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NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE CASE TEACHING NOTES for “History of the Atom: From Atomism to the Nuclear Model” by Jack F. Eichler, Department of Chemistry, University of California–Riverside INTRODUCTION / BACKGROUND This is a “clicker case study” that combines the use of inclass personal response systems (clickers) with a lecture that presents a historical case study. The case lecture is delivered via a PowerPoint presentation that has periodic clicker questions embedded within the slide show. As the case progresses, students learn about the various observations and sources of evidence that have been used to develop atomic models of matter. The presentation and clicker questions are structured in a way that gives the students a chance to discover how the various models of the atom have been constructed and have changed over time. This is done by providing the students with the data/evidence obtained in the historical experiments and then having them interpret this data in the clicker questions. This is different from the traditional lecture approach, which simply shows the students the various experiments and then tells them how these experiments were interpreted. This case should be taught in the first two weeks of a first-semester/first-year general chemistry course, or in a general science course that introduces the topic of atomic theory. The case introduces students to the discovery and description of subatomic particles and pre-quantum theories of the atom. In the course of covering these broader topics, students will be introduced to the topics of atomic number, atomic mass, and average atomic mass. Additionally, by having the students engage in the interpretation of data, critical thinking and higher order thinking skills also will be developed. Objectives By participating in this case study, students will: • Learn the fundamental particles that act as the building blocks of mater. • Learn about the historical experiments that provided the evidence used to develop the different models of the atom over time. • Learn about the pre-quantum model of the atom, and Case Teaching Notes for “History of the Atom” by Jack F. Eichler be able to work with concepts such as atomic number and atomic mass. • Be able to use data/evidence to arrive at a valid conclusion, in particular by using historical evidence to discover the various models of the atom that were developed over time. CLASSROOM MANAGEMENT / BLOCKS OF ANALYSIS This clicker case is designed for use in a large enrollment lecture course and has been taught at the University of California-Riverside (UCR) in classes with 300–500 students. The clicker questions are typically done in duplicate: once where students answer the questions individually and a second time where the students are allowed to work in small, informal collaborative groups (the students are not given the correct answer after the first question, though they are shown a histogram of question responses). Instructors could choose to ask each clicker question only one time (with or without the use of collaborative groups). If this case were administered to small enrollment classes, it could be done with more formal small group learning. I teach the case over two class periods, using the entire 50 minutes of a single class period to begin the case, and finishing it up at the beginning of a second class period. When I teach the case this way, I usually restart it at Slide 18 and finish in the next 30–40 minutes of the lecture period (instructors may wish to complete this case over the course of two 50-minute lecture periods, depending on how the discussions are facilitated). The next unit related to quantum theory and electronic structure is immediately started after the clicker case is completed (this course is taught in the “atoms first” approach at UCR, therefore this is the next unit in the general chemistry curriculum). If this clicker case is taught in a 75-minute class, it can usually be completed in one lecture period. The case can be used with students who have not done any prior reading or preparation, although it is ideal if Page 1 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Slide 1: Title slide. Slide 2: Case setup. The instructor should note to the students that they are going to go on a “journey through time” as they learn about the history of how atomic theory was developed. Slide 3: Excerpt from the Democritus/Atomism reading. Instructors can decide to give students some time to read this in class, or move directly to the next slide if they made the pre-class reading mandatory. Slide 4: Clicker Question #1 related to the Democritus/Atomism reading (answers to the clicker questions are provided in the Answer Key for this case). Slide 5: Discussion of Clicker Question #1. The concepts needed to answer this question are highlighted in the red boxes. The instructor should discuss these with the students, and explain how the philosophical arguments made by Democritus led him to the conclusion that matter must be made up of indivisible units called atomos (atoms). It is ideal if the instructor can state some of the logical arguments and then ask student volunteers to interpret specific aspects of the arguments. If the instructor is pressed for time, he/she can simply provide the explanations for the students and move on to the next slide. Slide 6: Summarizes the general conclusions of Dalton’s Atomic Theory. Slide 7: The content of this slide (and repeated on slide 9) was adapted from “A Letter from Dalton” (Groh 2001). Sample data that mimic data available at Dalton’s time are provided. The PowerPoint slide is animated so that each step of the calculation is revealed in succession, allowing the instructor to go over the calculation with the students and explain how the formulas for samples 1 and 2 could be deduced. This data is used to demonstrate to the students how the whole number ratio of elements could have been observed in Dalton’s time. The key is to highlight how when the mass of chromium is normalized between sample 1 and 2, the mass of oxygen between the two samples varies by a factor of 3. This should begin to reveal to the students that there are units (atoms) of matter that combine to form the two different forms of chromium oxide (this will be the basis of Clicker Question #2 in the next slide). Slide 8: Clicker Question #2. This requires the students to explain how the data in Slide 7 would have helped Dalton conclude that matter must be made up of atoms. Slide 9: The data from Slide 7 is shown again, and the instructor can use this to lead a brief discussion about how this data could have allowed Dalton to conclude that matter is made up of atoms. The key for the discussion is showing the students how the relative masses of an element in a series of related compounds such as this was found to differ, but only by whole number ratios. This suggested to Dalton that matter must possess some unit (the atom) that allowed to masses to change in such a way. Slides 10–11: These slides provide a picture of a magnetic field altering a cathode ray, as well as a diagram showing how cathode rays are made up of negatively charged particles that can be altered by a magnetic field. The instructor should lead a brief discussion about how this experiment demonstrated the existence of negatively charged particles called electrons. It is suggested that the instructor highlight the facts that the cathode ray is emitted from the negative electrode and received by the positive electrode. The instructor should also highlight how the external electromagnetic fields altered the cathode ray, again revealing how the cathode had negative charge. Slide 12: Clicker Question #3. Students are asked to determine which atomic model can be proposed based on the evidence from Slide 11. After the clicker question Case Teaching Notes for “History of the Atom” by Jack F. Eichler Page 2 the students have read the excerpts from the chapter on Greek atomism from the book, Readings in Ancient Philosophy: From Thales to Aristotle (Cohen, 2000). The points earned from the clicker questions in this case count toward a quarter-long compilation of clicker points, which is then used to assign extra credit (instructors may choose to have the clicker questions from the case count as a separate homework or quiz grade). Pre-Class Preparation A copy of an excerpt from the book Readings in Ancient Philosophy: From Thales to Aristotle (Cohen, 2000) is made available to students using the Blackboard course management site for the class, generally one to three days in advance of the class in which the case is run. Specifically, I ask them to read the preface to the chapter on Greek atomism as well as the numbered paragraphs 3, 4, 5, 8, 10, 11, 12, 13, 15, 18, and 19 in that chapter. Students can still successfully participate in the case study without reading this excerpt, but they are told that reading it will help them successfully answer one of the clicker questions in the upcoming lecture. Teaching the Case NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE is done, the instructor should briefly discuss how the cathode ray tube can lead to the conclusion that atoms must have negatively and positively charged particles (“Plum Pudding” model), but that this data provides no evidence regarding the exact structure/organization of the subatomic particles. It should be emphasized that the cathode ray tube provided direct evidence for negatively charged particles, and since it was known that matter is neutral under normal conditions, also provided indirect evidence that there must also be positively charged particles present in atoms. This is often a point not immediately noticed by students, and students often pick answer choice A as a result. Slide 13: This slide provides an illustration of Rutherford’s gold foil experiment. The instructor should lead a brief discussion about how the experiment worked, and what kind of data it provided. The instructor should emphasize that it was known (based on calculations of the mass of electrons) that alpha particles are about 7,000 times more massive than electrons (which suggests that electrons would not be able to deflect the alpha particles). The instructor should also highlight the fact that most of the alpha particles passed directly through the foil, but some were deflected as shown in the diagram. Slide 14: Clicker Question #4. Students are asked to determine which atomic model can be proposed based on the evidence from Slide 12. Instructors may want to highlight the fact that since alpha particles were known to be much more massive than electrons (approximately 7,000 times more massive), it was not likely that electrons would be able to deflect the alpha particles. This suggests that the deflections were caused by positively charged particles that had significantly larger mass than electrons. Slide 15: After the clicker question is done, the instructor should use the picture shown in this slide to briefly discuss how the gold foil experiment can lead to the conclusion that atoms must have a small, dense cluster of positively charged particles (protons) that are surrounded by the negatively charged electrons (“Nuclear” model), but that this data provides no evidence regarding the exact structure/organization of the electrons that surround the nucleus. Slides 16–17: The instructor can use these slides to briefly discuss how neutrons were discovered, and reemphasize what the nuclear model of the atoms is. Slides 18–19: These slides can be used by the instructor to transition from a discussion of atomic structure to a discussion of atomic mass. The instructor should highlight how data from the late 1700s and early Case Teaching Notes for “History of the Atom” by Jack F. Eichler 1800s could have been used to determine the ratios of atoms that combined to form compounds (using the example of water as described in Slide 19). Slides 20–21: After students answer Clicker Question #5 (Slide 20), the instructor should lead a discussion that relates how knowing the “parts” of each element that combine to form a compound (e.g., how there are two parts by volume of hydrogen and one part by volume of oxygen that combine to form water), along with the masses of each element used in the reaction can allow one to calculate the relative masses of the atoms of each element. Slide 22: The instructor can use this slide to talk about how atomic masses, along with periodic trends, were used by Mendeleev to organize the known elements into what we now know as the periodic table. The instructor will want to emphasize how Mendeleev found instances where the atomic mass did not increase as expected, and explain how he used the periodic trends to justify these exceptions. Slide 23: The instructor can use this slide to discuss how mass spectrometry was developed in the early 1900s and began to provide data related to the masses of atoms for the different elements, and in detecting different isotopes of elements. In particular, the instructor will want to emphasize that different isotopes of elements arise due to differing numbers of neutrons in the nucleus, and that the identity of elements arises due to the number of protons in the nucleus. This discussion can act as a transition into moving from using the average atomic weight of elements to arrange them in the periodic table to using atomic number. Instructors are encouraged to consult the article by F. McLafferty titled “Century of Progress in Molecular Mass Spectrometry,” Annual Reviews of Analytical Chemistry, 2011, issue 4, pg. 1–22. Slide 24: Describes how Moseley was able to experimentally determine the number of protons in the nucleus of atoms. The instructor will want to quickly explain how the number of protons in the nucleus will impact the energy required to excite core electrons in the atom, which can be detected using the X-ray spectra. The instructor will also want to re-emphasize how the number of protons, or the atomic number, is used to identify the different elements. Slide 25: Clicker Question #6. This question requires that students apply the concept that the atomic masses of elements are based on the protons and neutrons present in the nucleus. The number of protons (atomic number) increases from one element to the next; however, since Page 3 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE the number of neutrons may increase differently from one atom to the next, and since elements may have different abundances of various isotopes (forms of the element with different numbers of neutrons), the average atomic mass may not always increase from one element to the next. This explains why Mendeleev found some exceptions to the increasing atomic mass in his periodic table. Slide 26: This slide can be used to highlight the difference between atomic mass and atomic number during the discussion of Clicker Question #6. Slide 27: This slide allows the instructor to discuss in more detail the difference between different isotopes of an element. The instructor will want to emphasize that the identity of hydrogen is based on its atomic number (the number of protons), though it can exist as three different isotopes that have differing numbers of neutrons. The instructor will also want to explain how the mass symbols are used to identify the atomic number and mass number of each isotope. Slide 28: Clicker Queston #7. This question requires students to think about why the average atomic mass is not simply an average of the masses from the different isotopes. Given that the average atomic mass is very close to 1.00 amu, this suggests that the hydrogen-1 must be much more abundant than the other two isotopes. After the question, the instructor may also want to explain a couple of other examples (e.g., the average atomic mass of oxygen and its major isotopes) to demonstrate how higher abundances of a particular isotope will cause the average atomic mass of that element to be closer to that isotope mass. Slide 29: This slide can be used by the instructor to summarize the journey from the basic structure of the atom to the concepts of atomic mass and atomic number, and how these concepts were used to help create the modern periodic table. Slide 30: Conclusion slide. The instructor will want to summarize how we arrived at the nuclear model of the atom, and how this model of the atom allows us to identify the different elements, as well as the different isotopes of the various elements. The instructor will want to point out that we still have not discussed any evidence that allows the students to explain the electronic structure. New evidence that allows us to predict the electronic structure and come up with a more complete model of the atom will be discussed in the chapter related to quantum theory (this is the next topic in an “Atoms First” curriculum, and may be covered later in the Case Teaching Notes for “History of the Atom” by Jack F. Eichler quarter/semester if a more traditional general chemistry curriculum is used by the instructor). ANSWER KEY Answers to the questions posed in the case study are provided in a separate answer key. The answer key also includes the answers to the pre-case assessment questions. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to [email protected]. REFERENCES Cohen, S. Marc. 2000. Atomism: Leucippus and Democritus. In: Readings in Ancient Philosophy: From Thales to Aristotle. Indianapolis: Hackett Pub. Co., pp. 64–71. [I ask students to read the preface to the chapter as well as numbered paragraphs 3, 4, 5, 8, 10, 11, 12, 13, 15, 18, and 19.] Groh, Susan E. 2001. A Letter from Dalton. ProblemBased Learning Clearinghouse at University o f D e l a w a r e , h t t p : / / w w w. u d e l . e d u / p b l c , item#54122785113. [Registration required.] McLafferty, F. 2011. Century of Progress in Molecular Mass Spectrometry. Annual Reviews of Analytical Chemistry 4:1–22. • Acknowledgements: This material is based upon work supported by the NSF under Grant No. due-0920264. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Originally published December 7, 2012. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work. Page 4