2511 Atomic Hydrogen and Line Emission Spectra

ATOMIC HYDROGEN AND LINE EMISSION SPECTRA

Last Name	First Name		Partner Name(s)	Date

1.0 OVERVIEW

One of the most tantalizing puzzles at the beginning of the 1900s for scientists was to describe the make-up of the atom. In this activity, you will look at spectra predicted by different models of the atom. The models you will test include:

• Niels Bohr’s Shell model
• Louis de Broglie’s Electron Wave model
• Erwin Schrödinger’s Quantum Mechanical model

2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

Simulation. No safety precautions.

3.0 CHEMICALS AND SolutionS

Simulation. No required chemicals.

4.0 GLASSWARE AND APPARATUS

The simulation can be downloaded and run offline on a computer that has Java installed or a smartphone with internet connection. Unfortunately, the simulation does not run on iPads.

5.0 PROCEDURE AND ANALYSIS

1. Access the PhET website.
2. Search for Models of the Hydrogen Atom and run it.
3. Turn on the White light gun with Experiment highlighted in the top left corner. The white light is shining into a transparent box with a “?” containing hydrogen gas molecules.

Why, with white light, the light photons passing up through the box have different colors?

4. Check the Show Spectrometer box. Notice that the color of the photons coming from the box corresponds to a wavelength of UV, visible, or IR radiation.
5. In the Light Controls, click on Monochromatic. Notice that the incoming photons are now all the same color. A spectrum slider appears that allows you to change the energy of the incoming photons. Move the slider across the spectrum from ultraviolet (UV) down to the infra-red (IR). Notice the color of the lamp and the photons moving up the screen. Describe how you can distinguish between UV and IR photons. Record that information below. (You may want to move the Slow...Fast slider all the way over to Slow.)

6. Switch the Light Controls back to White light
7. Move the Slow...Fast slider all the way over to Fast, Reset the Spectrometer, and let the simulation run for 1 minute.
8. After 1 minute, click on the Spectrometer camera to take a snapshot of the Experiment.
9. Describe what is happening to the spectrum. Include in your description the colors, estimated wavelengths, and relative numbers of stacked colored balls. These colored balls correspond to photons emitted by the ? box.

10. Slide the snapshot off to the right of the screen for later comparison with the models.
11. In the top left corner, switch from Experiment to Predict and highlight the Bohr model, click on Show electron energy level in the upper right-hand corner, and set the speed to Slow. 
NOTE: These three next steps are critical to your understanding of the Bohr atom model.
    1. Describe what you see in the atom diagram.

2. Describe what you see in the energy level diagram

3. Describe how the atom diagram and energy level diagrams are related.

4. Notice that the electron decays in the energy level diagram are colored. What is the meaning of the colors?

12. Move the Slow...Fast slider all the way over to Fast, reset the Spectrometer, and let the simulation run for 1 minute.
13. Click on the Spectrometer camera to take a snapshot of the Bohr model. Compare it to the Experiment. Slide the snapshot off to the right for later comparisons.
14. In the Bohr model, electrons can exist only at certain energy levels (also called shells), not at any energy levels between them. Shells in this energy diagram go from 1 to 6. An increase in energy level (say from 1🡪6) can only occur if a photon of incoming light with enough energy is absorbed. A decrease in energy level (say from 3🡪1) is accompanied by the emission of a photon as the excited electron releases its excess energy.
    1. Calculate the wavelength of incoming light that must be absorbed to make a 1 🡪 6 transition.

2. Calculate the energy of the photon emitted when an electron makes a 3 🡪 1 transition.

15. In the Help menu, select Transitions to see all possible transitions. Does this match your calculations?

16. Move the Slow...Fast slider all the way over to Slow.
17. Set the Light Controls to Monochromatic.
18. Set the monochromatic light source to 122 nm by clicking on the slider box and type in “122.” This provides photons of just the right energy to raise (excite) the electron from n=1 to n=_______ shell. Describe the atom diagram and the energy level diagrams.

19. Set the light source to 103 nm. This provides photons of just the right energy to raise (excite) the electron from n=1 to n=_______ shell. Describe the atom diagram and the energy level diagrams.

20. Just to make sure you understand this model, set the light source to 97 nm. This provides photons of just the right energy to raise (excite) the electron from n=1 to n=_______ shell. Describe the atom diagram and the energy level diagrams. How do the transitions differ from the two above?

21. Set the light source to 656 nm. This corresponds to a transition from 2🡪3. What happens? Explain why.

22. Louis de Broglie was the first atomic theorist to incorporate the ideas of Planck and Einstein that electrons can be both waves and particles. He developed the de Broglie hypothesis stating that any moving particle or object had an associated wave (wave-particle duality). De Broglie thus created a new field in physics called wave mechanics, uniting the physics of light and matter. For this he won the Nobel Prize in Physics in 1929.

23. Set the Light Controls back to White. Switch to the de Broglie Electron Wave model. Describe what is the same and what is different about the de Broglie and Bohr models of the hydrogen atom.

24. Move the Slow...Fast slider all the way over to Fast, reset the Spectrometer, set Light Controls to White, and let the simulation run for 1 minute.

25. Click on the Spectrometer camera to take a snapshot of the de Broglie model. Compare it to the Experiment. Slide the snapshot off to the right for later comparison with other models.

26. Erwin Schrödinger began thinking about wave mechanics in 1925. His interest was sparked by a footnote in a paper by Albert Einstein. Like de Broglie, he began to think about explaining the movement of an electron in an atom as a wave. By 1926 he published his work, providing a theoretical basis for the atomic model that Niels Bohr had proposed based on laboratory evidence. The equation at the heart of his publication became known as Schrödinger's wave equation.

27. Move the Slow...Fast slider all the way over to Fast, reset the Spectrometer, set Light Controls to White, and let the simulation run for 1 minute.

28. Click on the Spectrometer camera to take a snapshot of the Schrödinger model. Slide the snapshot off to the right for later comparison with other models.
29. Switch to the Schrödinger model with White light and speed set to Fast. Describe what you see in the atom diagram and the energy level diagram.

1. The l value in the top left-hand corner corresponds to the l value we covered in lecture. What is this value telling us in the Electron Energy Level Diagram?

2. Describe what you see in the atom diagram when n=1 and l=0.

3. Describe what you see in the atom diagram when n=2 and l=1.

4. Compare the atom diagrams when n=2 & l=1 and n=3 & l=1.

8.0 CONCLUSIONS

Spread the Experimental snapshot and the model snapshots across the screen. Now that all of the models are side-by-side, describe in detail how each model’s spectrum compares to the other models and most importantly to the real hydrogen spectrum. Compare the number of photons in each region of the spectra. What types of transitions are best predicted by which model(s)? Why?