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CORK-BALL EXPERIMENTS ON CRYSTALLINE AND MOLECULAR STRUCTURE NORMAN DAVIDSON California Institute of Technology, Pasadena, California
B s c ~ u s sof the increasing emphasis on structural chemistry in freshman chemistry courses, the aut,hor believes that it is desirable to provide some laboratory experience with the subject at this level. It is not possible, in freshman laboratories, to study experimentally theelectronic structuresof the elements oranycrystalline or molecular structures. However, the construction of models may be used to give the student a concrete three dimensional appreciation of the geometrical arrangements of atoms in various molecular and crystalline structures, and to stimulate him to calculate quantitative geometrical relations for some important structures. In devising the experiments described here, it was desired to provide the student with materials with which he could assemble models realistically illustrating the packing of atoms in different structures, at least to the extent that the models should consist of spheres in contact, rather than open structures. An economically feasible way of doing this is to use cork balls for atoms, and double-ended pins to join them. "X-quality" cork balls can he obtained from the A~mstrong Cork Company a t prices per 1000 halls of $32.50 for the 1-in. size, 820.00 for 3/4-in. size, and $9.25 for '/%-in.balls (other sizes are also available). The double-ended pins are made from ordinary household pins by cutting off the heads obliquely with a side-cutter. If this wedge-shaped end of the pin is inserted into a cork ball with pliers, a second cork ball may be pushed on to the sharpened end of the pin by hand without driving the pin deeper into the first ball. Each student is supplied with a pair of pliers and a wide-mouth bottle containing 20 one-inch balls and a cork "pin-cushion." For models requiring balls of more than one size, S/4-in.and '/%-in.balls are dispensed in test-tube containers. In the first exercise, on the crystal structure of some of the elements, the students assemble a cube and cal-
A Body-Centered Cubic Structure
Chloroform. '/l-h B d b for Hydro-. J/4-in. a d s fer carbon, 1 . 3 ~ . B~II. for chlorine culate the values of the face diagonal and body diagonals in terms of the cube edge. These preliminary calculations assist t,hem in understanding the structures studied subsequently. They next construct several unit,s of a hody-cent,ered cubic structure, and calculate the relation between the cube edge and the atomic radius. They now derive the relation between the density, Avogadro's number, atomic weight, and the atomic radius (or the edge of the nnit cube). A similar study is made of cubic closest packing; in addition, the relation between two dimensional closest packing in a hexagonal array and the two kinds of three dimensional closest paclcing is developed. In an exercise on ionic crystals, the sodium chloride structure is constn~cted,and the type of coordination and the relat,ion to cubic closest packing noted. The ionic radii of Na+ and C 1 are 0.96 and 1.81 A; their ratiois 0.57,so 1/2-in.and 1-in. hallsrepresent the relative sizes of the two ions rather vell. The 3/4-in. and 1-in. balls are good for the construction of the cesium chloride structure, since the minimum radius ratio of cation to anion, to avoid anion-anion contact for coordination number eight, is 0.732. Trans C2H,C19
Several Hexagons1 Lmyof a Close-Packed Strusture
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Sodium Chloride, '/%.and ]-in Bells
JOURNAL OF CHEMICAL EDUCATION
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pairs of a noble gas octet around an atom are a t the corners of a regular tetrahedron, that a double bond involves sharing the electron pairs a t two of these corners, and a triple bond requires sharing the electron pairs a t three of the corners. I t is our experience that most of the students enjoy these laboratory exercises and that the exercises are essential and effective in getting the students to understand three dimensional relationships in structural chemistry. The idea that three-dimensional relations are readily comprehended and analyzed comes as a surprise to many students. This knowledge will presumably encourage them to think more frequently about the spatial geometrical aspects of problems they encounter in their field of specialization, whatever that may be. The author will be d a d to send a c o w of the instruct,ions for these experiments, as used a t Caltech, to interested individuals. A
"Giraffio Chloride"
In the study of molecular structures, the relation of a cube to a tetrahedron is first noted. This makes it easy to calculate the t,etrahedral bond angle and the ideal angle between a single and a double bond. Double bonds are represented by two bent pins. The geometrical structures of a variety of simple molecules are deduced in terms of the principlrs that the four electron
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ACKNOWLEDGMENT
The author acknowledges with pleasure that he first saw cork balls being used for the construction of molecular models in the laboratories of Professors Irving IClotz and Pierce Selwood a t Northwestern University. The models were photographed by Mr. Richard Bachard.
