edited ..~bv . . ,-
GEORGEL. GILBERT Denisan University Danville. Ohio 43023
A Lecture Demonstration Model of the Quantum Mechanical Atom S,mwmm
sr:
George Wlger California State UnlvarslW Domingwz Hills. CA
Melvln L. Dunon Callfanla State Collsge Bakersfield. CA C ~ c n r BY: n
Leonard C. Grolz University 01 Wisconsin at Waukesha Waukesha. WI 53186
T h e understanding of abstract concepts is one of the most difficult obstacles which freshman chemistry students must overcome. A major cause of this problem is the lack of appropriate and effective models related to the new concepts which can easily be used for lecture demonstrations. A particular case is the Bohr planetary orbit pictureof theelectronic structure of the atom versus the more descriptive quantum mechanical model. Although students can be taught about the theoretical shapes of s,p, and d orbitals, an elementary understanding of electron motion within the orbital and its probabilistic nature is difficult because of absence of simple, visual models. On the other hand, a supporting model for the planetary orbit system is visualized easily by the earth and moon analogy. In an attempt to simulate the quantum mechanical 1s orbital, we have constructed a very simple apparatus. It consists of a clear plastic, spherical shell, 18 in. in diameter. T h e electron is a ping pong hall. T h e motion of the electron is caused by a pressured air jet a t the bottom and is randomized by a seriesof small holes drilled in the sphere. Since thesowce of air pressure can be a moderate-sized tank, the demonstra-
tion is portable. Also, the size of the model makes it usable in large lecture halls. Preparation
Our model was constructed usine an 18-in. diameter clear plastic terrarium available a t mos't department stores (see figure). We found the diameter of these plastic spheres vary from 8 to 24 in. so models can be constructed that would he suitable for individuals or large lecture rooms. We chose the 1841. diameter sphere because it seemed most suited for viewing in a classroom of 50 to 75 students. T h e plastic terrarium consisted of two hemispherical sections which were glued together. A . ping-pong ball was inserted into the sphere .. through-a hole present in the bottom of sphere and then a polyethylene funnel was glued in place. A jet consisting of a 0.25-in. (id.) of comer tubine was inserted into the . . niece . funnel and connected to ;he air source. Without the jet, we have been informed that there is considerable difficultv with the Bernoulli effect. Approximately 15114-in. holes were irilled around the surface of the sphere which act as air vents and create random air flow within the cavity. The flow of air can be controlled by a needle valve or regulator and adjusted to provide "random" motion of the ping pong ball inside the sphere. Different types of air flow patterns are developed within the cavity as a function of incoming air pressure. We found it was necessaw to adiust the flow to obtain the best nattern. If the incoming air pressure is insufficient the electron w i l l spend too much time in the lower hemisnhere. At hieher " Dressures. . we observed an undesirable "orb~ing"of the ping-pong hali about the sphere For this reason, we suggest testing various air dispersal modes and velocities for the best operating conditions based on the size of the model. T o insure against "orbiting" a piece of tubing can he glued around the equator of the sphere. We have used this model following an introductory lecture into thecharacteristicsof theauantum mechanicalatom. Its mode of operation is explained and the "electron" is set in motion. Because of the design, the motion of the electron can he controlled by regulating the air flow. After a few moments of observation, the class is asked to state where the electron is located. The answer is generally, "inside the sphere." From this point, the discussion can be expanded to the notion of the
rested aemarshefiom is a m m l y featve designed to pesent lechve demonstrations and experiments in a format convenient lor classroom use. Readers interested in either submining or checking demonstrations should contact the column edita. An outline 01 format requirements was given on p. 166 of the March 1976 l r s w of THS JOURNAL. George Gilbert's inlerest in chemistry demonrtrations traces lrom Hsnover Hiah
..
chemical education include reif-pacing. TIPS. and project-style laboratories. He eansuitr with science museums and is active in fostering
C e w a y representallon of IJm quantum mechanical a t m .
Volume 58 Number 10 October 1981
801
Is electron's probability density. This is normally introduced bv askine. "Does the electron soend the same amount of time ai all po&s in the volume?"?he students are asked to consider the tvpe of plot that would he derived if the instantaneous posiikms o? the electron were recarded overa long period of time. Most students are able to see that the electron spends more time in some locations than others and that if these positions were plotted, the results would be a spherical cloud which is darker in some areas than others due to the higher probability of the electron occurring in those locations. We are well aware of the approximate nature of this treatment and of the model. We can not claim originality for its design since we are probably not the only chemists who have spent time in a Keno lounge. Rather, we present the idea because it has proved to be an inexpensive instructional aid for presenting an abstract concept and it may be helpful toothers. We will be most happy to provide any additional details which others may require. ~~
~~
~~
~
A Modified Thermit Lecture Demonstration S ~ s l l l n BY: ~o George B. Kauffman California Slate University. Fresno Fresno. CA 93740 CIEWEO BY: Marvln K. Kemp Universiw 01 Tulsa Tulsa. OK 74104
T h e recent note on the thermit lecture demonstration' prompts me to report some modifications of this well-known d e m o k r a t i o n that 1 have used for the past two decades to make ic more dramatic and interesting for students. I tm have employed the KXlnO4-glycerin ignition mixture (which, incidentally, seems to work hetter if the finely.powdered over the .. elvcerin rather than the reverse) KXlnO,. is sorinkled . . and :
