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
1
Acknowledgements
Faculty:
Postdocs:
Michael Dubson
Sam McKagan
Noah Finkelstein
Linda Koch
Valerie Otero
Grad Students:
Kathy Perkins
Wendy Adams
Steven Pollock
Jack Barbera
Carl Wieman
Chris Keller
Joe Redish (visiting)
Pat Kohl
Programmers:
Noah Podolefsky
Chris Malley
Sam Reid
Chandra Turpen
Ron Lemaster
Danielle Harlow
John deGoes
Kara Gray
Charlie Baily*
2
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
3
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)
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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!
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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?
6
Model Building:
Why do we
believe this
stuff?
Reasoning
Real World Examples:
Development: PMTs, discharge lamps,
How do we make
inferences from
observations?
fluorescent lights, lasers,
alpha decay, STMs, LEDs,
CCDs, MRIs, BEC
Physics 2130
Peer
Instruction
Modern Physics for
Engineering Majors
Interactive
simulations
~200 students
Collaborative
Homework
sessions
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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.
8
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
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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.
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Models used to describe Hydrogen atom
90%
80%
70%
60%
50%
40%
30%
20%
Fall 05
Schrodinger
deBroglie
Bohr
Schrodinger
0%
deBroglie
10%
Bohr
Percentage of Students
100%
Multiple Models
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!
Spring 06
11
Review Curriculum
Topics
• Models of the atom
through deBroglie
Not enough
explicit
• Electron waves
connections!
• Schrodinger Equation
~ 5 weeks
• Other applications of
Schrodinger Equation
• Schrodinger model of atom
Redesign Curriculum: Add focus on
connections between Schrodinger model
and other models, model-building exercise.
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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
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http://phet.colorado.edu
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Models used to describe Hydrogen atom
Multiple Models
One Model
Summary:
• ~80% of students use
Schrodinger model.
• Only ~10% use Bohr
model alone.
90%
80%
70%
60%
50%
40%
30%
20%
Fall 05
Spring 06
Fall 06
Schrodinger
deBroglie
Bohr
Schrodinger
deBroglie
Bohr
Schrodinger
deBroglie
Bohr
Schrodinger
0%
deBroglie
10%
Bohr
Percentage of Students
100%
 Teaching Bohr
model does not
prevent students
from learning
Schrodinger model.
Spring 07
15
Further results…
Percentage of students who…
60
40
Fa05
Sp06
Fa06
30
Sp07
(a)
(b)
(c)
50
20
10
56
38
34
28
28
14 14
0
...use multiple
models
36
18 18
10 10
...explicitly discuss …mention limitations
models
of Bohr model
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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
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