The Historic Atom: From D to Q Richard Smith Cabrillo Middle School, 1115 Badger Circle, Ventura, CA 93003 Most high school chemistry courses include a description ofcurrent viewsofatomicstructure (1-4). Of 10 hieh-schoollevel chemistry hooks reviewed by the author, nine treated atomic structure from a historical perspective. Historical models usually included are those of ~emocritus,Dalton, Thomson,Rutherford, and Bohr, as well as the orbital model from quantum mechanics. This article describes a versatile Styrofoam model of the atom that has been used to show middle school and high school students the historical development of the atomic model. The model consists of a dark hlue Stvrofoam snhere with a hookat either end. Splitting thesphere reveals first one dark hlue face with vellow neeative charges emhedded in a sea of white positive eharges. ?he secondface has two fuzzy dark blue circles centered on a black map pin, standing out against a background of white. Finally a cloverleaf assembly of pink, yellow, and green Styrofoam eggs completes the model by attaching to the second face. This model is useful for showing the appearance of an oxygen atom according to four of the six historical models. While this manuscript was in preparation, i t was suggested that removahle faces could he used to present the other two historical models. One removable face would have two sharp dark hlue circles centered on a hlack maD in. standine out aeainst a white on the cirbackground, with y e ~ i negative ~d signs cles. The second removahle face would have a hlack man pin in the center of a hlue face, with yellow negative signsicattered around the face. The model is used by presenting the origin of the atomic concept, displaying the apparently solid sphere. Discuss Dalton's work, attaching hooks to the sphere for molecular bonding. If two models are availahle, they can he attached toeether with these hooks to demonstrate hondine. Thomson's model is displayed with the hlue inner face containing vellow "neeative" dots surrounded hv nositve siens. sienifving electrons emhedded in a sea of piskive charges. Rutherford's discovery of the nucleus is shown by covering the Thomson display with aremovable face showing the nucleus as a black maD nin, surrounded by a cloud of electrons (yellow negatiie rharges). Reverse ;he removable face to reveal rings representing the next advance, Bohr's electron energy levels. The rings show electrons traveling in orbits around the nucleus. It is worth mentioning the distortion in scale hetween the nuclear and atomic dimensions necessary for visualization of any classroom model. In this case the enlargement of the orhits is 1356 million, while the enlnrgement of the nucleus is 1:750 trillion, a difference in magnification of approximately 2,000 times. Finally, demonstrate thequantom mechanical orhiral model with the inner faceof the second hemisohere. Two blue rinns on a white background represent the Is orbital of the first electron energy level. and the outerlvine 2s orhital. The rings have rounh edges suggesting the-probahility functions associated with the orhitals. Richard Shronshire's method of presenting the concept of quantum eneigy levels using standing waves should he included here (6). Show the 2p orhitals by attaching an egg assembly to this face. The continuity of the 2p orhitals across their nodes are demonstrated by the use of three colors, each orbital having a separate color. Emphasize that the use of different colors is only to show continuity ~
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across the nodes, and does not imply different attributes to 2px, 2py, or 2pz electrons other than their spatial orientation. This model is useful for demonstrating historical and current concepts of atmoic structure and facilitates comparison hetween them. The model allows visualization of "s" and "p" orhitals, and the spacial relationships between them. This is also useful in visualizing sigma and pi honding between atoms if two models are available. The "p" orbitals on each model can be lined up, and the bonding between the orbitals can easilv he visualized. Other aids that can be used in teaching atomic history and structure include an excellent method for demonstrating the "d" orbitals by A. E. Saieed (7),and a series of classroom demonstrations of effects explained bv five of these important historical models, written by R. Records (5). Materials needed to construct the model include a 6-in. Styrofoam ball, three Styrofoam eggs (2Y4X 1Y4in.)of different colors (I used pink, yellow, and green), pipe cleaners, white glue (I used Arlene's Tacky Glue), a crosscut saw, paint (I used Design Master Color Tool, deep hlue #743 spray paint, marketed by Colorado Dye and Chemical, Inc., Boulder, CO 80306,and white, black, and yellow Ceramcoat by Delta, an opaque acrylic craft paint), and sandpaper. Begin by cutting a %-in. slice out of the middle of the hall usineacooine or crosscut saw. and ~ a i n t i n the e hemisoheres deeihlue; both the cut face and the outer &face. ~ l s paint o the edges and one face of the slice. Leave one face of the slice unpainted. Be careful t o keep the spray paint can 1-2 f t away from the Styrofoam t o avoid dissolving it. The pieces fit together t o form a sphere in order to illustrate Democritus's and Dalton's concepts. Dalton's concept of honding can be shown bv fashionine two hooks out of nine cleaners and inserting these in opposite ends of the &embled sphere. Prepare t o make the egg assembly by tracing one of the circular faces on a piece of cardboard. Cut two of the eggs lenethwise. s h a ~ i n ethe narrow ends of these t o allow the four halvesto he assembled into a cloverleaf arrangement on the cardboard tracing. The outer edge of the cloverleaf should lie about 1' 4 in. within the circle previously traced out. Fashion the narrow end of the third egg so as to allow i t t o rest upright on the center of the cloverleaf. Glue these egg pieces together, and set aside t o dry. The distance from the center ofthis arrangement to the fat end of each egg should he about 2% in. (see Figs. 1and 2). When attached to one of the inner faces, these eggs will represent an atom's 2p orbitals. T o show Thomson's concept. paint eieht small vellow ('11in.) dots on the inside face o;o;e of thehemispheres. ~ a & these dots with black neeative siens. and scatter eieht white positive signs around theblue back&ound (see ~ig.3). Place small lengths of pipe cleaners in this face t o hold the slice on. Prepare the Rutherford display by painting eight yellow negative signs on the painted face of the slice. This face can then he attached by a hlack map pin in its center to the hemisphere used previously for the Thomson concept. The negative signs should he scattered about the center 6 represent Rutherford's cloud of electrons surrounding the nucleus. On the unpainted oppositeface of the slice paint two thin Volume 66 Number 8 August 1989
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Figure 3.oiagram of m o m m ' s raisin pudding atom
Figure 1. Top view of egg assembly.
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Figure 4. Dlagam of qusnhm medanical atom.
Figure 2. Side view of egg assembly.
blue circles with diameters of 3/4 and 3 in. Add two yellow negative signs to the inner circle, and six yellow negative signs to the outer circle to represent the electrons in their orbits, according to the Bohr concept. The same map pin used with the Rutherford overlay is used on this face, attaching it in the center to the as yet unused hemisphere. Prepare the quantum mechanical atom by scraping away most of the blue paint sprayed on the inner face of the remaining hemisphere, leaving dark rings of about '14-in. thickness and with rough edges at a distance of 314 and 3 in. from the center (see Fig. 4). The edges of the rings should be left rough, with the outer ring continuous with the painted outside of the hemisphere. Insert the map pin usedpreviously in the very center of this face. Complete the model by placing small lengths of pipe cleaners in this face to hold the overlay or to attach the egg assembly (see Fig. 5). I t is hoped that the versatility, ease of manufacture, and low cost will enable the model presented here to be widely used in teaching about the atom. Acknowledgment This article was written under the encouragement of Connie Sparks, and the model was developed while attending Project RISE at UCLA.
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Journal of Chemical Education
F l g w 5. Model of quamum a t m wkh s and p orbitals.
College Syatem.
4. Oillnpie. R. J.: ~umphreya,0. A. Chsm i d ~ e w 197% s (on),wz. 5. Records. R M. J. Chem. Edue. 1982.59.307. 6. Shmpshire,R.Sci. Teoeh. 1985,52,2&30. I. Ssieed,A. E. J.Chem.Educ. 1980,51,805.
