Download “History of the Atom: From Atomism to the Nuclear Model”

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
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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
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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
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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.
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