How do students apply the octet rule and how do they justify this application?

Goal

For this study the following research questions will be evaluated and answered:

1. How do the students apply the octet rule in the pre-test?

2. How do the students justify their choice for deciding whether the octet rule is satisfied or not?

3. How are the differences in the students’ answers after the intervention?

Sample

Although especially pre-service chemistry teachers are of interest in our research, we also integrated other first-year students in our study to ensure a sufficient number of participants. Pre-service chemistry teachers should know how to apply the octet rule, because, as stated above, this rule is commonly used at school. Overall, 148 students participated in the pre-test. Only if the students completed the survey they were included in the study; for the pre-test, 96 students completed the survey. Only 20 students participated in both pre- and post-test. Table 1 gives an overview on the sample of this study.

Design of the tasks (including the intervention)

For the intervention study, an identical questionnaire for the pre-and post-test has been developed; for four different structural formulas the students had to answer a question with yes or no and then to justify their choice (see Table 2; for the questionnaire see Supplementary Material).

Table 2:

The tasks for the intervention study.

Structural formula	Task
	Is the octet rule satisfied for the nitrogen atom (N)?
	Yes/no
	Justify (2–3 sentences)
	Is the following statement correct?: “The chlorine atoms (CL) satisfy the octet rule?”
	Yes/no
	Justify (2–3 sentences)
	Is the octet rule satisfied for the nitrogen atom (N)?
	Yes/no
	Justify (2–3 sentences)
	Is the following statement correct?: “The right oxygen atom (O) does not satisfy the octet rule?”
	Yes/no
	Justify (2–3 sentences)

For the design of the tasks, two main points were taken into account:

1. The formulas and the elements that are part of the formulas should be most certainly familiar for the students, because from previous studies it is known that most students that are studying chemistry or another STEM subject had chemistry lessons during their schooldays (Hermanns & Schmidt, 2019).

2. The molecules should be quite small, because it is known that students often struggle with chemical formulas (Taskin & Bernholt, 2014). By choosing small molecules, the students can focus on the relevant information. It also reduces the cognitive load of the task at hand (Kayler, 2014; Sweller, 1988; Tiettmeyer et al., 2017).

The intervention consisted of a step-by-step procedure for applying the octet rule. Step-by-step procedures are quite common for this sort of application; several examples can be found in the literature (Ahmad & Omar, 1992; Ahmad & Zakaria, 2000; Carroll, 1986; Miburo, 1998; Packer & Woodgate, 1991). All known step-by-step procedures are very complex and therefore in our view not very user friendly. Therefore, for this study, a new step-by-step procedure has been developed (see Supplementary Materials). This step-by-step procedure combines the periodic table of elements with the application of the octet rule which was also not done in the other step-by-step procedures. By doing so it is made transparent to the students that they can find relevant information for the application of the octet rule in the periodic table. The first version of the tasks and the step-by-step procedure had been tested with students studying life or nutritional sciences; 16 students participated. They worked by themselves and could only use the step-by-step procedure, no other support was allowed. In the questionnaire the students were asked what part of the procedure was supportive or not and why. In an open item they could write down what they wanted to contribute. Because the students felt supported by the step-by-step procedure and no concrete information on the improvement of the material was given, the step-by-step procedure remained the same for the study.

The intervention consists of three steps: the step-by-step procedure for the application of the octet rule, tasks for practicing the application and the solutions to these tasks (in total four power point slides). An overview on the intervention is shown in Table 3 (for the complete intervention material see Supplementary Material).

The use of the tasks and the intervention

The intervention was conducted in those courses where the teachers of the course agreed to participate; in total four teachers participated; each course had another teacher (one professor, three post-docs). The teachers used the intervention instead of their own teaching materials for the octet rule; the topic “octet rule” was part of the content in their courses. This ensured that no course time would be used only for the study. The students were informed on the use of the data and agreed with the evaluation of those data. Ethical guidelines have been followed. The slides used for the intervention were made available to the students using the Moodle platform that was used in the courses.

In the pre-test, the students received during the course the questionnaire with the four tasks (see Table 2 and Supplementary Material) as online questionnaire. The students had approx. 7 min. for completing the questionnaire. In the intervention, the course leaders used four power-point-slides for teaching and explaining the step-by-step procedure. The solutions of the tasks in the pre-test have not been discussed. After the explanation (first two slides) the students solved the tasks of the intervention by themselves (third slide). The solution was then presented and discussed with all (fourth slide). The students then received the questionnaire as post-test, again as an online questionnaire. The students had again approx. 7 min. for completing it.

The coding process

The tasks include the application of the octet rule to choose whether the question should be answered with “yes” or “no” and the students’ written justification for this choice. Therefore, two rounds of coding were needed for evaluating the study. The method of qualitative content analysis (Kuckartz, 2016) has been used for the coding. Approximately 20% of each round has been coded also by the corresponding author. The first round of coding evaluated only whether the students’ explanations were technical correct or not. Both coders discussed and compared their assignments until 100% inter rater agreement was reached (Saldaña, 2013). The second round of coding took also into account the linguistic quality of the justifications. The coding consisted of the marking of students’ written explanations with points and has been done comparable to marking students’ explanations at school. For a task on justification, the marking would focus on both the linguistic quality and the technical correctness of the answer; only students who wrote a correct justification would receive full points. The coding scheme for this study has been developed by us, supported by a linguist. The inter coder reliability after Brennan and Prediger (1981) was sufficient (κ = 0.86). Tables 4 and 5 give an overview over both rounds of coding with examples from the students’ justifications (translated from German to English).

For evaluating and discussing the assessment of both rounds of coding, the statistical software SPSS has been used.

Research question 1: how do the students apply the octet rule in the pre-test?

For answering the research question for all tasks, the arithmetic means and standard deviations for the technical correctness of students’ justifications have been calculated; an overview gives Table 6.

When comparing the results for the four tasks for all students, it is noticeable that the arithmetic mean for the technical correctness of task 1 is the highest (M = 1.12 or 0.97) with a standard deviation of 0.945. The arithmetic mean is the highest regardless of whether the students answered the question correctly or not. For both cases, the students’ answers differ quite widely. Although it can be assumed that the students know the valence bond formula of NH₃ by sight, not all are competent in explaining the correctness of this formula.

Because the arithmetic means for all groups differ and the standard deviations are quite high, a Mann-Whitney test has been calculated to explore whether the differences in the rating are statistically significant or not. First, both groups of the pre-service chemistry teachers (first year vs. second year undergraduate students) have been compared. For all data, see Table 7.

The results for all tasks are statistically significant (effect sizes between d = 0.459 and 0.976); the technical correctness of the answers to the tasks from the second year students is overall more correct than the technical correctness of the answers from the first year students. Although the octet rule is part of the school curriculum, the application for Lewis formulas seems not to be common for the first year students. Because the results of the second year students are significantly better, it can be assumed that the students learn the application at University, although especially the arithmetic means for tasks 3 and 4 show that the second year students also are not very secure in applying the rule for charged molecules. When taking into account whether the students answered the questions correctly, the data are even more statistically significant.

Second, the first year pre-service chemistry teachers (major chemistry students) are compared with both groups of students, who are not studying chemistry as major students (minor chemistry students). The differences between those groups are statistically not or only somewhat significant (see Table 8). However, the data for task 4 are the most significant of these data. In particular, negatively charged molecules seem to be problematic for students who are not studying chemistry as their major subject. Two reasons can be assumed: the students did not enroll in chemistry courses during their school days or their motivation for technical content in chemistry is less distinctive than for the major students, who are pre-service chemistry teachers.

Not only the students can be allocated in two groups, this is also of interest for the tasks. Task 1 and 2 include molecules without charge; in task 3 and 4 both molecules are charged (one positively and one negatively). Therefore, a Mann-Whitney-test has been calculated to evaluate whether the differences of the results between both groups of tasks (without or with charge) are statistically significant. Table 9 gives the arithmetic means (M), standard deviations (SD), significance (p) and effect size (d) for the technical correctness of all tasks (ratings of the tasks have been added; scale therefore between 0 and 4).

For both cases (without and with answering correctly), the arithmetic means for the tasks without charge are higher than with charge. Statistically significant is this difference for the case “with answering correctly”. Both fits with our observation that the students have especially difficulties with charged valence bond formulas. Although they know in principle how to apply the octet rule, the charge is unsettling them. Significantly less students decide correctly if there is a charge. For teachers this means that they should include charged examples when the students are learning how to apply the octet rule and explicitly discuss why atoms are positively or negatively charged. Therefore, the intervention included an example of a charged molecule. Reasons for the problems the students have with applying the octet rule to charged molecules can be found in their answers as the following quotes illustrate (all for task 4): “the octet rule is not satisfied for the right oxygen atom, because if you add all bonding and not-bonding electrons, the total sum would be nine” and “one electron too much” (comment: the negative charge was interpreted as an electron) or “the octet rule is satisfied, but the bonding is unlikely, because the molecule has a negative charge”. Some students are insecure if the molecule is charged. Therefore, the intervention discusses this explicitly.

Because the students have to apply the octet rule for all tasks, correlations between all tasks have been calculated. As can be expected, between all tasks significant correlations can be observed (r _S = 0.312^**–0.491^** without regard to answering correctly; r _S = 0.280^**–0.525^** with regard to answering correctly).

Summarizing, our observations that students have difficulties with the application of the octet rule, especially for charged molecules, have been confirmed by evaluating the results of the pre-test.

Research question 2: how do the students justify their choice for deciding whether the octet rule is satisfied or not?

Because students should know from their school days what to do if they should justify their choice in a task, the linguistic quality of students’ answers has also been evaluated. For answering the research question, for all tasks the arithmetic means and standard deviations for the linguistic quality of students’ justifications have been calculated; an overview gives Table 10.

As observed for the technical correctness, the linguistic quality is also lower for those tasks that include charged molecules. Overall, the linguistic quality decreases from task 1 to task 4; probably because the students do not see the necessity of repeating the same or similar arguments for all tasks. This will also be discussed in the limitations and conclusion and outlook sections.

As for the technical correctness, for the linguistic quality a Mann-Whitney test has been calculated (see Table 11).

The differences between both groups are statistically significant. Only small differences can be observed whether the students answered the question correct or not. The effect sizes for both cases are high (d = 0.981–1.376 vs. 0.967–1.195). Although not trained explicitly, the students seem to gain argumentative skills during their University study.

For the linguistic quality, the differences taking into account whether the students answered correctly or not are for both groups of tasks (without or with charge) statistically significant (see Table 12). It seems that the students’ uncertainty with charged molecules influences their argumentation. The linguistic quality is significantly better for the tasks without charges, the effect sizes are 0.345 and 0.344 and therefore in the range between a small and a medium effect.

As for the technical correctness, also for the linguistic quality between all tasks significant correlations can be observed (r _S = 0.397^*–0.673^** without regard to answering correctly; r _S = 0.309^**–0.639^** with regard to answering correctly).

The linguistic quality of students’ answers does not correlate with the technical correctness without taking into account whether the students answered the question correctly or not. If this is taken into account, small correlations can be observed for tasks 2 and 3 (r _S = 0.280^** and 0.215^**). If the students answer correctly, they tend to better justify their decision. Whether the reason for this is because those students are more certain, cannot be answered by the data.

The data have also been analyzed with a focus on the problems the students have while applying the octet rule or which alternative concepts they use. They often mix up features from ionic bonds and covalent bonds, for example by writing that one atom gives the other atoms its electrons. They are not sure what valence electrons are and they confound chemical bonds with chemical compound. Both words are quite similar in German: Bindung (bonds) and Verbindung (chemical compound). The differences between oxidation state and charge or between charge and partial charge are also not clear. As observed before, they use the existence of a charge in the molecule for deciding that the octet rule is not satisfied, for example by writing that an electron is missing and therefore the octet rule is not satisfied.

Summarizing, the students’ linguistic quality is not as could be expected. The assumption that the students are able to write a justification, because the verb “justify” is known to them from their school days could not be confirmed. One possibility is that the students knew what they should do, but were not capable of doing it for the tasks at hand. Because the linguistic quality of the students’ justifications depends on whether there is a charge or not, the quality of the students’ arguments seems to be influenced by their knowledge of the octet rule although there is only either a small or no correlation between technical correctness and linguistic quality. When comparing the first year major students with the first year minor students, the differences between both groups are statistically not significant.

Summarizing, the linguistic quality differs between the groups, but overall the linguistic quality is not as good as could be assumed taking into account that the students should know what to do from their own school days. This assumption is supported by the observation that the second year pre-service chemistry teachers’ argumentations were rated the best, presumably because they gained the competences needed for the tasks at University and not at school.

Research question 3: how are the differences in the students’ answers after the intervention?

Over all groups, 20 students participated and completed both questionnaires. Unfortunately, this represents only 20.8% of the students that participated in the pre-test. Because the courses were online (via Zoom) and the questionnaires were also provided online, there was no direct contact with the students. Explaining the study and motivating them to participate is in such a setting more difficult than in a face-to-face setting. However, some conclusions can nevertheless be drawn. Table 13 gives the arithmetic means (M), standard deviations (sd), significance (p) and effect size (d) for the technical correctness of all tasks. A Mann-Whitney test for comparing pre- and post-test also has been calculated (see Table 13).

For the students who participated in both pre-and post-test the results are positive; the arithmetic means for all tasks are significantly higher in the post-test than in the pre-test, regardless whether the students answered correctly or not. For task 4, the difference is the most significant; p = 0.004 without taking into account whether the students answered the question correctly and even 0.002 if this was taken into account. The effect size is then 1.126 and therefore large.

For comparing the two tasks without charge with the two tasks with charge, for both groups of tasks all statistical data have also been calculated (ratings of the tasks have been added; scale therefore between 0 and 4); see Table 14.

The differences between both groups of tasks are statistically not significant. Therefore, the comparison of the pre-and post-tests between “with or without regard to answering correctly” has been calculated for both groups of tasks (see Table 15).

Regardless whether the students answered the questions correctly or not, for the tasks with charge, the differences between the pre- and the post-test are statistically significant. The students use the intervention as learning opportunity especially for the tasks with charged molecules. For teaching the subject “octet rule” this means that charged molecules should be in the focus and the application of the rule for such molecules should be taught and trained explicitly (Table 16).

The differences between the pre- and the post-test are statistically not significant. This is not surprising, because the training of argumentation skills was not the main focus of our study. For future teaching, this should be taken into account. As discussed in the literature (Lieber et al., 2022b), argumentation skills can be trained explicitly.

As for the technical correctness, for the linguistic quality the data taking into account whether the students answered the questions correctly or not have been calculated (see Table 17). The differences between both groups of tasks are also statistically not significant.

Summarizing, the small intervention was successful as all students’ results were better in the post-test than in the pre-test. The results for task 4 were statistically most significant. This supports our observation that the students have difficulties with applying the octet rule to charged molecules. However, if the students have the opportunity to train this application, as in this intervention, their competences increase.