Lesson 10 of 10
Objective: SWBAT interpret flame tests as evidence of the Bohr Model of the Atom
This is a classic chemistry lesson, flame tests as evidence of quantized electron energies as posited in the Bohr model of the atom. In the past, I have utilized this lesson to focus on the quantitative amount of energy in the atom. However, this focus has been at the expense of using it to reinforce the understanding of the Bohr model and electron levels. HSPS1-1 will ask students to Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
This will be the first time we have focused on empirical evidence for electron energy levels. In doing so, I hope to engage students in Science and Engineering Practice 6: Constructing explanations. I want them to engage in the the flame tests and spectral tubes and then generate a reason why there are specific lines in their spectrographs.
For the activity, you need:
- 1 spectrum tube per pair of students or Spectra sheets
- Incandescent bulb
- Fluorescent bulb
- 4 glass petri dishes
- Ethyl alcohol (safer than the methyl alcohol that often comes with lab kits)
- Solid Lithium chloride
- Solid Sodium chloride
- Solid Potassium chloride
- Assorted Spectrum tubes and power supply
This lesson is the Tuesday following the Labor Day weekend. As such, it is important to engage students immediately. I begin class by asking students about their weekend, and ask someone to recap where we left off the Friday before.
I then explain that today we will be connecting the two main realms of Chemistry: energy and matter. I ask students how we most commonly experience energy, looking for the response of light. I ask students what they know about light and colors of light. Many students will reference the rainbow, or that white is the mixing of all colors.
I then pass out the Activity sheet from BrainPOP, and ask them to watch the short video on Color. When the video is done, I give students a minute to complete the activity sheet. We quickly go over the sheet, and add that light combines to make white, whereas pigments combine to make black. I also ask students which is the highest energy color in the rainbow (Violet) and the lowest energy color (Red). I conclude by asking students, "What colors should be present in the classroom lights?" Since the lights are fluorescent white, they expect all colors to be present.
Now I ask students to get a partner, and to come up and grab a spectroscope, and one colored pencil or marker for each color of the rainbow. (To speed this up, I will often rubber band marker and pencil sets ahead of time)
I orient students to the spectrometers, that they look through the square or round opening depending on the model, with the slit side towards the light source. I ask all students to look at the nearest overhead light and inspect the side of their tube for the color pattern they see. While they are looking at the overhead lights, I pass out the Light and Atoms Investigation Sheet. Students should report seeing distinct lines of each color of the visible spectrum with black space between.
Next I explain the Investigation Sheet- that they will record the general color they see with the naked eye, and either draw or describe the light as seen through the spectroscope. I then ask them to look at the incandescent bulb I have available. Now they should see a continuous rainbow with no space between colors. We do the first two lines of the Investigation Sheet together, recording that both light sources gave off a white light, but that the overhead shows a distinct line for each color, whereas the light bulb shows a continuous spectrum.
I then explain that students will visit four stations: 2 spectrum tubes in the back of the room and two flame tests in the front of the room. I tell them they are to fill out the sheet as they did for the first two sources, recording the color of visible light, and then the colors and number of lines viewed through the spectroscope. I instruct students to not touch any lab apparatus, due to shock risk in the back of the room and burn risk in the front of the room.
I indicate a starting station to each group, and explain the direction of rotation so everyone is clear before shutting off the lights and beginning.
Flame Tests & Spectral Tubes
Students will begin at a specific spectrum tube as assigned, usually by table. Students will have 3 minutes to look at the tube and describe or sketch the spectral pattern they see in the corresponding space on the Investigation Sheet before rotating to a new station. Students will have opportunities to visit Helium, Neon, Krypton and Hydrogen tubes.
The final stations are in the front of the room, and controlled by me. I have 4 glass petri dishes, each with a different metal salt (LiCl, NaCl, KCl) in the dish and ethanol. The fourth dish is our control and only contains ethanol. When students are with me, I ignite the ethanol using an aim-and-flame lighter.
As the ethanol burns down, it will change color to correspond with the metal salts. I have students use their spectroscopes to record the color pattern and general color of each salt.
When a dish burns off all of its ethanol, it is important to allow it to cool before adding more ethanol to avoid a flash fire. The risk of this is lessened by using ethanol instead of methanol, but it is important to still allow the dish to cool before adding more ethanol to the dish.
Students may not see all of the samples in the period, which is acceptable.
Now I have students return to their seats and turn on the classroom lights. I ask students to come up with an explanation for the colors they saw based on the Bohr Model picture (the pictures are present on the Light and Atoms document, or accessible with searching at the previous link) of each atom next to their spectral diagram. Depending on the lighting, and their perseverance to get accurate diagrams, they should resemble these. In this particular lab, my results turned out more like this and this.
While students are working, I circulate the room, listening to their discussions and prompting groups that are stuck. Common prompts will include:
- How does the number of color lines compare to the number of electrons in the atom?
- Does the number of levels of electrons seem to affect the number of color lines in your observations?
- How did elements with the same visual colors compare in their diagrams?
- What does the electricity and heat provide to the electrons?
- How might an electron with extra energy behave?
Next I ask each group to share the explanation they have generated. We capture these on the board, and I look for correct aspects inside of the explanations.
I then summarize the correct findings from the class: that electrons gained energy from the electricity or heat, that they would then move up a level or more, and that they give off light when they fall back to their normal (ground) state. Students may cite a pattern between the number of lines and number of electrons, especially in the valence shell. This may not be a consistent pattern, but present in the limited data set.
Since student data varied, student conclusions varied tremendously as well. Many students were unable to connect the absorption of energy and emission of the light as being proof of the electrons being arranged in layers.
I close the lesson showing the video Fireworks: The Chemistry in the Color from the American Chemical Society. I narrate over the video, clarifying some of the vocabulary such as "vapor state" and connecting the video explanations to the student explanations. This is the closure of the lesson.
If there is extra time at the end of class, I share this video of a drone flying through fireworks, just for the awe and beauty of it. Unfortunately, they had to change music due to copyright, so I would show it without volume at this point.