overhead projector
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DORISKOLB Bradley University Peoria, IL 61625
Close Packing of Identical Spheres Daryl L. Ostercamp Concordia College Moorhead, M N 56562 The noble gases--as well a s a majority of the alkaline earth metals, the transition metals, and the lanthanidescrystallize in either hexagonal close-packed (HCP, or cubic close-mcked tCCP, structures.',' Ordinary marhles can be placed in a wooden rack on the stagc ot'anbverhead projector to illustrate the close-pack coordlnatlon number of 12, that is, the arrangement k i t h 12 nearest neighbor atoms. This can also be used to show two types of alternative stacking of successive layers. ABABAB (as seen in HCP) and ABCABC (as seen in CCP)
Figure 2. AB layers of close-packed spheres.
A rhombus with a 3%-in. inside dimension, with 60' and 120' oblique angles, was assembled from a wooden stock (3/4 x 1/2 in.) cut in a miter box. Marbles with masses of 5.0-5.6 g were selected from several bags purchased in a local toy store.3 The erno on strati on^ The wooden rack with a n initial layer Aof 36 marbles is shown in Figure 1. Here each marble is in contact with 6
Figure 3. ABA triad of hexagonal close-packed spheres. are asked to confirm a coordination number of 12 for each atom in the close-pack arrangement. closeFieure .. 3 shows the ABA triad of the hexagonal .. pack repeatingntructure with its array ofoctahedral holes. Altcrnativelv, one mas arrange the 16 marbles of the third layer to eclipse the o&hedril openings, thus illustrating the ABC triad of the cubic close-pack arrangement.
Figure 1. Single close-packed layer of spheres. other snheres. Addition of laver B (25 marbles) brines ., three additional spheres inconiact with each interior marble in laver A. This oroduces thc central oattern of 16octahedral gales displayed in Figure 2. At tkis point students
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Wyckofl, R. W. G. CrystalStructures,2nd ed; Interscience: New I , p 10. York, i963; VOI. Oxtoby, D. W.; Nachtrieb. N. H.; Freeman, W. A. Chemistry:Science of Chanoe: Saunders: Philadebhia. 1990: D 862. ~~~ro~im~ half t Loft ethe l ~marble's wire used. Individual masses were 3.9-6.3 g. For an active demonstration of close packing, see Knox, K. J. Chem. Educ. 1990,67,700.
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Journal of Chemical Education
An Overhead Projector Demonstration of Nuclear Beta Emission John J. Fortman Wright State University Dayton, OH 45435
Students often find P-decay processes difficult to understand because they are not accustomed to thinking of electrons in the nucleus. I recommend this easy overhead projector demonstration to illustrate a n analogy for beta emission, with its concurrent conversion of a neutron to a proton. As shown in the figure, I have used a large washer with a positive sign to serve a s a proton. A smaller washer, which fits in the hole of the larger washer, shows a negative sign. When the smaller washer is inserted in the hole
