Paper-and-Glue Unit Cell Models W

Chemistry for Everyone
be used to show the packing in the hexagonal crystal system.
Note that the hexagonal unit cell has no 6-fold axis, but 3fold
and 2-fold axes are easy to identify from the models.
Students can compare the number of unit cells required to
make up a full atom in comparison to the cubic systems (6
vs 8 unit cells).
Finally, we have constructed a unit cell template for the
sodium chloride structure (Figure 5). This unit cell shows
the packing pattern very nicely but has a deficiency in that
the ion at the body-centered position ( 1 /2, 1 /2, 1 /2) is not visible.
Thus, students cannot use this model to correctly count
the number of ions per unit cell.
Full-sized versions of the templates, which can be printed
on 8.5-in. × 11-in. paper and result in cube sides of about 6
cm, may be found on JCE Online. W Students cut out the figure
and fold the paper along each of the lines. The tabs are
then glued inside the cube. The unit cells are easy for students
to construct and can be used in high school and college
chemistry courses.
The cubic paper-and-glue templates (Figures 1 and 2)
were designed to help students understand the structure of
cubic unit cells in the context of a crystalline solids unit of
instruction. We have incorporated the models into an intro-
A
B
Figure 3. Templates for body-centered cubic unit cell.
duction to crystalline solids in our general chemistry lecture
course. One lecture before introducing solid-state chemistry,
we hand out the template sheets to the students. We tell the
students to cut them out, to tape or glue them into cubes,
and to bring the assembled models to the next lecture (Figure
6). Glue sticks provide the simplest approach to fastening
the sides to the tabs during assembly of the models.
Most of the students cooperate and arrive with their
models in hand. We show microscopic diagrams and animations
of various crystalline solids and discuss the structures
of the unit cells that comprise these solids. When we introduce
the simple cubic and face-centered cubic unit cells, the
students bring their models to the front of the lecture hall
and add their models to others to build three-dimensional
crystalline solids (Figure 7).
Even with 200 students, this can be done in 5–10
minutes. Not only is the model effective at showing how the
eighths, quarters, and halves make up whole spheres in the
crystal, students are also more involved in creating the model.
Additionally, students can examine the holes in the structure
(white areas) and consider how the structure of the unit cell
dictates the shape of a particular hole (cubic, octahedral, or
tetrahedral). We then invite the students to take their cubes
Figure 4. Template for a hexagonal unit cell.
Figure 5. Template for a sodium chloride unit cell.
158 Journal of Chemical Education • Vol. 80 No. 2 February 2003 • JChemEd.chem.wisc.edu