Demonstration of Close-Packing Phenomena The subject of the close-packing of atoms is frequently discussed at the freshman chemistry level a3 well as at the juniorsenior inorganic chemistry level. When the sire of the claw exceeds the number of students that can conveniently see the instructor's model a t close hand, it becomes diificult to make clear the general features of close-packing as well as the distinction between cubic and hexagonal close-packing. Use of the blackboard and other printed visual aides is not very successfol. A simple, hut effective device for i h s t r a t i n g close-parking Figure 1 . Projection of a Figure 2. Projection of o has been used here with good response from our large freshman single layer close-pocked double layerclose-pocked chemislry classes (100+ students). A model of one layer of model. model. closmacked at,oms is constnlcted from stvrafoam halls hv "gluing" the halls together with chloroform or acetone. Using an overhead project,ar the one layer close-packed structure can he projected onto a screen as shown in Figure 1. The coordination number of any atom and the number of holes surrounding every atom in a. layer are clearly depicted. By placing a second layer of loose halls atop the first, layer and again projecting onto s. screen (Fig. 2) the blocking of one-half the holes can effectively he seen. With the third layer two alternatives are possible. If a single row of styrofoam halls is placed above one row of the remaining holes in the lattice then thelight will he completely blocked out on the screen. This is the 123123. . . layered structure corresponding to cuhic close-packing. On the other hand a hall can be placed directly above a ball in the first layer. Although the students can see yon form the third layer on the surface of the projector there is no apparent change in the projeotion an the screen. This is the 121212. . . layered structure corresponding to hexagonal close-packing. I n a similar fashion using a thin pointer the presence ot tetrahedral and octahedral holes can he illustrated. One dificulty with this presentation is that as the model exceeds the diameter of the overhead lens the holes in the first 2 ltlyers become distorted since the projector is no longer directly overhead. This can be seen far the hales near t,he edges of the model in Figure 2. This effect can he minimized by using small styroioam balls or marbles and cancentrat,ing on the center of the projection. In spite of this difficulty the sharp clear projections dramatically distinguish between the two difierent close-packed structures. Part of t,he reason for the effectiveness of this technique lies wit,h t,he fact that the students can see both the styrofoam model and the projection of the same model. As a result the differences between the two close-packed structures appears less a? a sleight of hand and more as a real structural phenomena. ~
~~~
.~~ ~~~~~
~~~
~
~
EDWARD R. B l a ~ n a u ~
Nmv MEXICOSTATEUNIVERSITY L.\s CRUCFIS, NEWMEXICO 88001
