out more than a bonding pair, takes up more room, and repels more strongly:
- Lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion.
- NH₃’s one lone pair squeezes the bond angle from 109.5° to 107°; H₂O’s two lone pairs squeeze it further, to 104.5°.
Five and six electron domains
Central atoms in period 3 and beyond can have 5 or 6 electron domains (the expanded octets of lesson 2.4.4). Five domains form a trigonal bipyramid (PCl₅, with 90° and 120° angles); lone pairs take equatorial positions first, giving seesaw (SF₄), T-shaped (ClF₃), and linear (XeF₂) molecules. Six domains form an octahedron (SF₆, 90°); one lone pair gives a square pyramid (BrF₅), and two lone pairs sit opposite each other to give a square planar molecule (XeF₄). Chinese textbooks focus on 2 to 4 domains; AP expects all of these.
Choose a molecule and drag the model to see it from every side. Turn on the electron-domain lines to compare the electron geometry with the molecular geometry:
Drag the model to see it from any angle
Example 1: Ammonia
Predict the shape and bond angle of NH₃.
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Lewis structure and lone pairs. Nitrogen in the center, bonded to three H atoms. By the formula ½(a − xb), with a = 5 valence electrons on N, x = 3 bonded atoms, and b = 1 for H:
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Electron domains. 3 bonds + 1 lone pair = 4 domains.
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Electron geometry. Four domains repel one another into a tetrahedral arrangement.
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Molecular geometry. Count only the atoms: one corner of the tetrahedron holds the lone pair, so the three H atoms and the N form a trigonal pyramid, with N at the top.
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Bond angle. The lone pair repels more strongly than bonding pairs and squeezes the H–N–H angle from 109.5° to 107°.
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A common mistake. Calling it tetrahedral because there are 4 domains. The electron geometry includes lone pairs; the molecular shape counts only atoms.
Example 2: CO₂ and SO₂
Why is CO₂ linear while SO₂ is bent?
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CO₂’s lone pairs. Central C has a = 4; it bonds to 2 O atoms, each with b = 2:
2 domains and no lone pairs: linear.
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SO₂’s lone pairs. Sulfur, in the same group as oxygen, has a = 6:
2 bonds + 1 lone pair = 3 domains, a trigonal planar electron geometry.
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SO₂’s shape. One of the three positions holds the lone pair, so S and the two O atoms form a bent molecule with an angle below 120°.
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Conclusion. The difference is whether the central atom has a lone pair: sulfur has 2 more valence electrons than carbon, which is exactly one extra lone pair.
- CO₂
2 bonding domains, no lone pairs.
- BF₃
3 bonding domains on boron and no lone pairs: trigonal planar.
- H₂O
4 domains, 2 of them lone pairs: bent, 104.5°.
- NH₃
4 domains, 1 of them a lone pair: trigonal pyramidal, 107°.
- CH₄
4 bonding domains, no lone pairs: tetrahedral, 109.5°.
- H₂S
S is in the same group as O: 2 bonds and 2 lone pairs, bent like water.
- PCl₃
P is in the same group as N: 3 bonds and 1 lone pair, trigonal pyramidal like ammonia.
- NH₄⁺
4 bonds and no lone pairs: tetrahedral.
- SO₃
3 bonding domains and no lone pairs on sulfur: trigonal planar.
Common mistakes
Mistake 1: “Water’s Lewis structure is a straight line, so water is linear.” A Lewis structure doesn’t show shape. Oxygen has 2 lone pairs, so its 4 domains are arranged tetrahedrally, and the two H atoms and O form a bent molecule.
Mistake 2: “NH₃ and BF₃ both have 3 bonds, so both are trigonal planar.” Nitrogen in NH₃ also has a lone pair, giving 4 domains and a trigonal pyramidal shape; boron in BF₃ has no lone pair, so only BF₃ is trigonal planar. Always count lone pairs as domains.
Mistake 3: “Electron geometry and molecular geometry are the same thing.” They match only when the central atom has no lone pairs. Water’s electron geometry is tetrahedral, but its molecular geometry is bent, because the lone pairs aren’t atoms.
Quick check
Which molecule is trigonal pyramidal?
NH₃ has 4 electron domains, one of them a lone pair, so it is trigonal pyramidal. BF₃ is trigonal planar, CH₄ is tetrahedral, and CO₂ is linear.
How do the bond angles of CH₄, NH₃, and H₂O compare?
All three have 4 electron domains, but 0, 1, and 2 lone pairs. Lone pairs repel more strongly, squeezing the angle from 109.5° to 107° and then to 104.5°.
Summary
- VSEPR theory: the electron domains around a central atom repel each other and spread as far apart as possible. Each bond (single, double, or triple) and each lone pair counts as one domain.
- 2, 3, and 4 domains give linear (180°), trigonal planar (120°), and tetrahedral (109.5°) electron geometries; ignoring the lone pairs gives the molecular geometry, such as bent or trigonal pyramidal.
- Lone pairs repel more strongly and shrink bond angles: CH₄ 109.5°, NH₃ 107°, H₂O 104.5°.
On to AP (for later)
AP U2.7 asks you to predict shapes and ideal bond angles for 2 to 6 electron domains, and to explain that lone pairs make angles slightly smaller than ideal. It also links the number of domains to hybridization: 2, 3, and 4 domains correspond to sp, sp², and sp³. Chinese textbooks teach hybrid orbitals too, with the same correspondence.