Sevinç Nihal
Yeşiloğlu
Department of Science and Mathematics Education – Chemistry Education, Gazi University, Ankara, Turkey. E-mail: nihalatalay@gazi.edu.tr
First published on 29th June 2019
The first purpose of this study was to introduce a laboratory modelling activity focusing on teaching the concepts of radioactive elements/atoms, radioactive decay, and half-life. The second was to investigate pre-service chemistry teachers’ understanding of these concepts. Fifteen pre-service chemistry teachers who had enrolled in a Physical Chemistry Laboratory course participated in the study. The participants simulated the radioactive decay process of an imaginary radioactive element called Cornium during the laboratory modelling activity. In this way, pre-service chemistry teachers were able to visualize and model what is meant by radioactive decay and half-life. Data sources included responses given to open-response conceptual questions, participants’ drawings about radioactive decay and half-life, semi-structured interviews, activity worksheets, and observation notes. Qualitative analysis of data revealed that the pre-service chemistry teachers had misconceptions about the definition of radioactive elements/atoms, radioactive decay, and the half-life process before the activity. The role of the modelling activity in exposing and eliminating these misconceptions was discussed.
Other researchers argue that radioactivity's coverage in the media is in fact one of the sources of misconceptions about the topic (e.g., Millar et al., 1990; Çelik Yalçın and Kılıç, 2005; Colclough et al., 2011; Mork, 2011). For example, statements in various media reports such as “a cloud of radiation” and “it contains radiation” (Millar et al., 1990, p. 338), may have encouraged one of the most common misconceptions: the equivalency of radiation and radioactive matter. There are many other prevalent misunderstandings: that an object exposed to radiation becomes radioactive (Prather and Harrington, 2001); that once a material is radioactive it is radioactive forever (Nakiboğlu and Tekin, 2006); that radioactive substances will never vanish (Suzuki, 2012); and that radio isotopes that have a long half-life are more dangerous (Suzuki, 2012).
Some of these misconceptions can be traced to a lack of clarification in mainstream discussions of these topics. Many students and adults do not understand the difference between ionizing and non-ionizing radiation (Rego and Peralta, 2006). For example, X-rays and nuclear radiations are not differentiated (Boyes and Stanisstreet, 1994). Nuclear radiation is thought of as machine-made (not natural) and hence is linked with technological advancements such as lasers (Boyes and Stanisstreet, 1994; Cooper et al., 2003). Another source of the misconceptions is the lack of knowledge about the particle nature of matter and its role in radioactive decay. Prather (2005) claims that one must have a fundamental understanding of how the atom (or atomic nucleus) behaves during the decay process to properly account for radioactive phenomena. In addition, Suzuki (2012) argues that because there are various formula for calculating the activity, decay constant, and radioactive decay law in the textbooks, students usually memorize these equations without understanding that radioactive decay is a random process. In 1944 Erwin Schrödinger explained the nature of radioactive decay as follows: “…But if you are given a single radioactive atom, its probable lifetime is much less certain than that of a healthy sparrow. Indeed, nothing more can be said about it than this: as long as it lives (and that may be for thousands of years) the chance of its blowing up within the next second, whether large or small, remains the same” (p. 78). In brief, the radioactive decay equation can be derived, as an exercise in calculus and probability, as a consequence of two physical principles: a radioactive nucleus has no memory, and decay times for any two nuclei of the same isotope are governed by the same probability distribution (Huestis, 2002). It is difficult for students to understand this characteristic of radioactive decay through formulated equations (Suzuki, 2012).
There is one way to experimentally investigate certain characteristics of radioactivity such as half-life—you can measure the activity of a radioactive material in each of a series of time intervals, plot the data as a function of the accumulated time on semi-log paper, and then measure the slope of the graph. The materials required to do this experiment are as follows: a Geiger counter, a stopwatch, and a radioactive material such as metastable Barium-137 with a 2.55 minute half-life. However, most schools lack this equipment, and the use of radioactive material in the classroom is not always practical or advisable (Hughes and Zalts, 2000). In addition, this experiment alone may be inadequate to demonstrate the random nature of decay to students. Therefore, to teach the radioactive decay and half-life process—taking into account the particulate nature of matter—and eliminate the existing misconceptions, the imaginary sub-micro or molecular level should be brought to the forefront. The present study was designed to introduce a laboratory activity that models the radioactive decay process through a molecular-level simulation exercise. It also examines pre-service chemistry teachers’ understanding of radioactive elements/atoms, radioactive decay, and half-life before and after the modelling activity.
Prather (2005) emphasized in his study that to correct student misconceptions about irradiation and contamination, it is critical to investigate students’ understanding of the role the atom plays in the radioactive decay process (p. 346). There are some tools to help students better understand radioactive decay and half-life—wet-lab experiments (Mak, 1999; Liguori and Adamsen, 2013; Pilakouta et al., 2016), model-based teaching activities (Schultz, 1997; Hoeling et al., 1999; Hughes and Zalts, 2000; Klein and Kagan, 2010; Bakaç et al., 2011; Claiborne and Miller, 2012), computer-based exercises (Crosier et al., 2000; Jesse, 2003; Mork, 2011; Jona and Vondracek, 2013; Sauter et al., 2013), and calculation exercises (Huestis, 2002; Ball, 2004). The exponential law of decay for radioactive atoms is usually taught with simulation exercises such as dice throwing or coin flips (e.g., Schultz, 1997; Jesse, 2003; Klein and Kagan, 2010; Bakaç et al., 2011). The simulation exercise used in this study is similar to dice throwing or coin toss simulations in terms of its analogical nature. The difference that sets it apart is its use in concurrence with the modelling activity described below.
The terms “laboratory modelling” and “laboratory simulation” are used in this study to distinguish from computer-based modelling activities and computer-based simulations. The number of studies examining the effect of computer-based simulations on students’ conceptual understanding has started to increase with the development of technology (Stern et al., 2008; Rutten et al., 2012), putting computers at the forefront in simulation research. In the chemistry classroom, computer-based learning environments attempt to make explicit the information embedded in traditional molecular representations, as well as provide a visual representation of molecular interactions for students to observe. However, a molecular simulation exercise does not have to be computer-based. In this study, one example of a laboratory molecular-level simulation exercise is illustrated.
(1) What are pre-service chemistry teachers’ understandings of radioactive elements/atoms, radioactive decay, and half-life?
(2) How does the use of a laboratory simulation exercise influence pre-service chemistry teachers’ understanding of radioactive elements/atoms, radioactive decay, and half-life?
Evaluate a model: In this stage, the laboratory simulation exercise (Chiappetta and Koballa, 2002) was implemented as an example of an authentic model of the radioactive decay process to evaluate the participants own models. In this exercise, each group was given one box (square, cardboard, with covers), which had a mark such as X on one of the inside walls. The researcher placed 100 kernels of corn into each box. It was noted that each kernel of corn has a pointed end. The groups were then asked the following question to discuss: What are the chances of a particulate kernel pointing to the side marked X? Then, all groups followed the following procedure:
• Shake the box with cover six times.
• Remove the kernels that are pointing to the side with the X. Determine how to label any kernels pointing at the X side's corners.
• Draw a data table to record the numbers of kernels taken out and the number left.
• Repeat this exercise for ten trials.
• Graph the results on the semi-log paper. How do you label the axes of the graph?
After all groups drew their graphs, it was discussed with the whole class that the laboratory exercise is also a model that simulates the radioactive decay process. Materials and events in the simulation exercise were compared with their representations (see Table 1). Scientific definitions of the following concepts—half-life, unstable atom, nuclear reaction, radioactive decay, decay rate, radiation, alpha, beta, and gamma decay—were addressed by the researcher. In addition, the “binding energy” and “band of stability” concepts, which had not been mentioned during participant brainstorming, were explained to clarify the theories about radioactive decay. It was emphasized that radioactive decay has a random nature. Any misconceptions determined in the early stages were debated among the whole class. Finally, the strengths and weaknesses of the simulation exercise (given in Table 2) were clarified.
Materials and events | Entities and events they represent |
---|---|
Corn kernels | Unstable atoms of Cornium element |
Each trial | Radioactive decay |
The chance of each kernel's pointed end pointing to the side marked | The random nature of radioactive decay |
The number of trials that it takes to use up half the kernels | Half-life |
Strengths | Weaknesses |
---|---|
It reflects the particulate nature of matter. | There is not an indication of the type of radiation (e.g., alpha or beta radiation). |
It represents radiations. | There is no representation of energy. |
It represents the random nature of decay very well. The decay chance of each kernel is the same. | Decayed corn kernels are taken out of the box. |
The half-life can be predicted. (You can predict it by graphing results; number of kernels remaining in the box versus trials.) |
It has been argued that although it has some weaknesses, this simulation activity is an effective model because it demonstrates the random nature of radioactive decay and predicts half-life. Before the groups graphed their results, they were asked what kind of line they expect. Then, they compared their expectations with their graphs. It was discussed that there was variation in the graphs; however, instead of an error, it may be due to the fact that the smaller the sample, the greater the variation. It was emphasized that the curve would be very smooth in an experiment measuring the radioactivity of a real radioactive material. Finally, groups were asked to find the half-life of the Cornium element, guided with the following question: how many trials did it take to use up half of the kernels that were left?
• Have you learned about radioactivity before? If so, in which class did you learn?
• What are radioactive elements and atoms?
• How can you tell if a substance is radioactive?
• What happens when a radioactive element decays? Please draw it and explain.
• Does a radioactive element/atom change when it decays? Please explain.
• Why do some elements/atoms decay?
• When does a radioactive substance/atom decay?
• What is “half-life”?
Open-response conceptual questions and activity worksheets were prepared by the researcher with the aim of probing pre-service chemistry teachers' understanding of radioactive decay process. The pre-service teachers responded to the questions in writing before and after implementing the activity. Their drawings about the radioactive decay process and explanations about them were obtained from the first activity worksheet and the second open-response conceptual question. Semi-structured interviews were conducted before and after implementing the activity with the aim of eliminating the unclear responses and gaining rich data for an in-depth analysis.
The data were analyzed and evaluated by the modelling constant comparative method (CCM) (Strauss and Corbin, 1998). The CCM was chosen to analyze the data to increase internal and external validity, because comparisons increase the internal validity of the findings. As Boeije (2002) states, “one criterion for qualitative research is that the researcher tries to describe and conceptualize the variety that exists within the subject under study. Variation or range exists by the grace of comparison and looking for commonalities and differences in behavior, reasons, attitudes, perspectives and so on. Finally, constant comparison is connected with external validity” (p. 393). The comparisons were made as follows: (1) comparison within single open-response conceptual questions; (2) comparison among respective open-response conceptual questions; (3) comparison between drawings and interviews; (4) comparison between open-response conceptual questions of different participants; and (5) comparison between drawings of different participants. During the comparisons, the participants’ understanding of the definition of radioactive elements/atoms fell into four emergent categories: scientific, not unscientific but insufficient, unscientific, and do not know. Understandings about radioactive decay and the half-life process obtained from the participants' drawings were coded under four categories as well:
(a) decaying and halving was understood as the same process;
(b) an object of radioactive decay was compared to the disappearing mass of a whole;
(c) the type of radiation emitted was not clear; and
(d) there was no indication or explanation of the probabilistic nature of decay and/or the predictability of the half-life.
Finally, understanding about radioactive decay process and half-life obtained from answers to open-ended questions were coded under the four categories; (a) changing after radioactive decay, (b) reason of radioactive decay, (c) nature of radioactive decay, (d) definition of half-life.
The reliability of the coding process was tested by an independent coder, who was an expert in the field of radioactivity. Reliability checks between researcher and a single outside expert have precedent in the field. Before the coding process, the coders discussed the category definitions and then compared their coding both qualitatively (20% of the coding) and quantitatively (calculating percentage agreement). The percentage agreement score was 95%.
Before the activity | No. of PCTs | After the activity | No. of PCTs | |
---|---|---|---|---|
Scientific | Material which contains unstable atoms which will spontaneously “decay” to form other types of atoms by emitting radiation in the form of particles and gamma rays. | 1 | Atoms that are unstable, meaning they have an imbalance of neutrons and protons, and will spontaneously “decay” to form other types of atoms by emitting radiation in the form of particles and gamma rays. | 6 |
Atoms that are unstable, meaning they have an imbalance of neutrons and protons. | 1 | Atom with a nucleus that falls outside the band of stability. | 5 | |
Not unscientific but insufficient | A radiant substance. | 4 | Atoms with n/p > 1 | 3 |
A luminescent substance | 1 | |||
Atoms whose nuclei change. | 1 | Atoms with an atomic number greater than 83 | 1 | |
High energy substances. | 1 | |||
Unscientific | A radioactive substance that vanishes over time and is transformed into energy. | 1 | 0 | |
A toxic substance. | 2 | |||
A substance that reacts under high temperature and pressure. | 1 | |||
A substance that occurs as a result of nuclear reaction. | 1 | |||
Do not know | 1 | 0 |
Before the activity, participants’ definitions of radioactive atoms were mostly not unscientific but insufficient. Only one pre-service teacher thought that radioactive atoms emit both rays and particles. One of the misconceptions was that “radioactive substances vanish over time and are transformed into energy.” Based on this misunderstanding, it may be concluded that the participant confused the concepts of radioactivity and radioactive material. Radioactivity is a characteristic feature of a radioactive substance and is exhausted over time. Radioactive substances are not vanished but transformed into a stable substance at the end of the decaying process. After the activity, participants had no misconceptions about the definitions of radioactive elements/atoms. It was observed that the pre-service teachers mentioned the particulate nature of matter more in the definitions. However, a few pre-service teachers still had insufficient understandings.
Only one participant could illustrate that the numbers of atoms in the radioactive substance decrease, noting the particulate nature of matter, but she could not illustrate in her drawing how they decrease or what happens to the decayed atoms (see Table 4, first individual drawing). One of the pre-service teachers illustrated the decay process by writing a reaction equation. It was observed that nine of the fifteen participants could not create a drawing that models radioactive decay and half-life processes. Before the simulation exercise, the understandings from the individual drawings and group drawings obtained from the activity were coded as follows:
(1) For the first-order process, a plot of the natural logarithm of the number of kernels remaining in the box versus number of trials is a straight line with a slope of −k;
(2) For the second-order process, a plot of the inverse of the number of kernels remaining in the box versus number of trials is a straight line with a slope of k.
Each group except one determined that the decay was a first-order process. The group that came to a different conclusion may have counted corn kernels incorrectly during the exercise; they noted that there might be a mistake in their data. Because the pre-service chemistry teachers already understood that the half-lives of the first-order reactions did not depend on the initial concentration, they also realized that the half-life in radioactive decay was not dependent on the initial concentration. They found the value of the rate constant, k, from the slope of the curve they obtained in the graph, and then determined the half-life of the Cornium element as 3 or 3.5 trials. The data, graph, and half-life value submitted by one group are given in Table 5 as an example.
Participants’ understandings about radioactive decay and half-life processes, as obtained from answers to open-ended conceptual questions, are given in Table 6. Those findings indicate generally that before the activity most of the participants had “no idea” (indicating declining to submit a theory or guess) about changes after radioactive decay, reasons for radioactive decay, nature of radioactive decay, or the definition of “half-life.” After the activity, participants offered responses for each category. Most of the misconceptions, before the activity, fell under the category of the nature of radioactive decay. Nine participants responded “no idea” about the nature of decay; some of the remaining participants thought decay was continuous and endless.
Before the activity | No. of PCTs | After the activity | No. of PCTs | |
---|---|---|---|---|
Changes after radioactive decay | A new substance is formed which is low in mass | 1 | A new substance is formed which is low in mass | 2 |
Chemical change | 2 | The numbers of protons and neutrons change | 2 | |
Numbers of neutron change | 2 | Numbers of neutron change | 2 | |
Reactivity of decayed material increases | 1 | Stable nuclei are formed | 5 | |
New substances are formed | 1 | The identity of the atom changes | 1 | |
No change | 2 | New substances are formed | 1 | |
No idea | 6 | |||
Reasons for radioactive decay | External effect | 1 | Reaching stable n/p ratio | 12 |
Instability | 3 | Having low binding energy | 3 | |
Characteristic feature of substance | 1 | |||
Not conservation of entropy | 1 | |||
Daylight exposure | 1 | |||
Electron exchange | 1 | |||
Not conservation of temperature and pressure | 1 | |||
No idea | 6 | |||
Nature of radioactive decay | Continuous and endless | 2 | Probabilistic nature | 5 |
Need to give energy | 1 | It continues until stable atoms are formed | 2 | |
Neutron bombardment | 1 | The decay time is unclear for a radioactive nucleus | 1 | |
When instability is maximum | 2 | It can happen at any moment | 4 | |
No idea | 9 | Random nature | 3 | |
Definition of half-life | Time for the mass of the radioactive substance to halve | 6 | This is the time for the number of undecayed radioactive nuclei/atoms to be halved | 11 |
It is half the time it takes for a radioactive substance to decay | 2 | Time for a radioactive substance mass of 2m to become mass of m | 4 | |
It's time the radioactive material started to decay | 2 | |||
No idea | 5 |
Suzuki (2012) mentioned in his study that many students think that a radioactive substance will never vanish because of their misinterpretation about half-life. That is, students believe that radioactive substances reduce to a half, then a quarter, then an eighth of their original levels at each half-life, never quite reaching zero. Based on the half-life definitions and drawings given by the participants in the current study, who think that decay is continuous and endless, it was observed that they explained half-life similarly to those in Suzuki's research. Some participants thought that there would be an external effect (e.g., energy or neutron bombardment) with radioactive decay. After the activity, all participants used expressions emphasizing the probabilistic nature of decay, such as “probabilistic nature,” “the decay time is unclear” and “random nature.” Before the activity, the pre-service chemistry teachers offered unscientific explanations of the reasons for radioactive decay such as “daylight exposure,” “electron exchange,” and a lack of “conservation of temperature and pressure.” Nakiboğlu and Tekin (2006) also found similar misconceptions in their study about radioactive decay rate—for example, “the radioactive decay rate depends on the physical conditions” (p. 1715).
After the activity, all participants were able to give causal explanations in accordance with the nuclear stability theory about the reasons for radioactive decay.
It was observed that some participants were not able to accurately define “half-life” before the activity. This may be due to a lack of knowledge about what exactly is “halving.” Some participants thought that half-life refers to the “time it takes for a radioactive substance to decay.” Some answers, such as “it's [the] time [when] the radioactive material started to decay” supported the idea that decaying and halving are being confused for one another—a misconception which emerged from the drawings as well.
One of the most significant findings, obtained from both the drawings and the open-ended conceptual questions about half-life, is the fact that the number of radioactive nuclei/atoms that remain after decay was initially ignored by all participants. “Half-life” should be defined as the time taken for the number of undecayed radioactive nuclei/atoms to decrease by half the initial amount. Conversely, the pre-service chemistry teachers drew the radioactive material as a whole; when this material halved, the mass of the whole also divided into two in their illustrations. After the activity, most of the participants were able to accurately define half-life in the open-ended conceptual questions. However, their modelling of the radioactive decay and half-life processes did not look as expected (see Table 7). The particulate nature of matter was rarely illustrated in the drawings. However, the explanations for the drawings illustrated a slightly stronger development of the participants’ understanding (see third individual drawing and its explanation, Table 7). As seen in the first and second drawing, the groups were able to illustrate the emitting radiation better than they could before the activity. Furthermore, after the activity, they determined that there should be a different drawing for each type of radioactive decay.
One explanation for relating decay to mass is the deficiencies in the definitions of half-life in the textbooks. One textbook describes it as “the length of time required for half of a ‘radioactive sample’ to disintegrate.” Another explanation is the phrasing of some questions and solutions in the textbooks. For example: If the half-life of 2 grams of a “radioactive substance” is 3 hours, how many grams was this substance 15 hours ago? The solution of this question is given below.
64 g ← 32 g ← 16 g ← 8 g ← 4 g ← 2 g |
Although we had explicitly discussed the weaknesses of the laboratory simulation exercise during the activity (see Table 2), these weaknesses may have also contributed to the issues some participants had no drawing the radioactive decay and half-life processes as expected. Removing decayed corn kernels from the box may have been misleading; in reality, decayed atoms transform into another atom—they do not go anywhere. The removal of corn kernels may have encouraged the pre-service chemistry teachers to draw the mass of the radioactive substance disappearing (see Table 7, third individual drawing).
After the activity, the participants were expected to create drawings that correctly illustrated the half-life process (see Fig. 1). In this drawing, the gray-colored particles represent the decayed radioactive atoms; white particles represent undecayed atoms. After each half-life, the number of undecayed atoms is reduced by half. The total mass does not change.
One implication of this study is a recommended change to the simulation in future implementations. In order to represent how total mass does not change, the kernels removed from the box should be replaced by an equal number of colored kernels. This will also indicate that decayed atoms are transformed into other atoms.
In the four-stage modelling activity, pre-service chemistry teachers had the opportunity to think and discuss the phenomenon studied in each of the stages. During the use of their model, they realized that they had no knowledge of when a radioactive substance would decay or how to predict its half-life. For this reason, the simulation exercise focused on teaching the probabilistic nature of radioactive decay, and as a result finding the half-life became more meaningful for them. Teachers or researchers using this four-stage modelling activity should be aware of students’ misconceptions and lack of knowledge, and follow their learning process through the stages of the modelling activity. Therefore, the results of the current study indicate that a laboratory simulation is more effective when used in conjunction with a modelling activity.
This journal is © The Royal Society of Chemistry 2019 |