Giant Atomic and Molecular Models and Other Lecture Demonstration Devices Designed for Concrete Operational Students Rubin Battino Wright State University, Dayton, OH 45435 The oversize lecture demonstration models we have developed appeal very directly to students who function in the concrete operational mode of thinking. These models also help formal operational students. In addition to describing the construction and use of these models, we will also describe our spdf sandwich board and an experiment using attrihute blocks. The common denominator is the appeal t o concrete operational students. AtomiclMolecular Models-Design and Construction These models come in two versions and share some components. Both versions use tetrahedral geometry since this is the design which covers most applications. The two versions will be described separately. Figure 1shows the heart of design 1.This is a 2 '14 in. aluminum cube with four tetrahedral holes, % in. diameter, drilled to the center off flattened corners. Drilling these holes accurately is a job for a trained machinist. On two opposite faces are thumb screws serving as set screws to hold the arms in place. The arms are shown in Figure 2. They are made of 'I7in. o.d, aluminum tubing, ... which has been anodized for aupc;lrilnce and prorectwn. (The dnorlizing is not neccsars.) One end of the rubinr 11m a ~ l uwhich r is thc same diameter is peggedin place. The other end has as the tube. This a specially shouldered plug which fits snug against the block. The "V" groove is at such an angle that the set screw hits it perpendicularly. The tuhing can he used without the plugs, but then much care must be taken in using the set screws since they could crush the thin wall tubing. The electron holders are made from 'I&. thick plexiglas, although other materials can be used. The two large diameter
holes are for insertion of our 3-in. square cross-section and 8or 12-in. long polyurethane (could also he foam rubber) "electrons." The longer "electrons" are useful for double and triple bonds. The plexiglas plates are fastened to the aluminum tubing in two ways. The simplest way is shown in Figure 3(b). We use a '12-in. bulkhead union whose center has been reamed to a %-in. d i e t e r all the way through the union. We have used brass Swagelok brass unions and nylon unions. The last are the least expensive and work quite well. The plate shown in Figure 3(c) was machined to allow the plate to swivel, although the knurled nut can he finger-tightened t o restrict movement. The flexibility is useful for illustrating double and
Figure 2. Armsthat fii in cubes. Oneend has aawivel bailJointso that theelectran holding plates are movable.
1
:
nrm
J
(c)
-1 3/4"1Figure 1. Aluminum cube for making tetrahedral models. 10-24 thumbscrews are used to lock the arms in place.
Figure 3. (a) 'I4-\". thick Plexiglas plate wim 3 in. holes tor electrons. (b)Plate showing '/Anylon bulkhead union. (c) Plate showing matching end of swivel ball joint.
Volume 60 Number 6 June 1963
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triple bonds. An assembled model of design 1is shown in the photograph in Figure 4. Design 2 is a tetrahedron made from 'I2-in. aluminum tubing. The end plates at the apexes of the tetrahedron were made in two ways. Our fancy version was machined from a llz-in. aluminum plate. The details are shown in Figure 5. The 10124 screws go through clearance holes on one side and into a threaded .---...~- nortion on the other. The %-in. bulkhead unions are used for the central hole to attach and adjust the 'I& aluminum tubine which holds the electron-holdine dates. The 112-in. aluminum tuhing which makes up theedges of the tetrahedron are 2 ft long and have both ends plugged and drilled with clearance holes for the screws. The structure is auite sturdv when assembled and colla~sesfor storage when the appropGate three linkages are opened. The secondversion of an apex end-plate (see Fig. 6) is much easier to make and does not require special machining capabilities. These are made from '/*-in. aluminum plate equilateral triangles which ~~
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Iis heldinblace using %-in. iylou cabli clamps (available from electronic supply houses). All hut the appropriate three sets of these cable clamps are pop-rivetted in place anchoring the tubing, but also permitting set-up and collapse of the models. Design 2 is shown assembled in the photograph in Figure 7. Atomlc/Molecular Models-Some Uses The models are used to illustrate: (a) ionic and covalent bonding; (h) single, double, and triple bonds; (c) electronic structure of atoms; (d) Lewis octet rule; (e) and the structure of molecules. I t is important in using these models to practice setting up and working with them. They are sufficiently large to be clearly seen even in large lecture halls. That these are models should be emphasized. The electronic structure of atoms can be illustrated using design 2 more readily than design 1. For this we use one 2.5 ft-tube in addition to the standard 1.5-ft tubes used for attaching the electron-holding plates. The short tubes are adiusted so that most of their leneth throueh the " nrotrudes . tetrahedron. These short tubes have one electron-holding plate at their outermost end and one half way between the end plate and the tetrahedron apex. The long tube extends into the interior of the tetrahedron and one electron holdinn d a t e is attached here. Then three electron holding plates &e attached svmmetricallv external to the tetrahedron The il;tarnitl platv h d d the twl, I t4ectnmi The t:rst h?er (11 plate5 lndds ~ I i etwo 2,s elertruns 311d six 21) elwlrlnls. 'l'he next layer of plates holds the two 3s electrons and six 3p electrons. The outermost plate holds the two 4s electrons. The Aufbau principle can beillustrated up through the element of atomic number twenty. Electrons are added one a t a time,
Figure 4. Design 1 assembled and showing tour electrons. Two units side by side.
486
Journal of Chemical Education
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Figure 5. %-in. thick aluminum end plate for apex af tetrahedron. For each section of 10-24clearance hole leads to a threadedsect~onso the unit can be easily dismantled.
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CABLE CLAMP
ROLL - 1
3 5"
Figure 6. '/&. thick aluminum end plate for apex of tetrahedron. %-in. nylon cable clamp holds 1 in, long 'Id,. roll pin whichgoesthrough filledend of %in. aluminum tube. Four of me cable clamp units are afixed with thumbscrews. The rest are rivetled.
Figure 7. Design 2 assembled.
naired when annronriate. and kent track of on an overhead . projector or blackboard at the same time. With two models, illustratina- sav - sodium and chlorine, ionization can he readily and directly illustrated. The charge on each atom is shown by a flash card (R112 in. X 11in.) which is hooked on to the model as +1 or -1, etc. Other ions and compounds can he formed readily. The Lewis octet rule is illustrated in that the outermost s a n d p electrons are central to chemical honding and interaction. The models are easily used here. We have also found that using the idea of a square dance with its octets to he a useful one in catching students' interest. A participatory demonstration could also he built around the square dance idea. Covalent honding and multiple bonds are readily shown for Fz, Oz, and Nz. See Figure 8 for N2 molecule. The longer "electrons" are used here to make the connections. Lareer molecules like ethane, propane, NazS04, can also be reazly built up and demonstrated. The design 1models can be used to show optical isomerism by attaching four different colored plates (groups) or painted polyurethane blocks to the ends of the tubes. Cis-trans isomerism is easily shown by constructing an ethylene molecule. The linearity of the acetylenic linkage is also easy to illustrate. Chair and boat forms for benzene can be shown, as well as ortho. uara, and meta substitutions. Although we have not described their construction in this paper, we also use the models for inorganic complexes and isomerism. For this we either use a 1 in. thick triangular plate with holes a t the apexes t o receive the arms shown in Figure 2 or a 3 in. on a side cube with holes drilled in the center of each face. The latter can be used for square planar and octahedral structures. The former is used for trigonal planar structures and one modification with bosses welded to the flat plate for a coordination compound with five ligands. Extra long pieces of urethane foam can he used to indicate bidentate ligands. We have found these models to have high visual and ideational impact. Students readily "see" what is happening and get a solid feeling from the models. The actual transfer of "electrons" in ionization is a terrific reinforcer. We do not spend agreat deal of time using the models but find that their use saves us much explaining time. The models can he readily assembled and rearranged while lecturing. For the more elaborate forms we set uo beforehand and also use student volunteers.
The board itself is made from two 22 X 34 in. 18-20 ,.eauee .. pierci u t g~~l\~riuized ihwt m ~ . t d.~ttilchedto ,I 1-in. aluminum :IIIYIV i r m ~ nr i shwm~in thr ulwtcrmwh in Fimre 9. The trnmc has two handles on it and hook eye;to attach the wide neck band. The hoard is assembled using pop rivets. A base coat of enamel is put down first, then the diagram is painted on the base coat. Colors are chosen for visibility. The letters are 2.5 in. high, and the arrows (electrons) are 2 in. high. The arrows are made either from stiff cardboard or more permanentlv cut C nrr painted ill tluoreirent irum 1 .-In. m:litmltt.. T ~ L . Fnrrowi c r h s and nre hacked with adhesive magnet nmeriol I No. liI):208 frvm Edmund Scient~fi~.('o.I In ndditim wc usc m a"netically backed 3-in. diameter circles with plus or minus signs painted on them. These are used to show the electrical charge on the given atom. For our use the boards fold and can he readilv stored and handled. ~ h e ~ u f b Principle au is easily illustrated by the pattern of plunking in electrons, element by element, following the increase in energy levels. The p, d, and f electrons are first put in unpaired and then are paired up. Hund's rule shows some of the transition metal characteristics, if required. Ionization is shown hv removinn an electron and addine a positive charge. isotopes can be shown by using the table showing the number of electrons, protons, and neutrons in that atom. Space is also provided for the atomic symbol of an element along with mass and atomic numbers. Comments could also be made about ionic size, ionization energies, and the energy levels in an atom. The Attribute Block Experiment Attribute blocks are sixty small plastic hlocks which come in five shapes (triangle, square, rectangle, circle, and hexagon), three colors (red, yellow, and blue), and two sizes and two thicknesses. They are commercially available (No. 12072 Damon Educational Division. 80 Wilson Wav. Westwood. MA 02090). For this experiment,'which is the very first one df the auarter at check-in time. the students work in nrouns of three or four. The experiments are done in sequence.
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A. The Attributes: The blocks are all dumoed out on the table. and
The spdt Sandwich Board-Construction and Use The spdf sandwich board is used to illustrate the Aufbau Principle, energy levels in atoms, distribution of electrons in energy levels, and ionic bonding. (Covalent bonding is talked about in principle for molecules like H2, F2, N2,02, CH4 and other carbon-based molecules, but showing i t is a hit much.)
Figure 8. The nitrogen molecule indicating a triple bond and using design 1.
Figure 9. The spdf sandwich board.
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Number 6
June 1983
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B. Attribute Difference Domino Game; One student starts by putting down any block. The next student puts down a block that differs in one attribute from the first block. Continue for three or four
testing the solutions the students are asked to develop a scheme for analyzing an unknown. Their procedure is checked and, if reasonable, they are given an unknown. T h e kind of reasoning used in the attrihute hlock experiment is similar to the reasonine reauired here. d . ~ unw hlwk. I h e " l ~ " . t u d ~171 h,>30pc,rtz r r.1111,lntc .rn..lhcr I,Iw k a n experiment in weighing and using the balance. The matenrxr I . , the tirst t ~ c 'I . he "11" .1udt 111 -:IY- " \ t ." t r ' n < j "< I t pending an whether the new hlock has an attridutematching the rials weighed are four different sizes of bolts in three lengths one in hisher mind. Testing with successive blocks will determine and appropriate nuts. Students can set up a simulated atomic the particular attributes. Students are asked to figure out ways weight scale with these sixteen different masses. The second of minimizing the number of test blocks. The "It" student repeats experiment has the students trying to figure out the formula the process with two and then with three blocks, each one with a of a number of holtlnut "molecules" in a sealed (but tared) specialattribute in hislher mind. After the group has figured out plastic coffee container. All the "molecules" in a'given con: the attributes, they are asked to place between the blocks a hlock tainer are alike. The additional information of a standard with the characteristics of both (and then all three) unknown weight in a second tared container gives them all the clues they blocks. (This is a good way to teach set theory.) require. The third experiment is one in which the students It is important first to run through this experiment with determine by chemical means the atomic weight of tin or your teaching assistants. An important by-product of this magnesium. Each of these experiments builds on the urevious exneriment is as an ice-breaker to eet students in the class to one. (Write-ups are available upon request.) know each other. Acknowledgment The attribute block exneriment is used as a lead-in to the second week's experimedt wherein students, again in small The assistance of Mr. H. R. DuFour in the design and congroups, are given three solutions (silver, mercury, and iron) struction of the models is acknowledged. The encouragement and two test reagents (hydrochloric acid and ammonia). By and suggestions of Dr. D. J. Karl are also gratefully acknowledged. Mr. Bruce Adams' art work is appreciated.
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Journal of Chemical Education
