Everyone Wants to be a Model Teacher Part Ill: Extensions to Atomic Structures and Bonding C. L. Schrader Dover High School. Dover, OH 44622 The ideas of making observations, noting patterns, and devising a model to explain patterns have been illustrated in simple concrete activities that have been described in the fust two articles of this series (J. CHEM. EDUC.61, 1001, 1086 (1984)). Atomic Theory
In the following activity students propose creative models of the atom that account for observed properties and predict additional experimental data. The approach is historical in the sense that the students begin by imagining that they are chemists in 1884, prior to the discovery of the electron. They are asked to develoo other models to account for afixed percent composition b; weight, valence, bonding, the differences in shaoes of molecules. and difference in activity of similar elements. The students first do three simple experiments to determine the empirical formula of a substance. In the first experiment, a measured amount of manganese metal is reacted with an excess of 3.00 M HC1. After the excess HCI and water of hydration are evaporated the percent Mn and percent C1 are calculated and the empirical formula is found. The effect of eva~orationof excess HCI (no residual mass remains) and the existence of water of hydration were investigated in separate ex~erimentsfrom two to six weeks earlier. Students are expected to connect relevant data from these past experiments to the one they are currently working on. For example, after observing the reaction that occurs upon the addition of HC1, the students place their beaker on a hot plate in the hood to evaporate the excess HCI and water. The following day the beakers contain a dry pink solid which the students weieh. The beakers are then heated for 2 or 3 min, cooled, and reweighed. Many students are surprised to find that the beaker weighs less after heating. I expect them to recall that many substances that look dry are in fact hydrates. which reauire heating to a constant weight to insure that the waters i f hydrationhave been removed. Once this drying process is complete, water is added to dissolve the MnCl2 residue so i t can be poured into the waste chemical jar. During this dissolution many students notice that the beaker becomes hot. This offers an excellent lead into a discussion of exo- and endothermic reactions. This experiment is repeated using magnesium in place of manganese. Only oral directions are given in order to determiniwhich understand the exneriment and which - - ~students ~ are using a "cookbook" approach. complete descrintion of these exoeriments and how they stimulate students to analyze, synihesize, and evaluate c& be obtained by writing to the author.)' The experiment is planned so that the students do not obtain a constant mass in the first lab period. 'When they weigh the beaker a t the beginning of the second period (prior to heating it), they find that it has gained mass. A gain of 0.5 to 2.5 g is common depending on the amount of magnesium chloride and the humidity. In a previous experiment on water of hydration, the students learned that some salts absorb water from the air. I t is interesting to observe the reactions of students when they record the increase in mass and. to observe how they analyze the reasons for this increase. The third experiment in this series is the oxidation of
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magnesium metal to magnesium oxide in a crucible. The magnesium reacts with both nitrogen and oxygen from the air. The magnesium nitride is decomposed by adding water. During ibis process, the odor of ammonia is wident, many students notice that the reaction isexothermic, and that the mass increases as the nitrogen atoms are replaced by oxygen atoms. Many of my students, and primarily the concreteoperational ones, expect the crucible and contents t o weigh less after the magnesium is burned. Because of the consistent nractice in cause-and-effect reasonine. some concrete-o~erational students are able to predict &rrectly that the magnesium oxide will weigh more than the magnesium. T o experienced chemists this is so obvious that i t is trivial, but to believers in the phlogiston theory as reported by Greyz and to beginners who are operating on the concrete level, this is not obvious. In this experiment, as in others, the students begin with simple concrete experiences that can be analyzed, then eeneralized. They then look for regularities and a model that will account for the observed Formula Prediction
After the experiments are completed, analyzed, and discussed, the studenu are given percent composition data for 27 binary compounds, and they calculare empirical f~mnulas for each compound. Since the students have just completed experiments u, determine the percenr compositiun of several co&unds, they can understand how such experimental data can be obtained. The formulas for the first 15 compounds are put on the board and the students are asked to look for patterns: NaF
MgFz
A1F3
NaCl
NazO
MgO
AI2O3
NazS
MgS
Na3N
Mg3N2
AIN
Na3P
Mg&
MgC12
Because the students have been writing formulas by using the r ; quickly recognize valences they memorized in ~ e ~ t e m b ethey that sodium can be assigned a valence of +1,magnesium +2, aluminum +3, fluorine -1, chlorine -1, oxygen -2, sulfur -2, nitrogen -3, and phosphorus -3. Students are then asked to predict formulas for several additional compounds based on these valences and to explain why that formula was predicted. A typical response would be: "If CuC12and CuS indicate a valence of +2 for Cu, and NaF and AIFt indicate a -1 valence for fluoride, then the correct formulafor copper fluoride would be CUF~:" When asked how they would determine if that formula is correct, the immediate response is usually, "We could ask the teacher or look in a reference book." I reply that although these may be good methods, what would they do if neithkr were available. At least one student will suggest doing an experiment similar to the ones we just finished. T o accimplish this, students are required to prepare a written procedure for the experiment they would follow and are asked to investigate
' Please send $2.00 to cover postage and copying expenses.
Grey, V., "The Chemist Who Lost His Head," Coward, McCann, and ~ e q h e b nInc.. , New York. 1982. Volume 62 Number 1 January 1985
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the propertiesof the materids and the methods they propose to use. This is a useful exercise in writing and in creative thinking and an eye-opening undertaking for some. For examnle. a student who planned to combine copper and fluorine directly learned that fluorine was extremely active, very difficult to isolate and handle, and quite toxic. Dalton's Role
Once the above activities are complete, students are asked
to propose a model that will explain why sodium has a valence
-.
nf + I . and - - oxveen and sulfur have valences of -2. etc. The students have not yet studied electrons, orbitals, energy levels, quantum numbers, electron configurations, or the periodic chart. Thus, the students are asked to imagine that they are chemists in 1884, when the electron had not yet been discovered but Dalton's atomic theory was well known. To facilitate this activity, students are given data for several hypothetical elements and compounds and are asked to determine the simplest formulas for the set. For example, element G is assigned an atomic weight of 12 and element R an atomic weight of 16. The simplest formula for the compound with 69% G and 31% R is determined to be G3R, while the compound which is 31.65%G and 68.4%P, has the formula G2P. From this informatiion, i t is deduced that the valence of R is -3, P i s -2, and G is +l.After carrying out asimilar analysis for 18 compounds including 18 different elements, students are asked to propose a workable model for bonding based on the determined valences. Their responses are typically quite creative. Listed below are several very interesting examples. A
Hook Model
Each atom of a given element has a number of hooks equal
to its valence and when atoms combine to form compounds all hooks must be hooked. In the G3R compound mentioned above, G has one hook and R has three hooks. A drawing would look like:
When I ask if the atoms must have hooks for this model to be valid, a student will reply, "They do not have t o have hooks. They only have to behave as if they do!' Arrow Model
Each atom of a given element has either a number of arrows or targets equal to its valence. All arrows must be in targets and all targets must contain one arrow. In the compound G3R, G has one arrow and R has three targets. A drawing would look like:
Other Models
Students also have suggested models based on hole and peg, lock and key, hook and eyes, and magnets. If all of these are not suggested by a given class, I will suggest them myself and ask a student to draw a diagram illustrating a compound such as G3R or G2P.
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Journal of Chemical Education
Once several possible models have been described and the students have practiced drawing diagrams, the students are asked to applya given model to other alternative situations. For example: Why do atoms combine to fonn compoundswith the same percent compositionby weight? Answer: In order to satisfythe hwk model three atoms of G must combine with one atom of R. Because all G atoms have the same weight (12) and all R atoms have the same weight (16), when they combine in this constant ratio ofnumbers (3 to 1) theweightratiomust beconstant. Thisisa subtle noint which I think is understood by. very few beginning students. Why do atoms comhine to form stable compounds? Answer: The hooks get hooked, the pegs stick in the holes, or the arrows get stuck in the target. Why dues element G with a valence of + I react with P faster than elrment Q whirh also hasa valence of + I ? Answer: C has larger hooks which can become hooked faqter or G has sharper arrows which can penetrate the target faster. I also want the students to he able to evaluate all the models to determine and understand their limitations. The following questions are typically used: Can the model account for the fixed percent composition by weight? Answer: Yes, all the above models can explain this data. Can the model explain why stable compounds are formed? Answer: The hook, magnet, hook-and-eye,and arrow-and-target models are strong in explaining bonding, but the loek-and-key and hole-and-peg models are not. Can the model account for the fact that some elements with the same valence react faster than others? Answer: The arrow-and-target,hook-and-eye, hook, and magnet models are good, hut the others are not. Can the model account for the fact that metals tend to comhine with non-metals? Answer: All models except the hook model can explain this ~
.
data.
Can the model account for the shape of the molecule? Answer: All models can account for the shape of the molecule by the placement of the hook, magnet, key, etc., on the atom. The culminating activity in this series involves the arrangement of the hypothet&l elements that were previously mentioned into a periodic chart. J. Bronowski3 considered Mendeleev's achievement one of the greatest in intellectual history; the exercise is conducted with this in mind. My students often first arrange the elements alphabetically, which leads to little if any correlation. An attempt to group the elements either according to valence or in order of increasing atomic weight is also not enough. Like the real periodic table, there are reversed pairs in atomic weight, and gaps for missing elements that must he considered. Once the students have decided on an arrangement, they are asked to predict the properties of these elements. Students are delighted to learn that their processes are similar to those used by Mendeleev. This year after the class had completed the study of atomic theory, electron configurations, bonding orbitals, and shapes and polarities of molecules, an excited student exclaimed. "The electrons in the outer shell of atoms play the role of h;oks!" Another student replied, "We must remember that electrons, too, are a model that mav or mav not bewue.' "It d w s not matter if the model is true & long as the atoms hehave ac if it is true. Students who can understand and agree with the last statement have a flexible model of nature that can be modified as it becomes necessary to explain additional experimental data.
Bronowski. J.. "Nature and Knowledge: The Philosophy of Contemporary Science." Little, Brown, and Company. Boston/Toronto, 1971.
