D. P. Mellor and V. shuk University of New South Wales Broadway, N. S. W., Australia
Close Packing of Atoms A lecture demonstration
In a recent elementary chemistry television program, we wished to show the hexagonal close packing of metal atoms and the effect of melting on the ordered atomic arrangement. The problem, to find some way of causing the model atoms to cohere, was solved in the following manner. Steel balls (l/a-in. diameter) representing metal atoms were placed in a perspex hexagonal prism of appropriate dimensions. Aft,er it was filled, the container was tapped to ensure that the spheres were in fact close-packed and was topped off if necessary. The top of the container was covered with a piece of cardboard, then inverted and placed over the pole of a permanent magnet (Fig. 1). The perspex container was then carefully wit,hdrawn leaving t,he magnetised balls to cohere in an otherwise unsupported closepacked hexagonal prismatic column (Figs. 2 and 3). To show the effect of melting, all that is necessary is to suddenly remove the magnet, and the whole structure instantly collapses (Fig. 4). While the experiment does not represent as well as one would like what actually happens on the atomic scale, it does show in a striking manner the phenomenon of close packing and the disappearance of the ordered array of atoms which t,akes place during the process of melting. I t is advisable to demagnetise the ball bearings between each demonstration by shaking them fairly violently in a suitable container. Care is needed in using the demonstration because it
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1 Jovrnd o f Chemical Education
could give rise to misconceptions. In the process of melting, t,he attractive forces between atoms do not disappear as they do between the steel halls in the model when t,he magnet is withdrawn.
Figure 3.
Figure 4.
