Explain how lone pairs compress bond angles
Three steps, the way the exam actually works: work through the lab, write down your own measurements, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own numbers will cost you the point — exactly as it would with a real reader.
Run the investigation
- 1Compare a tetrahedral molecule, a trigonal pyramidal molecule, and a bent molecule in the VSEPR viewer.
- 2Record the number of bonding regions and lone-pair regions around the central atom for each.
- 3Record the approximate bond angle for each structure.
- 4Use the comparison to rank how strongly lone pairs distort ideal geometry.
Booting the lab…
Record what you measured
These are your numbers, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Bond angle in the tetrahedral example | ° |
|---|---|
| Bond angle in the trigonal pyramidal example | ° |
| Bond angle in the bent example | ° |
| Lone pairs on the trigonal pyramidal central atom | |
| Lone pairs on the bent central atom |
Answer the free response
The VSEPR model predicts both shape and angle distortion. (a) Starting from the ideal tetrahedral angle, explain why the angle becomes smaller in a trigonal pyramidal molecule and smaller still in a bent molecule with the same electron-domain geometry. (b) Distinguish electron-domain geometry from molecular geometry using one of your examples. (c) Explain why lone-pair repulsion is stronger than bonding-pair repulsion. (d) A student says the shape of a molecule can be predicted by counting atoms attached to the center and ignoring lone pairs. Evaluate that claim using your recorded structures.
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