More on the Close-Packing of Atoms The subject of the close-packing of the atoms in crystals is important and fairly simple when properly presented. A recent demonstration is discussed hv Bimhaum in J. CHEM.EDUC... 149. A statement of the differ. . 678.. (1972)l. . ence hetween cuhir and hexagonal rlwe-parking is exrrprrunally difficult tu follow in m e ' s mind. .\ way tu illustrate rhls is to use the usual 3-7-3 layers of halls but make two adjaccnt outside balls black m the seven ball layer and two
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Figure 1. A hexagonal close-packed prism with the only 6-fold axis of symmetrv vertical.
Figure 2. A cube turned to show the cubic faces. One of three 4-fold axes of symmetry is shown vertical
Figure 3. A cubic close-packed cube with the same axis vertical (standing on a corner). It has now become one of three $-fold axes of symmetry.
halls black in the three hall layer. I t then heeomes quite easy to see the difference because in the cuhic the four black halls make a square diamond which is also a cuhic face. Also it is easy to see that a major 6-fold axis of symmetry is the vertical axis in the hexagonal prism and that this becomes a 3-fold axis of symmetry in the cuhic diagonal. See Figures 1, 2, and3.
H. Courtney Benedict Box 24 Windy Creek Road Glmdale. Oregon 97412
Volume 50. Number 6, June 1973 / 419
