Teacher-Student Interaction

Students will probably need help in determining coordination number, particularly with the wurtzite structure. If a set of layers that students construct are permanently glued together prior to the activity, the task of helping students with coordination number, layer orientation, etc., is greatly simplified.

Visit with students about the meaning of metallic and ionic bonding, and regarding the question on the density of two models of close packing of equivalent spheres. Considerable individual teaching can take place through such interactions. Students may also need help answering some of other questions. Encourage students to record observations and answer questions as they perform the activity.

Anticipated Student Results

It is not necessary to have data tables. Students may record observations directly on their lab sheets and answer the questions immediately. Most students can do the model building without difficulty. Some may have difficulty with coordination number until they have the concept demonstrated for them.

Answers to Implications and Applications

1. The coordination number is 12; the central atom has 12 nearest neighbors.

2. Face-centered cubic packing has a sphere in the center of each cube face, hence the name.

3. No. The two types of close packing both have a coordination number of 12.

4. The density would be the same since both use the same volume with the same efficiency.

5. Alpha-ferrite is body centered cubic with a coordination number of 8.
   Gamma-ferrite is face centered cubic with a coordination number of 12.
   Delta-ferrite is body centered cubic with a coordination number of 8.

6. Each sodium ion is most closely surrounded by chloride ions and each chloride ion is most closely surrounded by sodium ions.

7. The coordination number of each sodium ion is 6, which is also the coordination number of each chloride ion.

8. The coordination number of sulfide ion is 4; it's also the coordination number of zinc ion in the Wurtzite lattice.

   
   

Post-Laboratory Activities

Discuss student answers to the questions. Have models available to show students any points they might have missed. Use the laboratory results in class discussion concerning metallic bonding and ionic bonding.

Extensions

1. Build models from two-inch spheres with tetrahedral (four spheres) and octahedral (six spheres) structures. Determine which size spheres-three-fourth-inch or one-inch-will fit the "holes" in these structures.

2. Calculate the ratios of smaller-to-larger spheres in each case (0.75/2 and 1/2). Then calculate the diameter ratios of Zn2+/S2- and Na+/Cl- (Zn2+ diameter = 0.15 nm, S2- diameter = 0.37 nm, Na+ diameter = 0.19 nm, Cl- diameter = 0.36 nm). Are the two-inch, one-inch, and three-fourths-inch spheres good models for these ions?

   
   

Assessing Laboratory Learning

A good way to assess student learning in this activity is to check student laboratory reports. A good time to check for understanding is while students are performing the activity. If they can answer the questions asked, count nearest neighbors, and build the models, then they have demonstrated good understanding of the material. Here are two possible student questions:
1. Explain how face-centered, hexagonal close packed, and body-centered cubic crystals are different.

2. The chemically similar alkali metal chlorides NaCl and CsCl have different crystal structures (fcc and bcc) while the chemically different NaCl and MnS have the same crystal structures (fcc). Why?

   
   

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