Lesson Plan: Modeling Energy in Chemistry: Energy and the

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.

American Association of Chemistry Teachers

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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.


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


High energy Student D

Low energy

Electron movement from L2 L1 Electron movement from L3 L1

High energy



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. American Association of Chemistry Teachers

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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


122 nm

One emission at 122 nm (Experiment 2)

174 nm

No emission detected (Experiment 1)

326 nm


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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Lesson Plan: Modeling Energy in Chemistry: Energy and the

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