Research: Science and Education
Effect of Experience on Retention and Elimination of Misconceptions about Molecular Structure and Bonding James P. Birk* Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287 Martha J. Kurtz Department of Chemistry, Central Washington University, Ellensburg, WA 98926
Ever since Davenport (1) reported on “The Grim Silence of Facts”, researchers have become increasingly concerned about students’ misconceptions in chemistry. Studies have been done to determine student misconceptions at different levels of education. Treagust (2–4) reported misconceptions in bonding and molecular structure at the high school and college level. Boo (5) and Furio and Calatayud (6 ) have also investigated this area of misconceptions. Misconceptions about ten general chemistry topics were reported by Kurtz (7). Bodner (8) gave a conceptual knowledge exam covering a wide variety of topics and showed that misconceptions still exist in entering chemistry graduate students. A study by Gabel, Samuel, and Hunn (9) shows preservice teachers lack conceptual understanding of the particulate nature of matter. These articles also contain numerous references to other studies of misconceptions; the literature on misconceptions in chemistry is quite extensive. Study Design Although the published studies show misconceptions at all levels of education, their authors did not look at the retention of specific misconceptions over time. Consequently, we designed our study to diagnose student misconceptions over a large range of chemical experience from high school to faculty, to determine if and when the misconceptions disappear. The instrument we used was the diagnostic test on covalent bonding and structure developed by Peterson, Treagust, and Garnett (2). It consists of two-tier multiplechoice questions. The first part of the question asks the student to select an answer, generally based on recall or algorithmic knowledge; the second part requires a reason for the selection based on the student’s thinking in getting to that answer. The exam contains 15 question pairs, which examine understanding in six conceptual areas: bond polarity, molecular shape, polarity of molecules, lattices, intermolecular forces, and the octet rule (see box for examples). The test was administered to chemistry students ranging from high school to graduate school and to chemistry faculty. Subjects were allowed to work as long as they needed to complete the exam. One hundred thirty-nine high school students from four physical science classes and two chemistry classes in an average urban public high school took the test. The teachers of those courses assured us that they had covered the conceptual topics.
*Corresponding author. Email:
[email protected].
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The exam was also given to first-year and advanced college chemistry students. Of the first-year students, 244 took the test in their first semester and 271 after their second semester. Molecular structure and bonding are covered in the first semester. First-semester students were examined about one week after they had been tested on the same material by their lecture instructor. The 62 advanced undergraduates who took the exam comprised students who had completed a senior-level inorganic course or were undergraduate teaching assistants for general chemistry. Data from this group were collected from three large state universities. The graduate students who took the test were classified as “new” or “advanced”. Thirty-four new graduate students were given the test during an orientation program at the beginning of their graduate studies. The advanced group comprised 55 students from the same three universities, who had one or more years of graduate school experience. (It should be noted that there were no statistically significant universityrelated differences in test performance of either undergraduates or graduate teaching assistants from the three universities. The graduate students, of course, had received their undergraduate education from a large number of other institutions.) A group of 21 community college and university chemistry faculty who attended a two-year college chemistry conference were also given the exam. The data were processed with software that tabulated the percent of subjects who answered the first half of each question correctly and the percent who answered both parts correctly. These numbers give an indication of knowledge versus understanding in molecular structure and bonding. Results Graphs of exam performance for recall and conceptual knowledge versus chemistry experience are displayed in Figure 1. Experience is recorded as years of study, and experience classification labels mark each data pair. The figure indicates that, with experience, students gain better understanding of molecular structure and bonding. It is interesting, however, that the gap between knowledge and understanding has not completely closed even at the faculty level. One striking result is the linearity of the data for both recall knowledge and conceptual understanding. Another interesting finding is that high school students perform barely above the level of statistical guessing, presented as a baseline for guessing in the figure. These baselines were calculated by averaging the chance of
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
Research: Science & Education Examples of Diagnostic Test Questions 3. Nitrogen (a group 5 element) combines with bromine (a group 7 element) to form a molecule. This molecule is likely to have a shape that is best described as a(A) Trigonal planar b(B) Trigonal pyramidal (C) Tetrahedral 4. The reason for my choice on question 3 is a(A)
Nitrogen forms three bonds which equally repel each other to form a trigonal planar shape. b(B) The tetrahedral arrangement of the bonding and nonbonding electron pairs around nitrogen results in the shape of the molecule. (C) The polarity of the nitrogen–bromine bonds determines the shape of the molecule. (D) The difference in electronegativity values for bromine and nitrogen determine the shape of the molecule. 9. The molecule SCl2 is likely to be b(A) V-shaped a(B) Linear 10. The reason for my choice on question 9 is b(A)
Repulsion between the bonding and nonbonding electron pairs results in the shape. (B) Repulsion between the non-bonding electron pairs results in the shape. a(C) The two sulfur–chlorine bonds are equally repelled to linear positions as SCl2 has an electron dot structure shown as Cl
S
Cl
(D) The high electronegativity of chlorine compared to sulfur is the major factor influencing the shape of the molecule. 25. The “electron pair repulsion theory” is used to determine the a,b(B) Shape of a molecule (A) Polarity of a molecule 26. The reason for my choice on question 25 is (A) Nonbonding electrons determine the polarity of the molecule. For example, nonbonding electrons on the atom B in the molecule B A
C
cause B to become partially negative (δ ᎑). b(B) The theory states that the shape of a molecule is due to the arrangement of the bonding and nonbonding electron pairs around the central atom to minimize electron repulsion. (C) The theory states that the polarity of the molecule is dependent on the number of polar bonds present. a(D) The theory states that the shape of a molecule is due to repulsion between the atoms in the molecule. 29. Which of the following best represents the structure of N2Cl4 ? Cl
a(A)
Cl N
Cl
Cl
a(B)
N Cl
Cl
Cl
Cl N
a,b(C) Cl
N Cl
N Cl
N
Cl Cl a(D) Cl
N
N
Cl
Cl
30. The reason for my choice on question 29 is (A) The high electronegativity of nitrogen requires that a double or triple bond is always present. (B) The structure is due to repulsion between the five electron pairs (including bonding and nonbonding pairs) on the nitrogen atom. b(C) The structure is due to repulsion between the four electron pairs (including bonding and nonbonding pairs) on the nitrogen atom. a(D) The structure is due to the repulsion between bonds in the molecule. aAnswer/reason bAnswer/reason
relates to Figure 2. relates to Figure 3.
guessing the right answer over all questions. We constructed bar graphs to show the progression of the strength of holding each concept over time.The number of times a subject picked answer–reason pairs that support a statement indicates the strength of belief in that statement. High school students were not depicted because their responses were at the level of random guessing. Figures 2 and 3 show representative results. Figure 2 shows the percentage of each group who chose answer–reason pairs supporting the incorrect concept “the shape of molecules is due only to the repulsion between bonding electrons.” Since this concept is not an accepted one (shape is determined by repulsion between bonding and nonbonding electrons, according to VSEPR theory), a strength of zero is the desired result. A strength of zero means the individual never chose an answer–reason pair that supported the unaccepted concept. Figure 2 shows that as the subjects gain in chemistry experience, their strength of holding this concept goes to zero. Figure 3 depicts the accepted concept that shape is determined by repulsions between all electron pairs. Here, the desired result is for an individual to select an answer– reason pair supporting the concept each time it is possible, in this case, four out of four times. The figure shows that, most of the time, faculty correctly answer three or four of the answer–reason pairs, whereas first-year students correctly answer zero or one of the pairs. The box on page 125 provides examples of the kinds of questions used in the diagnostic test related to the two concept statements displayed in Figure 2 and Figure 3. The answer–reason pairs related to each statement are shown. The data were subjected to additional statistical analysis with SPSS/PC 4.0 (10). Answer–reason pairs were assigned to each concept statement tested in the exam. For example, the statement “the shape of molecules is due only to the repulsion between bonding pairs” had four questions with answer–reason pairs pertaining to it. SPSS was used to tabulate the extent to which students believed each statement by calculating percentages based on the number of times each student picked an answer–reason pair that supported that statement. Only three of the six categories tested by the exam were analyzed by concept: molecular shape, bond polarity, and polarity of molecules. The other three did not provide reliable results, since in many cases only one question related to those categories. Some questions tried to address two categories at once and consequently were difficult to interpret. Tables 1– 3 give the concept statements analyzed from the diagnostic exam and show the number of times an answer–reason pair reflected the statement. The percentage of subjects who selected answer–reason pairs supporting each statement 75% of the time or more is also shown. Some answer–reason pairs were not consistent with each other or were nonsensical. These are reported in Tables 1–3, as well. Subjects who selected nonsensical answer–reason pairs were most likely guessing. Again, the high school subjects were not included because their results were at the level of statistical guessing. Analysis of specific answer–reason pairs for the high school students confirmed this result. If students early in their studies do not have conceptual understanding, is it because they have a misconception or is it because they have no concept at all? The results shown in
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Research: Science and Education Recall Knowledge
Conceptual Knowledge
100
Faculty 90
1 yr Grad
2 yr Grad Senior Junior Begin Grad
80
% Correct
70
GenChem 2
60 50
GenChem 1
H.S.
baseline for guessing 40 30 20
baseline for guessing
10 0
10
12
14
16
18
20
Years of Study Figure 1. Test of recall and conceptual knowledge of covalent bonding and structure.
Figure 2. Strength of holding the misconception, “The shape of molecules is due only to the repulsion between bonding electrons.”
Figure 3. Strength of holding the accepted conception, “Repulsions between all electron pairs (bonding and nonbonding) result in the shape, according to VSEPR theory.”
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Table 1 for molecular shape indicate that students who do not consistently show conceptual understanding choose one of three other options. Instead of including both bonding and nonbonding electron repulsion for an explanation of shape, they select one or the other. Alternatively, students choose answer–reason pairs that make no sense, indicating no conceptual understanding. Students appear to understand that bond polarity does not determine shape. In looking at bond polarity (Table 2), however, one misconception stands out above all others: equal sharing of the electron pair occurs in all covalent bonds. The number of students holding this misconception may be exaggerated compared to other misconceptions because only one answer– reason pair was associated with it, so we were unable to examine consistency of holding this misconception. The other misconception of any significance was that ionic charge determines the polarity of the bond. Analysis of results for the polarity of molecules (Table 3) reveals two misconceptions. Students believed that nonpolar molecules form only when atoms in the molecule have similar electronegativities. They also believed that a molecule is polar solely because it has polar bonds. These two statements are consistent with each other and indicate that students ignore the effect of shape in determining molecular polarity. With these few exceptions, the extent to which students consistently hold the misconceptions uncovered in this study is small (generally 11–17%). Most students are somewhere between consistent performance in favor of the accepted concept and consistent performance indicating a misconception. (Consistent performance is defined here as selection of the appropriate statement 75% of the time or more.) This result agrees with the results of another study of misconceptions (6 ), which found that students’ answers to a misconceptions test at the beginning of first-semester general chemistry were classified as misconceptions 36% of the time, decreasing to 14% misconceptions at the end of the first semester. The motivation for testing students of differing chemistry experience was to see if a particular course provided the information necessary to overcome earlier misconceptions. This appears not to be the case: the students acquire both recall and conceptual knowledge linearly with experience. It is obvious from Figure 1 that experience in chemistry helps in gaining both recall and conceptual knowledge. An argument could be made that the gain is to be expected because only successful students continue to the next level. It is certainly true that students are selectively eliminated over time, but if this were the sole explanation for these results, then the data in Figure 1 would not be linear. The American Chemical Society Committee on Professional Training (11) provides data on enrollment in college chemistry courses, which gives an indication of the change in the pool of students that we are testing at each level. These figures suggest that of students in the first-year courses, about 30% take a second-year course, 3–4% a third year, and about 4% a fourth year. The pool of high school chemistry students is undoubtedly much larger, and the pool of graduate students is much smaller. Thus, it might be expected that decreases in misconceptions over time due to attrition of the student population would have significant curvature, accelerating with time, rather than the linear decrease that we measured. The results from this study provide some important
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
Research: Science & Education Table 1. Conception/Misconception Statements on Molecular Shape Q/A Pairs (No.)
Statement
Percent Selecting Pairs Supporting the Statement > 75% of the Timea
Undergraduate
Graduate
GC 1
GC 2
Adv.
New
Adv.
Faculty
Repulsions between all electron pairs (bonding and nonbonding) result in the shape (according to VSEPR theory).b
4
2.0
4.8
31
38
56
81
The shape of molecules is due only to the repulsion between bonding pairs.c
7
13.6
8.4
3
3
2
5
The shape of molecules is due only to the repulsion between nonbonding electron pairs.c
2
11.5
15.1
23
11
27
14
Bond polarity determines the shape of a molecule.c
16
2.0
1.1
0
0
0
0
Answer/reason pairs that make no sense.
26
10.2
11.8
18
3
2
5
aGC 1 is General Chemistry 1; GC 2 is General Chemistry 2. Percentages are based on the following numbers of people tested: 244 in GC 1, 271 in GC 2, 62 advanced undergraduates, 34 new graduate students, 55 advanced graduate students, and 21 faculty members. bAccepted concept. cMisconception.
Table 2. Conception/Misconception Statements on Bond Polarity Q/A Pairs (No.)
Statement
b
Percent Selecting Pairs Supporting the Statement > 75% of the Timea
Undergraduate
Graduate
GC 1
GC 2
Adv.
New
Adv.
Faculty
The electrons are closer to the more electronegative element.
3
17.3
29.2
74
71
82
90
Equal sharing of the electron pair occurs in all covalent bonds.c
1
51.6
41.0
18
24
20
5
The polarity of the bond is dependent on the number of valence electrons in each atom involved in the bond.c
4
3.3
1.1
2
0
2
Ionic charge determines the polarity of the bond.c
5
11.9
11.1
2
0
2
5
Nonbonding electron pairs influence the position of the shared pair and determine the polarity of the bond.c
4
0.8
2.2
0
0
0
0
The largest atom exerts the greatest control over the shared electron pair.c
1
4.5
4.8
2
0
2
0
3
0.4
2.2
0
0
0
0
4
0.4
0.7
0
0
0
0
Electrons have a positive charge.
c
Answer/reason pairs that make no sense.
0
aGC
1 is General Chemistry 1; GC 2 is General Chemistry 2. Percentages are based on the following numbers of people tested: 244 in GC 1, 271 in GC 2, 62 advanced undergraduates, 34 new graduate students, 55 advanced graduate students, and 21 faculty members. bAccepted concept. cMisconception.
Table 3. Conception/Misconception Statements on Polarity of Molecules Q/A Pairs (No.)
Statement
Percent Selecting Pairs Supporting the Statement > 75% of the Timea
Undergraduate
Graduate
GC 1
GC 2
Adv.
New
Adv.
Faculty
Nonsymmetrical molecules with polar bonds are polar.b
3
11.0
14.4
48
71
71
81
Nonpolar molecules form only when atoms in the molecule have similar electronegativities.c
7
17.3
11.8
6
6
7
0
Molecules of the type OF2 are polar as the nonbonding electrons on the oxygen form a partial negative charge.c
2
2.5
4.1
0
0
2
0
A molecule is polar because it has polar bonds.
11
15.6
14.0
6
0
7
5
Answer/reason pairs that make no sense.
10
4.1
2.2
0
0
0
0
c
aGC 1 is General Chemistry 1; GC 2 is General Chemistry 2. Percentages are based on the following numbers of people tested: 244 in GC 1, 271 in GC 2, 62 advanced undergraduates, 34 new graduate students, 55 advanced graduate students, and 21 faculty members. bAccepted concept. cMisconception.
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Research: Science and Education
insights into the teaching of chemistry and answer some questions raised about student misconceptions. At the high school level, students seem to have no understanding of molecular structure and bonding. Either they are not intellectually prepared to deal with abstract topics like this, or they have poor learning experiences. Students in their first year of college begin to show some understanding, but many of them respond inconsistently to similar questions, revealing a lack of comprehension. At the advanced graduate and faculty level, the misconceptions have disappeared for the most part, although performance is still not at 100%. A major difference between general chemistry students and advanced chemistry students is the study of organic chemistry, which places considerable emphasis on bonding and molecular structure. Unfortunately, we did not have the opportunity to test any organic chemistry students; but students who had this course did not show a significant decrease in misconceptions—the decrease seems to be linear with time. Since faculty seem to have few misconceptions, they are probably not transmitting misconceptions on molecular structure and bonding to their students, at least at the college level, although it is possible that teaching assistants are doing so. Significant changes in numbers of misconceptions seem to occur after the first year of college chemistry and during the first year of graduate school. This observation supports long-term anecdotal evidence that general chemistry topics are not thoroughly understood until an individual has to teach those topics to others. The difficulty of changing misconceptions and the slow pace of conceptual change is also supported by the literature (12, 13). Literature Cited 1. Davenport, D. J. Chem. Educ. 1970, 47, 271. 2. Peterson, R. F.; Treagust, D. F.; Garnett, P. J. Res. Sci. Teach. 1989, 26, 301–314. 3. Treagust, D. F. Int. J. Sci. Educ. 1988, 10, 159–169. 4. Peterson, R. F.; Treagust, D. F. J. Chem. Educ. 1989, 66, 459–460. 5. Boo, H. K. J. Res. Sci. Teach. 1998, 35, 569–581. 6. Furio, C.; Calatayud, M. L. J. Chem. Educ. 1996, 73, 36–41. 7. Kurtz, M. J. Using Analogies to Teach College Chemistry: A Multiple Analogy Approach; Ph.D. Dissertation, Arizona State University, Tempe, AZ, 1995. 8. Bodner, G. M. J. Chem. Educ. 1991, 68, 385–388. 9. Gabel, D. L.; Samuel, K. V.; Hunn, D. J. Chem. Educ. 1987, 64, 695–697. 10. Statistical Package for the Social Sciences (SPSS/PC+ 4.0); SPSS Inc.: Chicago, 1990. 11. Committee on Professional Training, American Chemical Society. CPT Newsletter 1995, No. VIII(Fall). 12. Posner, G. J.; Strike, K. A.; Hewson, P. W.; Gertzog, W. A. Science Educ. 1982, 66, 211–227. 13. Demastes, S. S.; Good, R. G.; Peebles, P. J. Res. Sci. Teach. 1996, 33, 407–431.
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