Download Energy and the Electron

Lesson Plan: Modeling Energy in Chemistry:
Energy and the Electron
FOR THE TEACHER
Submitted by
ACS High School
Professional
Development Team
Washington, D.C.
Instructional Notes and Answers
Introduction and Hook:
 What do we already know about the model of the atom?
 Have students discuss and share their prior knowledge about the
model of the atom. This will also serve as a review about subatomic
particles for students.
 Possible responses may be the atom is small, has proton, neutrons, and electrons etc.
The first theory of the atom was by Thales's, a Greek philosopher, who stated
“Everything is water”. Explain to the students that since that time scientists have
developed many theories on the atom. If you have already discussed historical models of
the atom, you may choose to briefly discuss those ideas. Today we are going to learn
about scientific arguments and specifically how evidence is used to make explanations.
Initial Ideas - Atomic Structure:
 These questions are designed to allow students to begin thinking about atomic
structure. They will discuss their ideas and draw their own diagram of the atom.
 Students should independently (or in pairs) answer the questions on their
handout. Once finished, consider grouping each pair with another pair (four students)
and have students share their answers. When everyone is finished, discuss the answers
as a group.
Exploration - Observations of a Hydrogen Spectral Tube:
 Guide students in making observations of the incandescent light bulb first without the
spectrometer/diffraction grating, and then with the spectrometer/diffraction grating. If
using spectrometers, students often need direct instruction in how they function.
 Next, guide students in making observations of the hydrogen spectral tubes first without
the spectrometer/diffraction grating and then with the spectrometer/diffraction grating.
Note: It is important to debrief precisely what students should be seeing – if students do
not use proper technique they will make incorrect observations.
 Students should then answer questions 1 – 8 with their groups. The teacher can circulate
and check in with each group individually to ensure they are making accurate
observations. In particular, ask guiding questions when necessary to help students
process the analysis questions.
 Stop after question 8 to share out group ideas. One strategy could be for students to
create white boards to summarize their answer to question 8. They can then share their
white board with the entire class, or with another group, depending on time constraints.
 After the class discussion, guide students in answering question 9 with their groups. By
emphasizing parts b and c of question 9, students can deepen their understanding of
each model and make predictions they can then compare to the actual collected data in
part d.
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Analyzing Proposed Models (read this before implementing the activity):
 Before beginning this activity, ensure students have an understanding of: light photons,
absorption of energy (as a photon), and releasing energy (as a photon). It might also be
helpful to review basic properties of light: namely, the inverse relationship between
wavelength and frequency.
 Guide students in analyzing the four student models. It may be helpful to discuss the
main points of each model collectively as a class. Then, guide student groups in making
predictions in question 10. At this point, their predictions should be based on what each
model suggests, not what they observed with the hydrogen spectral tube.
 Students may need additional scaffolding or support in understanding what the
screenshots in the handout are showing. These images were created using the PhET
simulation “Models of the Hydrogen Atom”
(https://phet.colorado.edu/en/simulation/legacy/hydrogen-atom). This simulation can
be demonstrated to deepen students’ understanding of the images in the student
handout. In particular to note:
 The “light beam” is shooting light energy of a specific wavelength through a
sample of hydrogen gas.
 The question mark in the box in the center represents a hydrogen atom.
 The “Light Controls” window at the bottom left shows the incoming wavelength
of light.
 The “Spectrometer” window at the bottom right shows the wavelength(s) of
emitted light.
 The data for 4 different experiments have been collected and are given in the
handout. Students are not conducting these experiments themselves.
 Guide students in discussing how the evidence supports the three claims given.
 Guide student groups in creating an argument with a claim, evidence, and reasoning to
explain why they selected the model they chose.
Experimentation:
 Screencasts have been created for each of the necessary experiments. These can be given
to groups of students electronically or as handouts, or they can also be presented to the
class as a whole. These screenshots and images were created using the PhET simulation
“Models of the Hydrogen Atom”
(https://phet.colorado.edu/en/simulation/legacy/hydrogen-atom). This simulation can
be used by students or shown to them; however the current simulation may contain
glitches in the Experimental Mode which may provide erroneous results.
 Students will work with their groups to gather the necessary evidence from these
additional experiments.
 Within their groups, students will use their newly collected evidence to further analyze
previous models and to analyze competing claims about electrons within the atom.
Questions 14 and 15 guide students in this analysis.
 In question 16, guide students in revising their selected model to better match the
collected data. They should construct a new argument to support their revisions. At this
point, each group can share their selection and argument with the class, if desired.
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Student Answers (shown in red):
Refer to subsequent pages for possible responses to the student questions. Keep in mind that in
many of the questions, the answers may vary and are not limited to what is provided in this
teacher’s guide.
Part I: Warm Up
1. Consider the following questions individually:
a. What do you know about the structure of the atom?
Atoms consist of three subatomic particles: protons, neutrons, and electrons. The
protons and neutrons are found in the nucleus, while the electrons are found
outside the nucleus.
b. Draw what you think an atom looks like. Label the different parts of the atom.
Answers vary. Typically students will have the protons and neutrons in the center
of the atom with the electrons somewhere outside.
c. What do you think happens to matter (atoms) when you add energy?
Answers vary. Students might respond that atoms start to move more quickly or
heat up (typically based on their previous knowledge of phase change.
Part 2: Exploration
Step 1: Look at the lights in the room or an incandescent light bulb through the spectrometer or
diffraction grating.
1. What do you notice about the incandescent light bulb as seen with the “naked eye” and
observations using the spectroscope or diffraction grating?
a. Observations – eyes only:
Answers vary. The light looks white.
b. Observations – spectrometer/diffraction grating:
Answers vary. Through the diffraction grating I see the entire rainbow. The
colors are continuous (not broken up).
2. Using colored pencils, draw what you see from the incandescent light bulb through the
spectroscope or diffraction grating.
(http://web.ncf.ca/jim/misc/cfl/)
STEP 2: Your instructor will show a gas tube filled with hydrogen gas (hydrogen spectral tube)
in a power source. The power source runs an electrical current through the tube at a high
voltage. Make observations of the hydrogen gas with your eyes only and then through the
spectrometer or diffraction grating.
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3. What do you notice about the tube of hydrogen gas as seen with the “naked eye,” and
observations using the spectrophotometer or diffraction grating?
a. Observations – eyes only:
The tube looks blue/purple and seems to be “vibrating.”
b. Observations – spectrophotometer/diffraction grating:
There are four bands: purple, blue, teal (light blue), and red.
4. Using colored pencils, draw what you see from the hydrogen gas through the
spectrophotometer or diffraction grating:
Students should have a picture that looks like this:
5. Compare and contrast your observations of the incandescent light with your observations of
the hydrogen spectral tube.
Student responses will vary. Similarities should include the observation of
colored light. Differences include the four distinct lines from the hydrogen gas
compared to the continuous “rainbow” of colors observed from the incandescent
light.
6. Which subatomic particle do you think is affected when energy is added to the atom?
Explain.
Student responses will vary. Students will likely connect the added energy to
what they saw when thermal energy was added to solids and liquids – namely
that the particles increased in speed. Given their background knowledge of the
atom, students should also make the connection that since the protons and
neutrons are “trapped” inside the nucleus, only the outer electrons are free to
gain the extra energy and speed up.
7. What do you think happens to this subatomic particle when it absorbs energy?
Student responses will vary. Students will likely say the electrons will speed up.
8. What do you think happens to this subatomic particle when it releases energy? Where does
this energy go?
Student responses will vary. If students think of additional energy as increasing the
speed of the electrons, they will likely say that loss of energy would slow down the
electrons. They might conclude the energy is lost to the environment. Some students
might be more specific and name the type of energy released. For example, they
might say the energy is lost to the environment as thermal energy.
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9. Two students are discussing the following questions: What does the observation of specific
colored lines (spectral lines) tell us about how the electrons are organized outside the
nucleus? Examine the responses of the two students below.
Steve
I think electrons can be found anywhere
outside the nucleus. They are not limited to
specific places. When energy is added, these
electrons just move around wherever.
Rebecca
I think electrons can only be found in specific
energy levels. When energy is added, electrons
can move to higher energy levels.
a. What is the main difference between the ideas of these students?
Steve’s model shows no organization with the electrons. Instead, the electrons are
found in random places throughout the atom. Rebecca, on the other hand,
believes the electrons are organized on to specific energy levels surrounding the
nucleus.
b. If Steve were correct, what would we see when energy is added to the spectral
tube?
If Steve were correct, when the electron gains energy it would be free to move to
any location outside of the nucleus. Therefore, when it releases energy, the
electrons would also move to any location and would release all colors of light.
c. If Rebecca were correct, what would we see when energy is added to the spectral
tube?
If Rebecca were correct, when the electron gains energy it can only move to
certain locations within the atom. Therefore, when it releases energy it would also
only move to specific locations and would release specific colors of light.
d. Which explanation is best supported by your observations of the hydrogen
spectral tube? Explain.
When we observed the hydrogen spectral tube, we saw only specific colors of light
being released. This supports Rebecca’s model for specific energy levels. Perhaps
the energy levels she describes correspond to the energies of the specific colors of
light we observed from the hydrogen spectral tube.
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Step 3: Four students are asked to propose a detailed model to represent the different energy
levels the electron in a hydrogen atom could take. Their responses are shown below:
I think there are two
possible energy
levels. The electron
can jump up to level
2 and then fall back
to level 1.
I think there are
three possible energy
levels. The electron
can jump up to level
2 or 3, and then fall
with the following
possibilities:
L3  L2
L3  L1
L2  L1
These energy levels
are all equally
spaced, so the light
released will be
equally spaced.
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I think there are
three possible energy
levels. The electron
can jump up to level
2 or level 3 and then
fall with the
following
possibilities:
L3  L2
L3  L1
L2  L1
Energy levels 1 and 2
are a little closer
together, and level 3
is a little further from
level 2, so two of the
colors of released
light will be low
energy and close
together, while one
will be high energy.
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I think there are
three possible energy
levels. The electron
can jump up to level
2 or level 3 and then
fall with the
following
possibilities:
L3  L2
L3  L1
L2  L1
Energy levels 2 and 3
are very close
together, and level 1
is significantly
further from level 2,
so two of the colors
of light will be high
energy and close
together, while one
will be low energy.
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10. For each student model, predict how many emission lines you would expect to see and their
relative locations. Draw these on the spectrometers below.
Model
Prediction of emission lines that would appear in the Spectrometer
Student
A
Energy released when
electron moves from L1
L2
High energy
Student
B
Low energy
Electron movement
from L3 L1 (twice
the energy released
from L3 L1 and
L2L1)
Electron movement from
L3 L1 and L2L1 (both
release approximately the
same amount energy)
High energy
Low energy
Student
C
Electron
movement
from
L3 L1
Electron
movement
from L2
L1
Electron
movement
from L3
L2
High energy
Student
D
Low energy
Electron movement
from L2 L1
Electron movement
from L3 L1
High energy
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Electron
movement
from
L2 L1
Low energy
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Step 4: Previously we added electrical energy to the hydrogen tube. However, in this
simulation, the energy going into the hydrogen gas is light. Recall that the light energy is related
to the wavelength. We will represent the amount of energy going in and released (emitted) in
terms of its wavelength (λ).
Using this simulation, students are able to expose a sample of hydrogen gas to light energy of
specific wavelengths. Four experiments are carried out with four different wavelengths (λ). For
each incoming wavelength, the students measure the wavelength of the light emitted, or given
off. Their results are given below:
Experiment 1:
Incoming λ: 174 nm
Emission λ: No
emission detected
Experiment 2:
Incoming λ: 122 nm
Emission λ: One
emission line at 122 nm
Experiment 3:
Incoming λ: 110 nm
Emission λ: No
emission detected
Experiment 4:
Incoming λ: 103 nm
Emission λ: Three
emission lines detected
at 103 nm, 122 nm, &
656 nm
11. Does incoming light energy of every wavelength (λ) result in light emissions? Explain.
Not every wavelength of light results in the releases of light. In Experiments 1
and 3, for example, no emissions were recorded. This must mean only certain
wavelengths are capable of causing the electron to increase energy levels.
NOTE: This data provide evidence that only photons of certain energy are
absorbed by the atom! You may wish to stop to have a discussion comparing
Experiment 2 results with experiment 3, to show that even though the photon in
experiment 3 has a higher energy; it must not have been absorbed by the atom, as
no emission was detected.
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12. Which student model does the experimental data support? Explain your reasoning.
We believe the data supports Student D’s model. While Experiment 2’s data
matches the prediction for Student A’s model, the results in Experiment 4
contradict it. This must mean that there is more to the atom than Student A
suggests. In Experiment 4, three emission lines were recorded: two of high
energy (low wavelength) and one of low energy (high wavelength). This supports
the model of Student D which predicted a similar result.
13. Build an argument to support your choice.
Student answers vary.
a. Claim
Student D’s model best represents the data.
b. Evidence
The results of Experiment 4 support this model. Specifically, Experiment 4
showed three wavelengths of light released. Of these three emission lines, two
were of high energy and one was of low energy.
c. Reasoning
The three spectral lines observed in Experiment 4 support the idea of three energy
levels. Additionally, Student D’s model places the first energy level fairly distant
from energy levels 2 and 3. These higher energy levels are closer to each other
than either is to level 1. This arrangement means a fall from level 3 all the way to
level 1 will release a significantly high amount of energy. Since level 2 is also fairly
far away from level 1, a fall from level 2 to level 1 would also release high energy,
though less than from level 3 to 1. On the spectrometer reading, this coincides
with the two high energy spectral lines observed. Finally, since in Student D’s
model level 3 and level 2 are considerably close together, an electron falling from
level 3 to 2 would release a low amount of energy; this result is also seen in
Experiment 4.
Part 4: Experimentation
You will now work with your group to further explore the model of the hydrogen atom.
Additional experiments using the same tube of hydrogen exposed to various wavelengths of light
have been conducted and the results are provided for you. With your group, examine and
discuss the results of each experiment. Use the data and information you collect to answer
questions that follow.
Complete the data in the table below. Results of the previous four experiments are already
recorded.
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Incoming Wavelength (λ) Observed Emission Wavelength (λ)
94 nm
Emission at: 94, 103, 410, 434, 486, 656, 780, many >800 nm
103 nm
Three emissions: 103 nm, 122 nm, & 656 nm (Experiment 4)
108 nm
No emission detected
110 nm
No emission detected (Experiment 3)
115 nm
No emission detected
122 nm
One emission at 122 nm (Experiment 2)
174 nm
No emission detected (Experiment 1)
326 nm
No emission detected
white light
Emission at: 94, 103, 410, 434, 486, 656, three >800 nm
14. Do the new data imply more, less, or the same number of energy levels as those present in the
student models? Explain.
The new data imply more energy levels than all of those presented in the student
models. Previous data only showed three spectral lines, indicating the presence
of only three energy levels. Experiment 7 (incoming wavelength of 94 nm)
showed between 11 – 15 spectral lines. This implies more possible combinations
than just three energy levels.
TEACHER’S NOTE: It is a good idea to demonstrate how adding more
energy levels increases the number of possibilities in a non-linear
way. For example, adding a fourth energy level creates more than
four possible electron drops: L4  L1, L4  L2, L4  L3, L3  L2,
L3  L1, and L2  L1. Students are quick to say: “There must be 11
energy levels!” without scrutinizing the situation in detail.
15. Do the new data support your selected model? Explain.
This data does not support the model I selected. Student D’s model only
consisted of three energy levels, but the new data implies there are more than
three energy levels.
16. How would you revise your model to account for the new data? Draw a diagram of your
revised model.
At this point students should create a model that
has more than three energy levels. Since the
simulation does not clearly show all possibilities,
they may not have a completely accurate model. In
actuality, there should be 15 spectral lines for
Experiment 7 – due to the non-linear scale in the
simulator, a few of these high energy and low
energy spectral lines overlap. If students identify
10 spectral lines, they are likely to identify 5 energy
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levels; if they count more, they may identify the 6 energy levels Bohr identified in
his model of the hydrogen atom. Student diagrams may look like the one below.
It may be necessary to remind students that our initial experimental data
supported the idea the first energy level was further away from the outer energy
levels.
17. Build an argument to answer the question: What happens to electrons when energy is added
to atoms? Be prepared to share this with the whole group.
Student answers will vary.
a. Claim
Electrons may be found in specific energy levels outside of the nucleus. When
energy is added to atoms, the electrons may move to a higher energy level if they
added energy is of a particular wavelength because they are gaining energy. When
electrons move back to a lower energy level, they will release the extra energy as
light. The wavelengths of the light emissions determine if the light is visible and
its color, or if it is too much or too little energy to see.
b. Evidence
In the simulator, only certain wavelengths of incoming light resulted in the
production of spectral lines and light emission. When these particular
wavelengths of light, light of a specific wavelength was released. These emissions
could be high energy (ultraviolet), low energy (infrared) or visible (colors). These
specific colors were also observed when the hydrogen spectral tube was exposed to
high voltage electrical energy. Experiment 4 showed three spectral lines: two of
high energy and one of low energy. Additional experiments showed 11 to 15
possible spectral lines.
c. Reasoning
Since Experiment 4 showed two spectral lines of high energy and one of low
energy, we can conclude that the outer energy levels are significantly further from
the first energy level than from each other. This is confirmed in Experiment 7
which showed a high energy line for every electron drop to the first energy level.
Experiment 7’s results support the idea of six energy levels. With this number of
energy levels, 15 drops are possible, which matches the data in Experiment 7.
d. Model Revisions
We had to change our initial model by adding more energy levels.
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