The Atomic Arrangement in the &lfur Unit Cell A Description of a n Illustrative Model WINTON BROWN Mellon Institute of Industrial Research, Pittsburgh, Pennsylvania
T
HE model described in this article attracted considerable attention at the New York World's Fair, where it was on display a t the exhibit of the Texas Gulf Sulphur Company in the Industrial Building. The problem in design was to show the arrangement of the atoms in the sulfur unit cell in a convincing manner, and, a t the same time, to prepare as novel and modern a representation as possible in order to keep in harmony with the underlying theme of the fair. A considerable measure of success in this aim was finally attained, without too much cost, by the use of the plastic, methyl methacrylate. The axes of the unit cell of rhombic sulfur have been determined by Warren (5) as follows: a = 10.48A., b = 12.92A., c = 24.55A. Other observers are apparently in good agreement on these values (4) (2). Warren has concluded that the unit cell is made up of sixteen puckered rings, containing eight atoms each. The bond angle between adjacent atoms in the molecule is 105'; the distance of closest approach i s about 2.1A., which is just twice the covalent atomic radius given by Pauling (3), so that apparently adjacent
two squares, one turned 45' with respect to the other, whose planes are separated by 1.15A. and whose sides are 3.38A. From rotation and oscillation photographs, Warren has calculated the atomic coordinates and worked out the positions of the rings in the unit cell. The molecules lie in four layers perpendicular to the c axis. The planes of the rings in any one layer are approximately parallel to one of the diagonals of the c face, while in the next layer the planes are all parallel to the other c face diagonal. It is this representation of the structure that the model portrays, as shown in Figure 1, a closeup view. Each atom is represented by drilling part way into the back side of a "Plexiglass" plate, using a '/*-inch bit. Viewed from the front and with the aid of suitable illumination, the holes give the illusion of spheres suspended in space without any visible means of snpport. The entire model is composed of twelve such plates, each of proper thickness and containing holes representing atoms lying in the same plane. Thus the effect of depth is obtained by superimposing the plates one behind the other, in the order indicated in Figure 2. A point of view has been chosen by which some of the puckered jngs appear almost as an end view; the others are seen in a side view. This arrangement means that the plate length corresponds to the c axis and the plate width to the diagonal of the c face. The templet for drilling each plate is shown in Figure 2. The distance from center ,line to center of hole is 0.46 cm. unless otherwise indicated. Other constant dimensions are shown only on Plate 1. All dimensions are in centimeters. Each hole should be drilled until the maximum diameter is attained even though in some cases i t means drilling through the plate and into the previous one. The design illustrated comprises one unit cell only, although the model can be extended to include as many atoms as desired by simply repeating the basic pattern. The finished product on display at the New York World's Fair represented more than the one basic unit cell. With the aid of proper edge lighting, the holes were made to appear as strongly illuminated spheres, owing to the diffusion of light from the drilled surfaces. The effect is best brought out by using a flush mounting on a panel board, preferably black. Both Warren (5) and Bragg (I) have demonstrated the structure believed to exist in the sulfur unit cell, atoms touch each other within the molecule. From in so far as it is possible to do so in a two-dimensional these data one is able to calculate the dimensions of drawing. The model described in the foregoing parathe molecule, which may be considered as consisting of graphs has been built up from the latest atomic data 182
PLATE 1.
PLATE 3, 0.25 THICK
0.64 THICK
PLATE 5.
0.25 THICK
PLATE 8. 0 . 8 3 THICK
PLATE 11. 0.25 THICK
F I G U ~2E(PLATES1-B).-DESIGN
OF
PLATES IWR CONSTRUCT~G A MODEL OP THE SULPURU N E CELL
on sulfur and may possibly afford a somewhat easier visualization of the entire unit cell. The method of design used here may, of course, also be extended to other avstals whose structure is known, but whose picturizakon is made the 'Omplexities involved. Credit is due Mr. R. P. Murray, formerly with Jenter Displays Company, but now with The Of New City* who original1y conceived this method of showing the structure of the
sulfur unit cell and who supervised the construction of the model illustrated. LITERATURE CITED
(1) . . B u o o . "The atomic structure of minerals." Corndl University Press, Ithaca, N. Y., 1937, pp. &. (2) MARK WIoNER,2,p h j b , C h , , 111, 39-14 (1924). (3) PAWLING, "The nature of the chemical band and the stnu ture of molecules and crystals," 1st ed., Cornell Unid sity Press, Ithaca, N. Y., 1939, pp. 164-6. (4) TRILLAT AND O K ~ T ~2. I ,Kvyst., 98, 334-43 (1937). (5) WARREN AND BURWELL, J. Chem. Phys., 3 . 6 4 (1935).
