C3 Atomic Emission Spectra and Bohr PDF

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assignment for chem 141 (intro chem)...

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CHEM 141 FALL 2015 ATOMIC EMISSION SPECTRA AND THE BOHR MODEL Group Name _______________

Open the PhET model, “Neon Lights and other Discharge Lamps”. Spend a few minutes familiarizing yourself with the discharge tubes model. Try clicking on different options, check all of the checkboxes, and get a general feel for how the model works. Investigate both the one atom and multiple atoms tabs.

Part 1: Configurable atom  Use the tab “One Atom”  Click on the spectrometer, squiggles, and run in slow motion.  Choose “Configurable” as your type of atom and “2 levels.” (This is a pull down on the upper right – the default reads “hydrogen.”)  Please check out the legend so that you can recognize electrons and visible, ultraviolet or infrared photon. We will use this “make-believe” atom to study the photons emitted when its electron gets kicked to a higher energy level and then returns to a lower level. It is important throughout this exercise to distinguish between those electrons that are streaming between the electrodes in the tube and those electrons that belong to specific atom being investigated.

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)

In each of the following, reset the spectrometer between observations and let each observation run for about 20 seconds. 1. Set energy level 2 midway down the chart by pulling down on the circled number 2.  Click on “continuous” electron production.  Adjust the slider on the battery so that the red arrow “energy of collision” lies between energy level 1 and energy level 2.  Note that the colliding electron is NOT absorbed but imparts some of its kinetic energy to the atom as it collides (prove this for yourself watching the animation)  Indicate colors and wavelengths (approximately) of any emitted photons. None

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What did you learn from this/what can you conclude about atoms, electrons, and light from this? Energy of collision must be above the energy gap form levels 1 to 2 for an electron to be promoted. Even LOTS of electrons at low energy cannot cause it.

2. Adjust the slider on the battery so that the collision energy lies at or slightly above and energy level 2.  Indicate colors and wavelengths (approximately) of any emitted photons. (Note: If the photon is not in the visible range, identify it not by “color” but as ultraviolet or “far UV” or “even farther UV” etc. and similarly for infrared (IR). ) ~460 (will depend on how you set your energy levels) 3. Adjust the slider on the battery to maximum collision energy.  Indicate colors and wavelengths (approximately) of any emitted photons. (Note: If the photon is not in the visible range, identify it not by “color” but as ultraviolet or “far UV” or “even farther UV” etc. and similarly for infrared (IR). ) Same as 2; excess energy in the colliding electron remains with the colliding electron as kinetic energy. 4. Move energy level 2 to a higher energy than in steps 1 and 2. Adjust the slider on the battery so that the collision energy lies at or slightly above and energy level 2  Indicate colors and wavelengths (approximately) of any emitted photons. ~380 (will depend on how you set your energy levels)  What do you learn by comparing steps 2, 3 and 4? As the energy gap between levels 1 and 2 increases, the wavelength of light emitted decreases. The energy of the colliding electron determines whether a photon is emitted, but does not affect the color of the photon. 5. Click on “3 levels.” Adjust the slider on the battery so that the collision energy lies at or slightly above energy level 3. Indicate the colors and wavelengths of all emitted photons. Also indicate the energy levels involved for each photon (2 to 1, 3 to 1, etc.)

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)

~far UV, 380, 650 (will depend on how you set your energy levels) Note that you should have 3 emitted photon wavelengths

Part 2: Hydrogen Change the atom type to Hydrogen. The energy levels are now fixed for Hydrogen. Before observing the spectrum, complete the following: 6. If the energy of collision is high, hydrogen will emit some photons in the ultraviolet, some in the visible and some in the infrared. For each of the following level changes, predict UV, IR, or visible: 

2 to 1 UV

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3 to 1 UV

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3 to 2 Visible

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4 to 1 UV

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4 to 2 Visible

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4 to 3 IR

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5 to 2 Visible

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5 to 4 IR

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6 to 1 UV

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6 to 3 IR

Adjust the slider on the battery so that the collision energy lies at or slightly above and energy level 6. 7. Check your predictions and complete the table below, indicating UV, IR or the particular color. If the wavelength is not available for UV or IR, label is far, farther or farthest. Feel free to add others you may observe. Record your observation in the following table:

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)

Energy Level Change 2 to 1 3 to 1 3 to 2 4 to 1 4 to 2 4 to 3 5 to 2 5 to 4 6 to 1 6 to 3

UV, IR or color UV UV Red UV Green IR Blue IR UV IR

Wavelength UV UV 650 UV 480 IR 430 IR UV IR

Part 3: Mercury 8. Change to Mercury, remove “run in slow motion” and adjust the battery to provide energy at or just above level 9. Let the PhET run for at least a minute and record the colors and wavelengths of the emitted photons. UV, 350, 405, 435, 545, 575, 680, IR

9. How does the Mercury spectrum differ from the Hydrogen spectrum? In particular, if you had to differentiate between the two, what might you look for? More visible peaks. Look for the yellow line

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)

Part 4: E/M properties 10. Paying particular attention to the energy diagram in the model, describe the process by which the emission of electromagnetic radiation (light) occurs. When an electron drops form a higher energy level to a lower energy level the excess energy is released as a photon.

11. What evidence do you see which suggests that light is emitted in quantized amounts? Explain. Only certain colors/wavelengths were emitted

12. Is it possible for a single electron to collide with the atom of hydrogen which results in more than one photon (particle) of light being emitted? Explain how. [Utilize the slow motion option for this!] Yes –It could drop from 4 to 2 and then 2 to 1 for example. Click on the “Multiple Atoms” tab on the model. Make sure that the Spectrometer and Squiggle options are checked. For each of the available gases given in the drop down menu on the right side of the model, record the colors and intensities of light in the visible spectrum emitted by each element. Hydrogen Emission Spectra

Mercury Emission Spectra

Sodium Emission Spectra

Neon Emission Spectra

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)

Part 5: The Bohr Model of Hydrogen The Bohr model of the hydrogen atom was the first quantized model of an atom. The Bohr model is able to successfully explain why the hydrogen atom has the absorption/emission spectrum that it does. No previous model of the atom could explain absorption/emission spectra. The Bohr model of the hydrogen atom involves the single electron orbiting around the tiny central nucleus (a proton). The electron could only be in one of the various quantized energy levels, each level having a different quantum number n with its associated radius. The electron could not exist between these fixed states. (An analogy to this is how you cannot plant your foot between rungs on a ladder, but only on the rungs, which have fixed positions on the ladder.)

Figure 1: The Bohr model of the atom Based on your observations above and using Figure 1, answer the following: 13. In your own words, what is meant by the term “quantized”? [e.g., What does it mean for the distance between the electron and the nucleus (thus the electron’s energy) to be quantized?] Only particular values are available. For the electron it can only be at certain energy levels and distances from the nucleus. (Stair-step model) 14. a.) What charge does an electron have? a proton? -1, +1 b.) Which is more stable, an electron close to or far away from the nucleus? Close (electrostatic attraction) c.) What is the relationship between the energy of an electron and its distance away from the nucleus? As distances increases energy also increases (becomes less negative) 15. Use the Bohr model to explain the results you observed above, specifically in Part 2. See Figure 1. As the electron moves between the various energy levels (states) energy is either absorbed or emitted. Only certain values are possible. (see pages 74-75 in Tro for a complete explanation)

ATOMIC EMISSION SPECTRA (modified from the PhET website and Ken Marr at Green River Community College)...