Relate 3D molecular shape to bond angle and polarity
Three steps, the way the exam actually works: work through the lab, write down your own observations, 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 evidence will cost you the point — exactly as it would with a real reader.
Predict before you look
- A molecule can have polar bonds and still be nonpolar overall. What has to be true of the bond dipoles for that to happen?
- CO₂ and H₂O each have two identical bonds to a central atom. Which one has lone pairs on that central atom, and what will they do to the angle?
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you found
This is your reading of the source, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Shape of H2O | |
|---|---|
| Shape of CH4 | |
| Shape of CO2 | |
| Approximate H–O–H bond angle | ° |
| Approximate H–C–H bond angle | ° |
| Approximate O–C–O bond angle | ° |
Answer the free response
The same atoms can produce very different molecular behavior because of shape. (a) Using your three molecules, explain why methane is nonpolar even though each C–H bond has some electron density difference, while water is polar. (b) Compare water’s bond angle with methane’s and explain what that difference says about lone-pair repulsion. (c) Explain why carbon dioxide is nonpolar even though each C=O bond is polar. (d) A student says molecular polarity can be decided from bond polarity alone without drawing a shape. Use your data to evaluate that claim.
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