5. Return to the model of hexagonal close packing built in Part I. Rearrange the layers so that the top three-sphere layer is rotated through 60° and is no longer directly over the bottom layer. The rearrangement changed the type of packing from hexagonal to face-centered cubic. To confirm this, rotate the structure until four spheres forming a square face outward. When this orientation is obtained, remove the top five-sphere layer of the second model (face centered-cubic) and place it over the four spheres facing outward. Note that this results in a fcc packing tilted at an angle to the desk top. Answer Implications and Applications questions 3 and 4.

Part III

6. Use two-inch spheres to build the layers shown.

The space between spheres in each four-sphere layer should be about 5 mm.

Place the single sphere in the center of one layer and then place the second layer directly over the bottom layer. Study the model carefully to decide about the appropriateness of the name body centered-cubic (bcc). Sodium, potassium and a few other metals pack in this manner. Answer Implications and Applications question 5.

7. To construct the sodium chloride lattice, you'll need the structure of Part II (fcc) and 13 one-inch spheres. The two-inch spheres represent the face centered arrangement of chloride ions. Insert the 13 one-inch spheres (representing sodium ions) into the holes between the chloride ions in each layer. Note that there is also a face-centered arrangement of the one-inch (sodium) spheres.

8. Determine the coordination number of each size sphere. The sodium ion has a diameter of 0.19 nanometers; the chloride ion's diameter is 0.36 nm. This gives a ratio of about 1/2, so one-inch and two-inch spheres give a good approximation of the NaCl lattice. Answer Implications and Applications questions 6 and 7.

9. Zinc ions have a diameter of 0.15 nm while sulfide ions have a diameter of 0.37 nm. Use two-inch spheres to represent sulfide ions and three-quarters-inch spheres to represent zinc ions. Use the Part I structure (hcp) to represent sulfide ions. Using connectors, secure a three-fourth-inch sphere above each of the two-inch spheres in each of the three hcp structure layers. Then place the largest layer on the table and fit one of the smaller layers on top so the smaller spheres fit into alternate depressions. Invert these two layers and fit the other small layer, small spheres pointing upward, above the larger layer so the top-layer spheres are directly above the spheres on the bottom layer. Study the structure carefully. Answer Implications and Applications question 8.

10. Return the spheres and connectors after dismantling the models.

11. Thoroughly wash your hands before leaving the laboratory.

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