A Two-Dimensional Working Model of the Neon Electron Configuration Clark E. Bricker University of Kansas, Lawrence, KS 66045
S t u d e n t s t o d a v do n o t Drogress very f a r in the physical sciences until they a r e exposed t o t h e i d e a t h a t t h e first 10 electrons in all a t o m s of t h e neon configuration a r e arranged s o t h a t t h e first two electrons a r e i n t h e first level or K shell a n d t h e n e x t eight a r e in t h e second level o r L shell. Furthermore, these shells o r levels a r e almost universally shown in elementary physical science books a s circular o r l ~ t t reminiss rent of t h B~ ~ h model r of t h e alom. When a more s o ~ h i s t i ..--..cated approach t o t h e electronic configuration of t h e first 10 electrons i s presented, q u a n t u m n u m b e r s a r e used a n d t h e electrons a r e t h o u g h t to exist a s clouds with n o fixed positions h u t with a densitv t h a t decreases with size. However, q u a n t u m n u m b e r s confirm t h e concept t h a t two a n d only two electrons can h e i n t h e first energy level or shell and only eight electrons c a n h e in t h e second energy level. T h e demonstration vresented here shows a dynamic two-dimensional working model of t h e arrangement of t h e 10 electrons found in t h e neon core. ~
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Apparatus Plastic dishpan about 15 in. in diameter. Close to its top, approximately 60 turns of #29 shellac-coated copper wire are wrapped around the outside. The two ends of this wire are attached to terminals that are fastened to the rim of the dishpan. Ten small permanent bar magnets, approximately 718 X X %s in. Each of these magnets is about one-third embedded in and fastened to the bottom of a cork so that the same pole of the magnet is exposed from each cork. A source of 16-V direct current with a rheostat to regulate the current. An HO train transformer works very well. If this demonstration is to be shown to a large audience, a 15- X 18-in. mirror together with the necessary ring stands and clamps to mount the mirror at approximately 45O angle above the dishpan are needed. Alternatively, a wooden stand that supports the mirror, shown in Figures 1and 2, provides a more permanent setup. T o make this demonstration even mare vivid, the tops of the corks to which the magnets are attached should he painted and numbered with fluorescent paint. Also, an 18-in. ultraviolet lamp should he mounted at the top of the mirror. Water to fill the dishpan to within about 3/n in. of the top.
one in the center. Eight corks form a heptagon with one in the center and nine corks form an octagon with one in the center. When the 10th eork is added, thecorks rearrange themselves so that twoare in the center and eight surround the two just like the arrangement of the neon core or the first 10 electrons in an atom. If the dc current is now reversed, all 10 corks will migrate to the edge ofthe dishpan. Again reversing the current, the corks willmove toward the center ofthe pan but always assume the 2 and 8eonfiguration. By decreasing the current flowing through the wire, the magnets occupy a larger area, indicating the relative size of fluoride or oxide ions. By increasing the current, the area of the magnets approximates the relative size of the sodium, magnesium, or aluminum cations. If the corks are numbered, it is very interesting to observe which two corks are in the center and then whieh corks are in the center after thev have been to the edge of the pan and back again. This ohservatkn illustrates that all &ctrons are exactly the same and it is never oossible to orediet which electron will he a t a particular location in the atom. ~~
Figure 1. Dernonstrat~onwith five corks in the pan and the other five corks lying on the stand.
Procedure Fill thedishpan with water so that the level of the water is as deep as the highest turn of wire surrounding the dishpan. Connect the source of de to the two terminals. If the ultraviolet light and mirror are used, the room should now be darkened. The direction of the dc current should he checked so that, when a cork with a magnet is added to the water, the cork will move toward the center of the dishpan and not toward the edge. Now, add acork to the dishpan. I t will go to the center ofthe pan. When a second eork is added, the two corks arrange themselves so that they are equidistant from the center. Three corks arrange themselves in an eouilateral trianele: four corks as a square: five corks as a pentagon.'~henthe sixtheork is added, a reg& hexagon may be formed, but sometimes a pentagon with one eork in the center is preferred. These two configurations of six corks appear to have about equal stability. Seven corks always form a hexagon with Presented at the 9th Biennial Conference on Chemical Education held at Bozeman. MT, July 27-31, 1986.
Figure 2. Demonstration showing how 10 corksarrange themselves in the pan.
Volume 64
Number 2
February 1987
133
Ifateacherwishes toillustrate the roleofprobability, thiademonstration is almost ideal. When the studenta see which two of the 10 electrons are in the center, ask them to predict which two will be in the center after the current has been reversed twice. (You can even have them wager a dime that they can guess correctly and you can agree to pay one dollar for a correct guess!) Students will learn quickly, after trying to guess several times which corks will be in the center and seldom being correct, that prohahility is important. There are 45 different wavs that two corks out of the 10 can he in the center; therefore, a student has only one chance m 45 ofguessing the correct answer. (A lesson on the risks of gambling can he rntrudured by now explaining that if you are giving 10 to 1 odds in return for only a 1in 45 chance of guessing correctly, the gambler will lase!)
134
Journal of Chemical Education
This demonstration works for no more than 10 magnets because if the 11th magnet is added, three magnets goto the center. In order to show the arrangement of more than 10 magnets or electrons, a much more complicated magnetic field than that used in this demonstration would presumably be needed.
Acknowledgment T h e author gratefully acknowledges t h a t a version of this demonstration was shown t o him some 20 years ago by Albert Burgstahler who in t u r n h a d observed Eugene Rochow perform a similar demonstration a t H a w a r d University about 15 years earlier.
