Why we should teach the Bohr model and how to teach it effectively

1. Why we should teach the Bohr model and how to teach it effectively Sam McKagan, Kathy Perkins, Carl Wieman University of Colorado July 30, 2007

2. Faculty: Michael Dubson Noah Finkelstein Valerie Otero Kathy Perkins Steven Pollock Carl Wieman Joe Redish (visiting) Programmers: Chris Malley Sam Reid Ron Lemaster John deGoes Postdocs: Sam McKagan Linda Koch Grad Students: Wendy Adams Jack Barbera Chris Keller Pat Kohl Noah Podolefsky Chandra Turpen Danielle Harlow Kara Gray Charlie Baily* Acknowledgements

3. Why teach atomic models? Content: Understanding atoms crucial for understanding basis of modern technology. Beliefs: Students can appreciate the beauty of the universe by understanding the fundamental building blocks of nature. Skills: Good example of model building – can be used to teach scientific reasoning skills. Relevant at all levels of education – grade school to grad school

4. Controversy – how to teach atoms • Researchers* claim that learning the Bohr model is an obstacle to learning true quantum nature of atoms. Should avoid teaching Bohr model entirely. • Many in PER take this idea seriously – incorporate into curriculum design. • Area of active debate among high school teachers. * Fischler and Lichtfeld, “Learning quantum mechanics,” in Research in Physics Learning: Theoretical Issues and Empirical Studies (1992), pp. 240-258. Fischler and Lichtfeld, “Modern physics and students’ conceptions,” Int. J. Sci. Educ. 14, 181-190 (1992)

5. Several Objections • Research poorly conducted. • Assumption that Schrodinger is only goal. • Practicing scientists use the Bohr model! • Bohr model taught in high school. • Research shows that avoiding discussions of misconceptions doesn’t work. Must explicitly address them!

6. Design a study to answer this question: • Is teaching the Bohr model an obstacle to learning Schrodinger model of atom? Method: • Can’t do a controlled study because no one will teach course without Bohr model. • Can we develop a curriculum focused on model-building, including treatment of Bohr model, that leads students to develop understanding of Schrodinger model of atom?

7. Real World Examples:PMTs, discharge lamps, fluorescent lights, lasers, alpha decay, STMs, LEDs, CCDs, MRIs, BEC Model Building:Why do we believe this stuff? Reasoning Development:How do we make inferences from observations? Memorization Physics 2130 Modern Physics for Engineering Majors ~200 students Interactive simulations Peer Instruction Solving complex 3D Schrodinger problems Special Relativity Collaborative Homework sessions

8. Instruction on Atomic Models • Lectures: focus on model-building, reasons for each model. • Homework: compare and contrast models, discuss the advantages and limitations of each.

9. Models of the Atom • Democritus – Billiard Ball (450 BC) • Why? Smallest indivisible chunk of matter • Problem: Atoms can be ionized. • Thomson – Plum Pudding (1904) • Why? Known that negative charges can be removed. • Problem: Rutherford showed positive charge was small core. • Rutherford – Solar System (1911) • Why? Scattering showed small core. • Problem: electrons should spiral into nucleus in ~10-15 sec. • Bohr – fixed energy levels (1913) • Why? Explains spectral lines. • Problem: No reason for fixed energy levels • deBroglie – electron standing waves (1924) • Why? Explains fixed energy levels • Problem: still only works for Hydrogen. • Schrodinger – quantum wave functions (1926) • Why? Explains everything! • Problem: hard to understand

10. Results • Essay question on final exam: “Use words, graphs, and diagrams to describe the structure of a Hydrogen atom in its lowest energy state .” • Question does not mention models. Which model(s) do students use? • Analyze responses from random sample of 50 students from each class.

11. Multiple Models Models used to describe Hydrogen atom One Model Summary: • More students are using Schrodinger model than Bohr model, BUT… • About 1/4 still use Bohr model only. • Only about 2/3 use Schrodinger model at all! We want to do better! 100% 90% 80% 70% 60% Percentage of Students 50% 40% 30% 20% 10% 0% Bohr deBroglie Schrodinger Bohr deBroglie Schrodinger Fall 05 Spring 06

12. Review Curriculum Topics • Models of the atom through deBroglie • Electron waves • Schrodinger Equation • Other applications of Schrodinger Equation • Schrodinger model of atom Not enough explicit connections! ~ 5 weeks Redesign Curriculum: Add focus on connections between Schrodinger model and other models, model-building exercise.

13. Redesign Curriculum Fall 06: • Add focus on connections between Schrodinger model and other models. • Add general model building exercise. Spring 07: • Add interactive computer simulations: • Models of the Hydrogen Atom* • Rutherford Scattering* • In homework, students work through sims, describe reasons for adopting each new model. *http://phet.colorado.edu

14. http://phet.colorado.edu

15. Multiple Models Models used to describe Hydrogen atom One Model Summary: • ~80% of students use Schrodinger model. • Only ~10% use Bohr model alone.  Teaching Bohr model does not prevent students from learning Schrodinger model. 100% New Curriculum Old Curriculum 90% +sim 80% 70% 60% Percentage of Students 50% 40% 30% 20% 10% 0% Bohr deBroglie Schrodinger Bohr deBroglie Schrodinger Bohr deBroglie Schrodinger Bohr deBroglie Schrodinger Fall 05 Spring 06 Fall 06 Spring 07

16. Further results…

17. For more info… See our article at: http://arxiv.org/abs/0707.1541 Download simulations (free!) from PhET website: http://phet.colorado.edu Course materials (lecture notes, homework, exams) available at: http://per.colorado.edu/modern