A Qualitative Investigation of Undergraduate Chemistry Students

Research: Science and Education edited by

Chemical Education Research

Diane M. Bunce The Catholic University of America Washington, D.C. 20064

A Qualitative Investigation of Undergraduate Chemistry Students’ Macroscopic Interpretations of the Submicroscopic Structure of Molecules Gayle Nicoll Department of Chemistry, University of Nebraska, Lincoln, NE 68583; [email protected]

Within the domain of chemistry, students are expected to be able to translate between three distinct languages: the macroscopic (white, crystalline powder), the symbolic (NaCl), and the microscopic (°•). Indeed, several research studies have documented the differences among these three languages of chemistry (1–5). Students appear to have less trouble grasping the macroscopic realm, that is, the realm they live in. However, the symbolic and microscopic realms tend to give chemistry students more difficulty (1, 2, 4, 5), giving rise to reported misconceptions (6–10). While the expert chemist has no difficulty translating between these three languages, and probably does so without realizing it, previous research has found that undergraduate students—and specifically general chemistry students—have a difficult time recognizing and switching between these languages (1, 4, 5). Professional chemists tend to have detailed microscopic representations of matter. They can draw microscopic representations, use 3-D models of molecules, and illustrate molecular interaction using animations. This may help them translate between the microscopic and macroscopic worlds. In contrast, numerous studies have highlighted students’ difficulties grasping the particulate nature of matter (11–23). It was illustrated quite early on (16–18) that young students have misconceptions associated with the particulate nature of matter (19). In fact, even science teachers at the primary and secondary level have misconceptions associated with the particulate nature of matter (20, 21). This results in a repetitive cycle, whereby the teachers pass on their misconceptions to their students. Tests have been developed to identify (22, 23) and correct (24) these misconceptions. However, concepts associated with the particulate nature of matter are resistant to change (25), perhaps because it is difficult for students to have mental models for phenomena they cannot directly observe. Indeed, recent research (26) on students’ conceptions about the electron has found that students tend to use macroscopic explanations to justify electron behavior. Griffiths and Preston’s paper (26) is particularly pertinent to this study, as students’ conceptions about how electrons behave were investigated. The research in this paper investigates the encoding that students use to develop molecular models. These models are macroscopic interpretations of the submicroscopic structure of molecules. Little research on this topic could be found at the undergraduate level. However, several studies have reported on high school students’ misconceptions dealing with concepts related to molecular structure (27–31). This study focused on how students, from freshman-level general chem-

istry though senior-level physical chemistry, translated between the symbolic and submicroscopic representations of molecules by building free-form models, given the symbolic formula for the molecule. The results of a qualitative inquiry to determine how students conceived of the submicroscopic world and whether these conceptions changed with increasing chemistry instruction are described. Participants All subjects were volunteers enrolled in a chemistry class at a large Midwestern university in the fall of 1998. Volunteers were solicited to represent the spectrum of courses available to chemistry majors: general chemistry for science and engineering majors, general chemistry for chemistry majors, organic chemistry, inorganic chemistry, and physical chemistry. Students were not solicited from analytical chemistry, as this course is normally taken concurrently with another of the courses listed above, so the same subject pool existed. Twenty volunteers were interviewed from general chemistry for science and engineering majors and six were interviewed from physical chemistry. The low physical chemistry numbers reflected the lack of volunteers from this course. Ten volunteers were interviewed from each of the other courses. A total of 56 students participated in the study. The study was limited to ten students from each section in order to focus on students’ depth and breadth of knowledge. Had more students been included, the study could not have developed such a detailed picture of how students represented their submicroscopic understandings. Since one of the goals of this study was to determine how students represented the submicroscopic world in a free-form format, it was decided to perform a qualitative study in order to capture a more detailed and accurate representation of students’ ideas. While this format limits the generalizability of the findings, it was a useful tool for this investigation. Procedures Qualitative interviews were conducted by the author once towards the end of the semester. This timing was done so that students, presumably, had the opportunity to internalize the material taught during the semester. Each student was given an individual, hour-long, private interview. Each interview was audiotaped with the consent of the participants. Field notes were made after each interview to record impressions, observations, and trends emerging from the interviews.

JChemEd.chem.wisc.edu • Vol. 80 No. 2 February 2003 • Journal of Chemical Education

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Research: Science and Education

The field notes then became part of the data. An open-ended, semi-structured interview style was adopted to allow students to express their ideas as fully as possible. A pilot study of 20 students was conducted prior to the actual study in order to perfect the interview protocol. The interview protocol was designed to probe how students translated between the submicroscopic and symbolic worlds and how students conceived of the submicroscopic world. In order to accomplish this goal, the interview protocol was broken into two questions. For the first question, students were given the chemical formula of formaldehyde and were asked to draw the Lewis dot structure. Formaldehyde was specifically chosen for several reasons. First of all, it is a relatively simple molecule that freshmen-level general chemistry students should be capable of drawing correctly. Secondly, it contains more than two elements, so that students have to think about the connectivity within the molecule. It also contains a double bond, which makes it a more difficult problem for less experienced students. Finally, based on how the students draw their Lewis dot structure, there are a variety of different geometries that they might predict for the molecule. Although the molecule is trigonal planar, students who do not realize that there is a double bond, for instance, may state that the molecule is pyramidal. Students had to go through the thinking process of how to draw the Lewis dot structure of formaldehyde, explain their drawings to the interviewer, and predict the geometry of formaldehyde based on their Lewis structure. Students were only given the chemical formula of formaldehyde and a periodic table. Results from the pilot study revealed that a slight modification in the question was necessary. In the pilot study, all students were given the chemical formula of formaldehyde in the more traditional form of CH2O. This resulted in all of the freshmen believing that the structure was simply water, H2O, with a carbon bonded off of the oxygen. It was decided that the carbon should come first in the formula to suggest that carbon was the central atom. Thus, the chemical formula was changed to COH2. In the second question, students were asked to use modeling clay (Play-doh) to build a model of a formaldehyde molecule. They were specifically given four colors of clay to have the freedom to use them, for instance, to represent each individual atom as a different color. Students were also provided with two different lengths of sticks, which they were told they could use if they wanted to. This gave the students flexibility, as they could choose to build a space-filling representation without sticks, or they could choose to designate the short sticks as double bonds and the long sticks as single bonds. Regardless of the representation they built, the students had to explain why they built the model the way they did and what the individual parts of the representation meant to them. This question was specifically designed to probe how the concepts of bonding and molecular geometry were linked together in the student’s mind and how the students used these links to solve this simple chemistry problem. After all the interviews were completed, the audiotapes were transcribed and imported into a qualitative data analysis program. A coding scheme was developed to embody the trends that came out of the data. The coding was done by the investigator using a qualitative coding program on the computer. The coding scheme was used to elucidate the dif206

ferent ways that students used the modeling clay and sticks to represent formaldehyde on a submicroscopic level. Students’ Lewis dot pictures were collected and became part of the data set. Students’ modeling clay structures of formaldehyde were also photographed and used in the qualitative coding scheme. For a list of the codes and definitions, see Table 1. Results and Discussion

Coding Table 1 was broken down into five main areas: arrangement, color, geometry, size, and sticks. These categories were used to describe the variances observed in the way students chose to build their model. Arrangement referred to the way students placed the atoms in space: whether they chose carbon or oxygen to be in the center of the molecule or whether they arranged the model in such a way that there was no center atom. This could be achieved by making the molecule linear, for example, or in a circle with four atoms. While ideally the carbon would be in the center with the other atoms in a trigonal planar arrangement, many other arrangements were observed. Color referred to how students chose to use the different colors of modeling clay. While some chose to use only one color, the most popular option was to represent each element with a different color. However, several students were creative and used the fourth color to represent lone pairs, which they included in their model. One senior even went so far as to make the intensity of the color represent the increasing electronegativity of the atoms. It should be noted that there was no expectation of how students would use the colors at the beginning of the study. However, the different colors were included to allow students as much flexibility in their models as possible. Geometry referred to the various geometries the formaldehyde molecule could be described as. Bent, linear, T-shaped, tetrahedral, trigonal planar (correct), and trigonal pyramidal descriptions were all observed in the study. Size referred to whether students varied the sizes of the different atoms in the molecule or whether they made all atoms the same size. When students did vary the size, it was observed that various students chose carbon, hydrogen, or oxygen to be the largest atom in the molecule. It was expected that students would follow the periodic trend that they had been taught in general chemistry. Therefore, hydrogen should have been smallest and carbon the largest. Sticks referred to how students chose to use the sticks in their model. Most students used sticks, but the way in which they used them varied considerably. Some students used only long sticks, while some used only short sticks. Some used two sticks for the carbon–oxygen double bond, while others used one short stick to represent the same thing. Still others decided there was a single or triple bond between carbon and oxygen. Once again, there was no specific expectation of how students would use the sticks in their models. While the most conventional method is to use multiple sticks for multiple bonds, it was also hoped that some students would make space-filling models without sticks. For some examples of the types of models and Lewis dot structures students created see Figure 1.

Journal of Chemical Education • Vol. 80 No. 2 February 2003 • JChemEd.chem.wisc.edu


Table 1. Qualitative Coding Scheme Developed to Categorize the Methods Students Used to Represent Their Models of Formaldehyde Category

Definition

Arrangement

Which atom is in the center, regardless of the geometry students used

Carbon center

Carbon is in the center

No center

A linear molecule or other arrangement that has more than one center

Oxygen center

Oxygen is in the center

Color

How students used color in the models

All same

Students used only one color of modeling clay

Different atoms

Students used different colors to represent different atoms

Electronegativity

Students used different colors to represent increasing electronegativity

Lone pairs

A different color is used to represent lone pairs

Geometry

What geometry students used to make their models

Bent Linear T-shaped Tetrahedral Trigonal planar Trigonal pyramidal Size

Reasons that students made the atoms in their model different sizes

All same

All the atoms in their model were the same size

Carbon and oxygen same

Carbon and oxygen were the same size but hydrogen was a different size

Carbon biggest

Carbon was the largest atom represented

Hydrogen biggest

Hydrogen was the largest atom represented

Oxygen biggest

Oxygen was the largest atom represented

Sticks

How students used the different lengths of sticks to build their models

C⫺O bond

What type of bond did students make between carbon and oxygen

Double

How students represented their double bond

One long

Using one long stick

One short

Using one short stick

Two long

Using two long sticks

Two short

Using two short sticks

Single

Students who believed there was a single bond between C and O

Triple

Students who believed there was a triple bond between C and O

Reason

Why students chose the sticks they did

Number of electrons

Based on the number of electrons in the bond

Bond type

Different sticks used to represent ionic versus covalent bonding

Distance

Students used different lengths of sticks because of distance considerations (intuition)

No reason

Students did not have a reason for their choice of sticks

Size

Students stated that the size of the bonds were different

Strength

Students used different lengths of sticks because of bond strength considerations

Use

How students use the sticks: their method of representation

Lone pairs

Students used sticks to represent lone pairs

Long and short

Students use both long and short sticks

Only long

Students used only long sticks between bonds

Only short

Students used only short sticks between bonds

Space filling

Did not use any sticks


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Qualitative Data The following discussion of the data focuses on illustrating both the commonalities and diversity within the students’ responses. The examples given are selected in some cases to illustrate the most popular responses that were observed; in other cases, examples are selected as unique responses to underline the diversity of the data set. In all cases, however, the coding scheme was used to classify and group these responses into common themes. Therefore, it is these

groupings, or codes, which are the key to seeing the trends in the data. Both the students’ clay models as well as the explanations and justifications they had for those models were taken into account when analyzing the data. Figure 1 illustrates some of the varied ways that students chose to represent their models. (Please note that all names used in this paper are pseudonyms.) Martin, a junior enrolled in organic chemistry, used different colors to represent the elements. He intentionally varied the sizes of the atoms with oxygen being the largest. When asked why he did it that way, he replied, ...I made the oxygen a little bit bigger than the carbon because it’s got more electrons. And so, as you go toward the right on the periodic table, and up, you’re going to, um, ah, as you go to the right and down, you’re going to have a little bit larger, ah, atoms. Because the electrons have, there are more electrons in higher orbitals and it’s just going to make the whole atom bigger.

Even though Martin had the atomic size trend incorrect, he could justify his choice. Martin also opted to use both the long and the short sticks in his model, using long sticks to represent single bonds and two short sticks to represent a double bond. When asked why he did this, he explained, “Um, the double bond is a stronger bond [than the C–H single bond], so it’s going to pull the atoms closer together, and that’s why I used a shorter, um, [stick].” In contrast, Evelyn, one of Martin’s classmates in organic chemistry, had a completely different representation. While the connectivity was incorrect, Evelyn did make a creative representation. She, too, made the oxygen bigger than the carbon, for the same reason as Martin. However, she chose to represent lone pairs with short sticks that had no modeling clay on the end. She also only used short sticks in her model. When asked to explain her reasoning, she stated, And I put two sticks here for the double bond, it’s just because, uh, I have a molecule set at home and that’s how it does it. Just so that when, when you turn it, it’s harder for it to spin, so that you know that it’s really not supposed to. Because double bonds can’t rotate. And, um, I put two sticks in here for the lone pairs because they’re still part of the molecule, even though they don’t always show up in the diagram, they’re still there.

Figure 1. Examples of the various types of models and Lewis dot pictures of the formaldehyde molecule that students created in the study: (A) Martin, a junior taking organic; (B) Evelyn, a sophomore taking organic; (C) Morgan, a freshman in general chemistry for science and engineering majors; and (D) Ben, a freshman in general chemistry for chemistry majors. The colors of the clay are represented by the following letters: g–green, b–blue, y–yellow, and p–pink.

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Morgan, a freshman in general chemistry for science and engineering majors, had a bent representation for his model. He, like Evelyn, thought that there were lone pairs coming off the oxygen and carbon that would make the geometry bent instead of linear. However, he did correctly identify the atomic size trend in the periodic table and intentionally made carbon larger than oxygen. Finally, Ben, a freshman enrolled in the general chemistry course for chemistry majors, was the only student in the entire study to make a space-filling representation. It should be noted that both the Lewis dot picture he drew and the connectivity of the molecule are correct. Not only did Ben make a space-filling model, but he also correctly hybridized both the carbon and the oxygen atoms in the molecule! Ben chose to represent the hybridized orbitals on carbon with green, while he represented the p orbitals with pink. He was at a bit of a loss as to how to represent the pi bond, however, and finally gave up as a bad job, verbally explaining what he



was trying to do instead of modeling it, “And then, but you still have these p’s out like this. And so, they can form a bond up here, up above and then down below. It’s just, that’s just the pi, and that’s the sigma. So, that, that’s, like, a double bond.” Note that Ben also represented the lone pairs on oxygen in pink in the picture. It should be noted that in all cases, the ways students drew their Lewis dot structures were identical to the way they built their models. That is to say, students were true to their Lewis dot pictures in building their models. In no case did any student change the arrangement or geometry of the molecule between drawing the Lewis dot picture and building the model (see Figure 1).

Trends In order to organize the data, the number of students enrolled in each class whose data fit into each portion of the coding scheme was tallied. The results are presented in Table 2. Table 2 is broken down by the classes that the students were enrolled in at the time of the study. This was done to preserve the common educational backgrounds that these students would have shared. While some students taking organic chemistry, for example, might be sophomores and some juniors, they had the common experience of being enrolled in organic chemistry at the same time. Thus, it was decided to classify based on chemistry class rather than on grade level. Arrangement Some interesting trends come out of the data presented in Table 2. In the first category, arrangement, most of the students (50%) correctly identified carbon as the center atom (Carbon center). However, an alarming number (39%) built their models with oxygen in the center (Oxygen center). Rhonda, a freshman in general chemistry for science and engineering majors, typified students’ explanations for putting oxygen in the center, “Um, well, I’m kind of going off the, the, ah, water...Basing it off of that and then just adding another, another molecule [atom] to it.” Looking at the distribution of students in each category based on chemistry level, it would also appear that further instruction in chemistry does not appear to have successfully dispelled this idea, as physical chemistry students were just as likely to put oxygen in the center as carbon. Color In the color category, the majority of the students (89%) used different colors to represent the three elements in the molecule (Different atoms). A few creative students also used the fourth color to represent lone pairs (Lone pairs). Elizabeth, a freshman in general chemistry for chemistry majors, talked as she built her model: “Actually, the lone pairs, I want green...And these [lone pairs] aren’t really, like, bonding. It’s just, they’re sticking on.” One exceptional student, Chris, used the colors’ intensity to represent increasing electronegativity for each element (Electronegativity). Chris was a senior taking physical chemistry who explained his reasoning as he built his model, “Okay. I’m using the colors to represent electronegativity...So, the darker the color, the more electrons it will have. Or, I guess the more electronegative it is...So I’m going to use the yellow for the hydrogens. They’re the least electronegative.” Two things should be noted at this time: First, some students opted to use sticks while others

used a different color of modeling clay to represent lone pairs. Second, the total numbers in this section add up to over 56 because some students were counted in both the “Different Atoms” and “Lone Pairs” categories. Geometry By far the most popular geometry (70%) was trigonal planar. However, this included both students who correctly put carbon in the center as well as students who placed oxygen in the center. The next most popular geometry was bent (11%), followed by T-shaped, and trigonal pyramidal (7% each). All students stated that they choose the geometry based on their Lewis dot structures. Once again, a comparison of the breakdown by the chemistry class shows no obvious improvement in the distribution of students who correctly identified the geometry as trigonal planar. While the majority of students at each level chose trigonal planar, there are students at every level who opted for a nonconventional geometry. This would tend to indicate that increased chemical instruction had not improved students’ understanding of this topic. Size As far as the size of each element in the molecule relative to the others, the most popular choice was to make oxygen (37%) the largest atom (O biggest). Martin’s explanation above typified students’ responses in this category; they all incorrectly identified the atomic size periodic trend. Twentyfive percent of the students did not make any differentiation between the sizes of the atoms (All same). When asked why, they usually responded as Julia, a junior in organic chemistry, did, “I didn’t think about it.” A surprisingly small number, 21%, identified carbon as the largest atom in the molecule (C biggest). All students who identified carbon as the largest atom correctly identified the atomic size trend on the periodic table. Interestingly, there were also those students (14%) who claimed that carbon and oxygen would be “about the same” size (Carbon and oxygen same). Mark, a junior taking organic chemistry, explained, “And I tried to make oxygen and carbon about the same size, because they’re fairly close on the periodic table and they have the same shell structure up to that point.” Once again, there appears to be no obvious improvement or change in the trends observed from freshman-level through senior-level courses. Students’ understandings of atomic size trends appear to be relatively stable despite increased chemistry exposure. Sticks Focusing on how students used the available sticks, there were several different ways to approach this data. First, of those students who did use sticks, 75% (42 out of 56 students) made a double bond between carbon and oxygen (C⫺O bond: Double). Only two students claimed there was a triple bond and twelve students (21%) claimed there was a single bond. Of those students that used a carbon–oxygen double bond (Double), they had a variety of ways of representing that bond. Most of the students in this category (67%) used two short sticks (Two short). Students in this category justified their choice like Bill, a senior enrolled in physical chemistry, “The bond lengths are shorter for double bonds. And that’s why I shortened up the bonds from the oxygen.” Seven students, 14%, however, used two long sticks (Two long) to represent the double bond. When asked why, the students had a variety of answers. Mark, for instance,


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Table 2. Number of Students Who Built the Model According to the Qualitative Coding Scheme Gen Chem Science

Gen Chem Majors

Organic

Inorganic

Physical

Total

Carbon center

5

8

6

6

3

28

No center

2

0

2

1

1

6

13

2

2

3

2

22

Category Arrangement

Oxygen center Color All same

0

1

2

0

1

4

Different atoms

20

9

10

8

3

50

Electronegativity

0

0

0

0

1

1

Lone pairs

1

3

0

0

1

5

Bent

2

0

2

1

1

6

Linear

0

0

0

0

1

1

T-shaped

2

0

1

1

0

4

Tetrahedral

0

0

1

1

0

2

14

9

5

7

4

39

2

1

1

0

0

4

14

Geometry

Trigonal planar Trigonal pyramidal Size All same

4

3

4

2

1

Carbon and oxygen same

3

2

2

0

1

8

Carbon biggest

6

1

1

4

0

12

Hydrogen biggest

0

0

0

1

0

1

Oxygen biggest

7

4

3

3

4

21

One long

1

2

1

0

0

4

One short

2

0

1

0

0

3

Two long

3

2

2

0

0

7

Two short

8

5

5

7

3

28

Single

5

1

1

2

3

12

Triple

1

0

0

1

0

2

Number of electrons

0

1

1

0

0

2

Bond type

1

1

1

0

1

4

Distance

8

3

1

4

2

18

Sticks C⫺O bond Double

Reason

No reason

11

4

3

1

2

21

Size

0

0

2

0

0

2

Strength

1

1

2

4

0

8

Us e Lone pairs

0

2

1

1

0

4

12

5

7

8

5

37

Only long

5

1

1

0

1

8

Only short

3

1

1

1

0

6

Space filling

0

1

0

0

0

1

Long and short

NOTE: “Gen Chem Science” are those students taking the general chemistry course for science and engineering majors, as opposed to the “Gen Chem Majors”, who are declared chemistry majors enrolled in the general chemistry sequence.

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chose to make the carbon–oxygen bond long and the carbon–hydrogen bond short. He explained, And again, it shows a double bond between the oxygen and carbon. I, I chose shorter bonds for the hydrogen because I think they’re small bonds. Once again, it might be because of electronegativity. And also, size, just general size. That allows them to be closer to each other, as opposed to this bond, which actually might be a little shorter because it is a double bond, it’s stronger.

Other students, like Ned, a sophomore in organic chemistry, said, “I was just picking the ones [sticks] up that I could see. I didn’t know there was a difference.” Four students used only one long stick (One long) in their representation. Blair, a freshman in general chemistry for chemistry majors, explained her reasoning, “The largest bond length I believe would be between the carbon and the oxygen...The shells would be the biggest between the two.” This was a perfectly viable consideration: she was trying to take the size of the atoms into account in representing the distances between the nuclei. In contrast, the last group of students (three) used one short stick (One short) for their double bond. Kelly, a freshman in general chemistry for science and engineering majors, explained, “And...the shorter stick represents a double bond because with a double bond it’s shorter and it’s stronger. Where with a long bond, it’s longer and not as strong.” In essence, Kelly felt that the type of bond was represented by the length of the stick alone, so that having two sticks between carbon and oxygen was largely extraneous. Since the classification simply coded what types of sticks students used, another subclass was necessary to classify why students actually chose the representations that they did (Reason). While the most popular answer when students were asked why they chose the sticks was a shrug and a “no reason” (No reason), the most prevalent explanation (32%) was because of the different distances involved in the bond (Distances). In order to be classified here, students had to use “distance” or “length” in their explanation, rather than “strength of the bond.” For example, Casey, a senior in physical chemistry, stated, “... shorter sticks I used for the double bond because it’s just been drove [sic] in that they’re shorter. They’re closer.” In contrast, students that were classified in the “Strength” category (14%) actually stated that double bonds are stronger than single bonds, as did Belinda, a sophomore in inorganic chemistry, I used the shorter sticks for the bonds between the carbon and the oxygen because double bonds, as I drew in the picture, uh, they’re stronger than single bonds. And because they’re stronger, they hold the atoms closer together. That’s why I picked the shorter sticks for that. And then the longer sticks I picked for the hydrogens because they’re just single bonds, which are weaker and longer than double bonds.

The remaining categories were broken down based upon nuances in students’ answers. For example, two students were very particular about stating that the reason they used shorter sticks for the double bond was because there were four electrons being shared instead of two. These students were categorized into the “Number electrons” category. Elizabeth explained her reasoning, “Um, I made the, ah, pi bonds the shorter ones because, ah, well, there’s just more electrons being

transferred and there’s more, there’s just a larger chance of, ah, them being closer together in my mind. And, um, I made the sigma bonds a little bit longer because, ah, there’s not as much electron, um, pull.” These students did not simply invoke the rule that double bonds are shorter than single bonds, but were attempting to reason out their answers. In contrast, students who were placed in the “Size” category, like Miguel, a sophomore taking organic chemistry, did not attempt an in-depth explanation. Miguel stated, “... it’s a bigger bond [C=O] than the carbon hydrogen bond generally is.” When asked why, Miguel said, “I’m not sure about that. But, um, I just studied that.” The last category in this classification was “Bond type”, which was for those students who believed there were both ionic and covalent bonds within the formaldehyde molecule. For example, Ian, a junior in physical chemistry, believed that the carbon–oxygen bond was covalent while the carbon–hydrogen bond was ionic, I tried to choose a, a longer stick [between carbon and hydrogen] because we talked about bond types earlier. Um, there are two different types of bonds that I see going on here...Um, we had our ionic bond that we were talking about. Ah, in this case, the hydrogen not having enough force to control its one electron pretty much lost it to, ah, the carbon and oxygen atoms.

Finally, the last category in Table 2, Use, dealt with the way that students used their sticks within their models. By far the most students (66%) used both long and short sticks (Long and short) in their models. This could have been to represent single versus double bonds. However, students also used varying stick length to represent lone pairs. Just as some students used a different color of modeling clay to represent lone pairs, four other students used the sticks to signify the position of lone pairs (Lone pairs). Eight students used only long sticks (Only long), preferring to use two sticks for a double bond. Similarly, six students used only short sticks (Only short) in their models. Only one student, Ben, used a space-filling model without sticks (Space filling). Conclusion In summary, this qualitative investigation reported on the rather diverse molecular models that students from freshman-level through senior-level chemistry majors built. Five assertions were made based upon the trends in the data: (1) Students do not necessarily have a developed or accurate mental image of how atoms are arranged in a specific molecule. The molecule used in this study, formaldehyde, is one that students are expected to be familiar with, and yet some students could not draw an accurate Lewis dot structure, much less a detailed model. This data would tend to indicate that when students are given free reign to represent molecules on the submicroscopic level, they do not inherently come to the same representation that an expert in the field— or a conventional model kit—would. (2) Students do not understand periodic trends. It was evident from the large number of students who reversed the atomic size trend that students are not internalizing the trends taught to them in general chemistry. Students also confused the trends for ionic size and electronegativity. Since all these trends are related, it would seem that students do not understand how these trends are connected nor the justification for the trends in


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the first place. (3) Students do not necessarily pay attention to bonding when building molecular models. Some students in this study did not appear to grasp the difference in strength between single and multiple bonds. Others paid no attention to bonding, using only one stick to connect their atoms. Certainly no one mentioned the different types of bonding while constructing their model and very few actually considered bond length based on both distance between the center of the atoms as well as the size of the atoms themselves. This may be because traditional model kits do not allow for this kind of flexibility. (4) Students do not necessarily improve in their understanding of these concepts as they progress through additional chemistry courses. The results reported in Table 2 tend to indicate that the number of incorrect models did not decrease with increasing level of chemistry exposure. In fact, the levels appear to be rather similar across the different classes. This seems to indicate that students’ submicroscopic models are rather resistant to change, despite increased educational level in chemistry. However, it could also be that the more advanced classes are not emphasizing modeling as much as general chemistry. This would be a potential avenue for further research. (5) Some students try to bring more to their models than are traditionally included in model kits. In fact, it would seem that conventional model kits tend to lead students towards the correct answer, with a fixed number of holes and a fixed geometry inherent in each piece of the kit. While conventional model kits have their benefits, they do not allow students the freedom they may desire to represent different nuances of molecules, such as bond length. Model kits also appear to have another down side: despite using these kits, students still appear to build their own, potentially unconventional, representations of molecules. In short, not all students are positively affected by the use of these kits. This is both good and bad. At some level, educators desire students to have a common submicroscopic representation of molecules, which is partially the aim of molecular model kits. However, this data tend to indicate that while some students produced something that represented traditional model kits, there were those students who had perfectly correct representations, but that represented points of the molecule which traditional model kits tend to overlook, such as lone pairs and bond length. In short, these students appear to have exceeded the expectations of the model kit and built more creative—and potentially useful—representations. Literature Cited 1. de Jong, Onno; van Driel, Jan. Prospective Teachers’ Concerns about Teaching Chemistry Topics at a Macro-Micro-Symbolic Interface; Paper presented at the 72nd Annual Meeting of the National Association for Research in Science Teaching; Boston: MA, March 28–31, 1999; ERIC ED430778. 2. Bradley, J. D.; Brand, M. J. Chem. Educ. 1985, 62, 318. 3. Davenport, D. A. J. Chem. Educ. 1978, 55, 93–96. 4. Wolfer, Adam J.; Lederman, Norman G. Introductory College Chemistry Students’ Understanding of Stoichiometry: Connections between Conceptual and Computational Understandings and Instruction; Paper presented at the Annual Meeting of the National Association for Research in Science Teaching; New Orleans: LA, April 28–May 1, 2000; ERIC ED440856.

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5. Shavelson, R. J. Translation among Symbolic Representations in Problem-Solving; Revision of a paper presented at the Annual Meeting of the American Educational Research Association; Washington: DC, April 20–24, 1987; ERIC ED301451. 6. Pfundt, H.; Duit, R. Students’ Alternative Frameworks and Science Education, 3rd ed.; Institute for Science Education: University of Kiel, West Germany, 1991. 7. Hinton, M. E.; Nakhleh, M. B. Students’ microscopic, macroscopic, and symbolic representations of chemical reactions Chem. Educucator 1999, 4 (5), 158–167. 8. Kwen, B. H. Research in Science and Technological Education, 1996, 14, 55–66. 9. Al-Kunifed, A.; Good, R.; Wandersee, J. Investigation of High School Chemistry Students’ Concepts of Chemical Symbol, Formula, and Equation: Students’ Prescientific Conceptions. 1993, ERIC #ED376020. http://www.askeric.org (accessed Dec 2002). 10. Ben-Zvi, R.; Eylon, B. S.; Silberstein, J. J. Chem. Educ. 1986, 63, 64–66. 11. Friedler, Y. Identifying Students Difficulties in Understanding Concepts Pertaining to Cell Water Relations: An Exploratory Study; Paper presented at the 58th Annual Meeting of the National Association for Research in Science Teaching; French Lick Spring: IN, April 15–18, 1985; ERIC #ED256623. 12. Haidar, A.; Abraham, Michael R. J. Res. Sci. Teach. 1991, 28, 919–938. 13. Stavy, R.; Tirosh, D. How Students (Mis)Understand Science and Mathematics: Intuitive Rules; Teachers’ College Press: New York, 2000. 14. Hwang, Bao-tyan. Students’ Conceptual Representations of Gas Volume in Relation to Particulate Model of Matter; Paper presented at the Annual Meeting of the National Association for Research in Science Teaching; San Francisco: CA, April 22– 25, 1995; ERIC ED387325. 15. Wolfer, Adam J.; Lederman, Norman G. Introductory College Chemistry Students’ Understanding of Stoichiometry: Connections between Conceptual and Computational Understandings and Instruction; Paper presented at the Annual Meeting of the National Association for Research in Science Teaching; New Orleans: LA, April 28–May 1, 2000; ERIC ED440856. 16. Hibbard, K. Michael; Novak, Joseph D. Science & Education 1975, 59, 559–570. 17. Novick, Shimshon; Nussbaum, Joseph Science & Education 1978, 62, 273–281. 18. Novick, S.; Nussbaum, J. Science & Education 1981, 65, 187– 196. 19. Samarapungavan, A.; Nakhleh, M. B. J. Res. Sci. Teach. 1999, 36, 777–805. 20. Gabel, Dorothy L. J. Chem. Educ. 1987, 64, 695–697. 21. Kokkotas, P.; Vlachos, I.; Koulaidis, V. Int. J. Sci. Ed. 1998, 20, 291–303. 22. Williamson, V. M.; Abraham, M. R. J. Res. Sci. Teach. 1995, 32, 521–534. 23. Smith, K. J.; Metz, P. A. J. Chem. Educ. 1996, 73, 233. 24. Russell, Joel W.; Kozma, Robert B.; Jones, Tricia; Wykoff, Joann; Marx, Nancy; Davis, Joan. J. Chem. Educ. 1997, 74, 330. 25. de Vos, Wobbe; Verdonk, Adri H. J. Res. Sci. Teach. 1996, 33, 657–664. 26. Mashhadi, Azam; Woolnough, Brian. Students’ Conceptions of the “Reality Status” of Electrons; Paper presented at the Annual Meeting of the Singapore Educational Research Association,


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A Qualitative Investigation of Undergraduate Chemistry Students

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