K20 LEARN | Emission Spectra of Excited Gases

Summary

In this lesson, students will observe the colors in emission spectra for different excited gases. They will compare the specific emitted wavelengths and explain how these specific emitted wavelengths are the result of energetic and physical transitions by electrons in excited atoms. They will then apply this experience with emission spectra to predict and compare the spectra of atmospheres for planets in our solar system.

Essential Question(s)

How do excited gas atoms emit specific colors of light, and how can these specific colors be used to explore the composition of planetary atmospheres?

Snapshot

Engage

Students hypothesize about the origin of "neon" colors, what types of gases would allow a planetary atmosphere to support life, and how we measure the composition of planetary atmospheres.

Explore

Students view glass tubes filled with various gases before and while they are excited by electricity, using spectroscopes to observe and record spectra.

Explain

Students work with simple Bohr models to formulate a physical and energetic explanation for the emission of specific colors of light. Students explore the relationship between light wavelength and energy by correlating specific light colors emitted with specific energy changes. Optional video and kinesthetic activities help students explain their observations.

Extend

Extension options for students include predicting the emission spectra of gases from different planets in the solar system, exploring and explaining the Northern Lights phenomenon, and performing Algebra 1-level energy calculations.

Evaluate

Evaluation options for students include writing a data table title that effectively summarizes the key lesson of the exploration, sharing the results of their extension research, or designing their own multi-colored electrified gas sign.

Materials

Lesson Slides (attached)
Lesson Handout (attached, one per student)
Lesson Handout with Teacher's Notes (attached)
Data Table handout (attached, one per student)
Sample Data handout (attached, optional; for your reference or to show examples to students)
Tweet Up handout (attached, optional; one half-sheet per student)
Spectrum tubes (The Vernier set includes He, H2, Ne, Ar, N2, CO2, and Air. If you have access to a different set of tubes, use those.)
Spectrum tube carousel (Vernier Carousel is available to check out upon request, but any electrification device that matches your available tubes will work.)
Spectroscopes (Triangular spectroscopes sold by Flinn or tube spectroscopes sold by EISCO are good options, though other brands and homemade work as well.)
Colored pencils or markers

Engage

Teacher's Note: Lesson Background And Rationale

This lesson represents a core opportunity to lay the groundwork for a unit in astronomy, physics, chemistry, or physical science classes. It is designed to be flexible and provide you with options to best fit your students' needs.

Different atoms and molecules produce different colors of light when electrons become excited after absorbing energy from an outside energy source, such as the electricity used in the lab or by the heat generated within stars. Light is produced the same way when solid chemicals are exposed to flames in flame tests, in fireworks, and in neon lights used for signs on businesses (which are becoming rarer these days!). By looking at the atomic details of light creation, this lesson presents an opportunity to demonstrate energy conservation within an atom.

Energy transformations are a disciplinary core idea found in the standards of nearly every science class. Electrons in their ground state (most common, lowest energy location) absorb specific amounts of energy to make specific physical and energetic jumps to excited states. Eventually, the electrons re-release this energy, and when they do they can create electromagnetic radiation (that can be viewed as visible light if of the proper energy).

For more advanced students, this demonstration can initiate a quantitative study of light, the frequencies and wavelengths of visible particular colors of light, and calculations using Planck's equation (E = hv). As you prepare to teach the lesson, choose the options that you will use, and then go through the slides and the lesson handout to edit or remove unneeded content.

Before you begin, pass out a copy of the attached Lesson Handout to each student. Use the attached Lesson Slides and Lesson Handout with Teacher's Notes as you guide students through the lesson. Briefly review slides 1–3, introducing the lesson title, essential question, and lesson objectives. The main goals of this lesson are for students to observe line emission spectra, explain how these spectra result from physical and energetic changes in atoms (electron transitions) and apply line spectra to astronomy as a tool for analyzing the composition of planetary atmospheres. Spectroscopy is a broadly used tool with applications throughout physics, chemistry, and biology. Depending on the course you're teaching, weigh the electronic structure and astronomy goals to fit your needs.

Direct students to the Engage section of the Lesson Handout. The next two slides provide an overview of the questions posed in this section. Move to slide 4, and let students consider questions 1 and 2 on their own or with Elbow Partners. Depending on your preference, ask them to record their answers either in their lab notebook or on the handout.

Display slide 5. Introduce the spectrum tube carousel and gas tubes that you'll be using in the next activity. Demonstrate the use of a spectroscope and show students an example of a spectrum. You might utilize the attached Sample Data sheet. Explain your plan for the lab workflow (see setup options below) and discuss safety concerns.

Explore

Teacher's Note: Setup Options

Before the next activity, assess your material supply and decide whether you will have students view the gas tubes as part of a whole-class demonstration or as a station activity. If you have access to only one spectrum tube carousel or excitation box, consider performing a demonstration or using this activity as one station in a lab with other rotations. For example, you could include this activity in a rotation that also includes a chemical flame test experience. If you have access to more than one carousel or excitation device, consider placing different gas tubes at different stations and having students rotate to view them independently. This approach has the advantage of making sure students who work at different speeds have sufficient time to record the spectra.

Distribute the attached Data Table and spectroscopes to the students. Depending on your spectroscope supply, form groups as necessary. Ideally, you'll have one spectroscope for every 1–2 students, but the lesson can be done with a more limited supply and larger groups. The larger the group, the more you will need to manage turn-taking.

Have students turn to the Explore section of their handouts. Let students get their bearings with the spectroscopes by following step 1 to view natural light and/or overhead lighting. Remind students not to look directly at the sun. The broad rainbow spectra are easier to see than the line spectra emitted by specific gases. Use slide 6 to show what spectra should look like as students work on their technique. (See additional tips below.) It can be surprisingly tricky to see the spectra produced by a spectroscope, and seeing the example will help many students.

Ask students to view the gas tubes with and without the spectroscopes, following step 2 in their handouts. If you are working with one carousel or excitation device, you will set the pace for viewing the tubes. Make sure to first show the tubes without electricity (under which conditions they will all appear colorless) to emphasize electricity's role in color production.

Teacher's Note: Activity Tips

Follow the tips below to help ensure that the activity runs successfully.

Lighting: For the best results, work in the darkest room available with minimized ambient and overhead lights.
Spectroscope Troubleshooting: Students sometimes have initial difficulties using a spectroscope. The spectrum appears to the side of the initial line of sighting. The key for most spectroscopes is to center the slit of the spectroscope on the gas sample and then, without moving the spectroscope, slide your eye to either side to find the spectrum. If you use the triangular spectroscopes, the top of the spectroscopes can be labeled so that all students will orient them the same way and can adjust using verbal directions. Projecting or showing students a sample spectrum can help them better understand what it should look like.
Choosing Gases: If you decide to show fewer tubes to improve students' focus, pick gases based on what would interest your class the most. Some suggestions might include: Neon (to relate to the Engage question about neon colors and neon lights), Helium and H2 (to show off the two most abundant elements in the sun), and air, CO2, and N2 (to talk about atmospheric composition that is consistent with life).
Recording Data: Make sure students are recording data. By monitoring the data acquisition, you will also know whether you are providing sufficient time for students to gather their observations. If tubes are viewed as a whole-class demonstration, then be patient to allow most students to record data before moving onto the next type of gas.

After the activity, return the ambient light to the room. Direct students to step 3 on their handouts, asking them to form peer groups to compare results and ensure that everyone has data.

Use slide 7 to introduce principles of scientific titles, and then ask students to add a title to their data table. Have students share their titles with you and other classmates as a mid-lesson formative evaluation or an Exit Ticket for the first class session. (Typically, it takes about one class period to introduce the lab and gather data.)

Optional: Tweet Up and Other Activities

Consider asking students to share their data table titles using the Tweet Up strategy, as described on slide 8 and in Evaluate option 1 in their Lesson Handout. You could allow them to create a tweet on their Twitter account and have the class use a common hashtag, such as "#excitedgases" so that you and their peers can easily find them. You can also use this strategy in an analog format by allowing students to compose their tweets on the attached Tweet Up handout. If you prefer to wait and have students complete this activity at the end of the lesson, that is an option as well.

(If you choose not to have students participate in the Tweet Up activity, skip slide 8 or hide it from view. To do so, navigate to the editing view of the Lesson Slides. Then, in the column of slide thumbnails, right-click slide 8 and select "Hide Slide.")

Explain

At the end of the first class period, at the start of the next class, or as homework, have students answer the Explain questions from the Lesson Handout. You can ask students to work individually or in small groups. Encourage them to take chances and make hypotheses using their own words.

Follow up with a whole-class discussion about the questions. Questions 1–4 are close to the data and should be relatively easy for students to address. Questions 5-10 will require additional discussion. Use slides 9 and 10 to assist with this discussion.

Teacher's Note: Explaining Light Emission Using Bohr Diagrams (Questions 5–10)

To discuss energy transitions that lead to the light, it can help to visualize the change using Bohr models. The thin circles represent energy levels for electrons in atoms. Though the idea of orbiting electrons shown in this model has long ago been discarded, the idea that electrons exist at specific energy levels (ONLY!) is very powerful and remains a standard convention. It is easy to show energy changes in the Bohr model, though other diagrams could also be used. The diagram on slide 9 is also included in the Lesson Handout.

In step 1, excitation occurs using the energy supplied by the electricity to move an electron from an inner shell to an outer shell. In step 2, light emission occurs and the potential energy lost by the electron as it moves closer to the positive charge of the nucleus is released as light energy. Electricity is involved only in step 1, and light is released only in step 2.

Slide 10 shows a second Bohr diagram that explains why hydrogen makes a green and red color (but not orange or yellow). The green jump is higher energy, as it is a higher energy color (higher frequency) than the red jump. Electrons can move between quantized electron energy levels only, so the entire rainbow of colors is not made. Different atoms have different energy levels available, which changes the color and number of lines observed for each element/molecule type. Throughout, "light" is used to indicate the visible colors we can observe directly in the lab. It is important to note that the atoms make many frequencies of non-visible electromagnetic radiation, too (question 10).

Optional Kinesthetic Reinforcement Activity

Try this activity to conceptualize and build a physical model for the energy transition for your students. Start by asking students why they don't stand up in class, and then inviting students, if they prefer, to stand instead of sitting (likely a few will, but not the majority). Explain that standing is the "excited state" for a student: it is less common, but possible, while sitting is the "ground state."

Ask students when energy needs to be put into their bodies: when going up or coming down? (Going up.) Ask students when they will get to release energy. (Going down.) Light is released when electrons go back to the ground state—not when they are excited by the external energy. Then, ask what bribe (extra credit? candy?) it would take to get them to stand during class. Point out that this represents what happens when we add an external energy source (electricity in neon signs or thermal energy in fireworks) to move electrons to an excited state.

Or, introduce a "token," such as a balled-up piece of colored paper representing the color of the light photon released during relaxation, to represent the energy that is needed to move to the excited state (step 1) and then released as light (step 2) to allow the electron to return to the ground state. When students have a token, they are allowed to stand. When they release the token, they need to sit. To address why only a limited number of colors are made, ask students why they are not sitting/levitating at any level beneath their chair seat or above the floor to introduce the idea that only certain states are allowed (quantization). This is why we see only certain lines in the spectra.

If you have surfaces at different heights (desktops), you can use the higher surfaces to represent higher energy jumps. Surfaces at varying heights can also be used to represent different elements that have access to different energy jumps due to their varying electronic energy structures. Conclude by returning to the diagram included on the handout to make sure that students see how this physical analogy applies to the electrons drawn in the diagrams.

Extend

Direct students to the Extend section of their handout. Assign specific questions from this section that best support your goals for the lesson. Students can complete the questions individually or in groups, in class, or as homework.

Core extension questions 1 and 2 give students a chance to see an astronomy application of the spectra observed in the Explore activity. Use slide 12 to orient students to these questions.
Optional extension question 3 allows students to connect a natural phenomenon (the Northern Lights) to the gas emission observations and explanations. It is best suited for classes with the available time and interests in earth and space sciences. Use slide 13 to introduce this question.
Optional extension question 4 gives students a chance to discuss qualitative and quantitative methods of measurement and scientific communication. This discussion is accessible to students at all math and science levels.
Optional extension questions 5 and 6 offer an opportunity for quantitative practice. The equation is a two-variable inverse equation (y=constant/x) that can be used in algebra 1 physical science classes. The equation is not introduced at depth in this handout, so it will be more accessible to students who have been previously introduced (at least qualitatively) to the relationship of energy and light frequency. Use slide 14 to introduce these questions. Slide 15 shows work for an answer to question 5.

Evaluate

Direct students to the Evaluate section of their handout. Depending on your goals for the lesson, you can choose to have students complete one, two, or all three of the evaluation activities, which are described below. All three options are suitable for students at various math and science levels.

Tweet Up Exit Ticket. The Exit Ticket described in the Explore section and on slide 8 can be used as an end-of-lesson assessment. You can have students share their tweets online or use the attached Tweet Up handout to write their responses.
Extend question discussion. Use the Three Stray, One Stays strategy described on slide 16 to have students share the results of their Extend question research in peer groups. Split the class into groups of four and assign each group one Extend question. Give each group time to discuss and take brief notes over their assigned question, coming to a consensus. Ask one student per group to stay, acting as a group representative to describe the group's answer and reasoning to other students as they move through the room. Ask each of the remaining group members to stray, each traveling to a different question group (that is, not all going to the same table). Ask students to interview the representative in their new group to gain a deeper understanding of the answer to the question. Students should take notes and then return to their original groups to share what they have learned. Ask each group to synthesize the information they learned and share it with the class.
Design a sign for the school. Invite students to get creative and design a sign that uses electrified gas tubes to produce the desired colors. Using the directions on slide 17, students should perform research to find gases that can make other colors that they haven't seen in the lesson, and then draw a diagram to show the design for their sign, making sure to label the gas used for each portion.

Resources

Atmospheric Optics. (n.d.). Glowing gases - aurorae. https://www.atoptics.co.uk/highsky/auror3.htm
K20 Center. (n.d.). Bell ringers and exit tickets. Strategies. https://learn.k20center.ou.edu/strategy/125
K20 Center. (n.d.). Elbow partners. Strategies. https://learn.k20center.ou.edu/strategy/116
K20 Center. (n.d.). Tweet up. Strategies. https://learn.k20center.ou.edu/strategy/130
K20 Center. (n.d.). Three stray, one stays. Strategies. https://learn.k20center.ou.edu/strategy/85
Northern Lights Centre (n.d.) Northern lights. https://www.northernlightscentre.ca/northernlights.html
NPR's Skunk Bear. (2017, July 3). The science of firework color [Video]. YouTube. https://youtu.be/dW5OBrB4MRM

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Emission Spectra of Excited Gases

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Summary

Essential Question(s)

Snapshot

Materials

Engage

Explore

Explain

Extend

Evaluate

Resources

Publisher

Downloads

Standards

Facilitator Resources

Learner Handouts

Authentic Lessons for 21st Century Learning

Summary

Essential Question(s)

Snapshot

Materials

Engage

Explore

Explain

Extend

Evaluate

Resources

Publisher

Downloads

Standards

Facilitator Resources

Learner Handouts

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