Shapes of molecules
Introduced| English | Meaning |
|---|---|
| VSEPR theory/ˈvespə ˈθɪəri/ | Valence shell electron pair repulsion: electron pairs arrange as far apart as possible, setting molecular shape. |
| lone pair/ləʊn peə/ | A pair of outer-shell electrons on one atom that is not shared in a covalent bond. |
| tetrahedral/ˌtetrəˈhiːdrəl/ | Having four positions arranged towards the corners of a tetrahedron around a centre. |
| bonding pair/ˈbɒndɪŋ peə/ | A pair of electrons shared between atoms in a covalent bond. |
| bond angle/bɒnd ˈæŋɡl/ | The angle between two bonds that meet at the same atom. |
| octahedral/ˌɒktəˈhiːdrəl/ | Describing six positions arranged towards the vertices of an octahedron around a central point. |
Why molecules have shape
- To predict a shape, use VSEPR theory 价层电子对互斥理论.
- Electron pairs around the central atom push apart as far as possible.
- Lone pairs 孤对电子 push hardest, squeezing the angles.
VSEPR theory says the shape of a molecule is set by:
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Like charges repel, so the electron pairs push apart to minimise repulsion, fixing the shape.
A molecule with four bonding pairs and no lone pairs (e.g. methane) is:
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Four pairs repel to ~109.5°.
Lone pairs push harder
- A lone pair (not in a bond) repels more strongly than a bonding pair 成键电子对.
- Each lone pair squeezes the bond angle 键角 by about $2.5°$.
- That's why $\text{NH}_3$ ($107°$) and $\text{H}_2\text{O}$ ($104.5°$) have smaller angles than $\text{CH}_4$ ($109.5°$).
A molecular model shows the shape of a covalent molecule
Shapes of molecules
electron pairs repel (VSEPR)
Electron pairs spread out as far as possible; lone pairs squeeze the bond angle.
Why is the bond angle in water (104.5°) smaller than in methane (109.5°)?
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Lone pairs repel more than bonding pairs; water's two lone pairs squeeze the H–O–H angle below 109.5°.
The shapes
| Molecule | Shape | Angle |
|---|---|---|
| $\text{CO}_2$ | linear | $180°$ |
| $\text{BF}_3$ | trigonal planar | $120°$ |
| $\text{CH}_4$ | tetrahedral 四面体形 | $109.5°$ |
| $\text{NH}_3$ | pyramidal | $107°$ |
| $\text{H}_2\text{O}$ | bent | $104.5°$ |
| $\text{SF}_6$ | octahedral 八面体形 | $90°$ |
The common VSEPR shapes, from linear (180°) to octahedral (90°)
Methane (CH₄) is:
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Four bonding pairs and no lone pairs give a tetrahedral shape at 109.5°.
Match each molecule to its shape.
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CO₂ = linear (180°); BF₃ = trigonal planar (120°); NH₃ = pyramidal (107°).
Match each electron arrangement to its shape.
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Electron pairs repel to spread out; lone pairs repel more, so they shrink the angle.
VSEPR predicts a molecule's shape from the number of electron pairs around the central atom, and lone pairs repel more strongly than bonding pairs.
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That extra lone-pair repulsion is why water (104.5°) has a smaller angle than methane (109.5°).
Don't forget lone pairs
- Lone pairs repel more than bonding pairs, so they squeeze bond angles down.
- Water (2 lone pairs) is bent at ~104.5°; ammonia (1 lone pair) is ~107°.
You've got it
- VSEPR: electron pairs repel and spread out as far as possible
- lone pairs push harder than bonding pairs (≈ $2.5°$ each)
- linear $180°$, trigonal planar $120°$, tetrahedral $109.5°$, pyramidal $107°$, bent $104.5°$, octahedral $90°$