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.
Run the investigation
- 1View the NaCl lattice and record the coordination number visible for each ion.
- 2View the CsCl lattice and record the coordination number visible for each ion.
- 3Compare the relative ion sizes implied by those two structures.
- 4Use the comparison to explain why the structures differ despite both being ionic solids.
Booting the lab…
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.
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