Connect ionic size ratio to lattice coordination
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
- Coordination number counts the nearest neighbors of opposite charge. Would a LARGER cation fit more neighbors around it, or fewer?
- NaCl and CsCl are both ionic solids with a 1:1 formula. If the formula is the same, what could possibly make their structures differ?
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.
| Coordination number in NaCl | |
|---|---|
| Coordination number in CsCl | |
| Structure description of NaCl | |
| Structure description of CsCl |
Answer the free response
Two ionic compounds can have different crystal structures even when both are made of positive and negative ions. (a) Compare the coordination numbers you observed in NaCl and CsCl, and explain what coordination number means physically. (b) Use relative ion size to explain why CsCl can support a higher coordination number than NaCl. (c) Explain why neither solid consists of discrete NaCl or CsCl “molecules” inside the crystal. (d) A student argues that stronger ionic attraction always means a higher coordination number. Evaluate that claim using your two lattices.
Sign in to have this graded and saved to your progress.