ch30 - CHAPTER 30 THE NATURE OF THE ATOM CONCEPTUAL...

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CHAPTER 30 THE NATURE OF THE ATOM CONCEPTUAL QUESTIONS ____________________________________________________________________________________________ 1. REASONING AND SOLUTION A tube is filled with atomic hydrogen at room temperature. Electromagnetic radiation with a continuous spectrum of wavelengths, including those in the Lyman, Balmer, and Paschen series, enters one end of the tube and leaves the other end. The exiting radiation is found to contain absorption lines. At room temperature, most of the atoms of atomic hydrogen contain electrons that are in the ground state ( n = 1) energy level. Since the radiation contains a continuous spectrum of wavelengths, it contains photons with a wide range of energies ( E = hf = hc / λ ). In particular, it will contain photons with energies that are equal to the energy difference between the atomic states in the Lyman series. When the radiation is incident on the atoms in the tube, these photons are absorbed by the electrons. When a photon, whose energy is equal to the energy difference for a transition in the Lyman series, is absorbed by a ground state electron, that electron will make a transition to a higher energy level. Every photon of this energy will be absorbed by a ground state electron and cause a transition. The wavelength of radiation that corresponds to that particular photon energy will, therefore, be removed from the radiation. When the radiation is analyzed, the wavelengths that correspond to transitions in the Lyman series will be absent. Since most of the atoms in the tube are in the ground state ( n = 1), the electron populations in the n = 2 and n = 3 states are extremely small. Therefore, any absorption lines resulting from Balmer or Paschen transitions will be extremely weak. When the radiation is analyzed, the only predominant absorption lines in the exiting radiation will correspond to wavelengths in the Lyman series. ____________________________________________________________________________________________ 2. REASONING AND SOLUTION Refer to the situation described in Conceptual Question 1. Suppose the electrons in the atoms are mostly in excited states. Most of the electrons are in states with n > 2; therefore, Balmer and Paschen series transitions will occur, and the absorption lines in the exiting radiation will correspond to wavelengths in the Balmer and Paschen series. Since there are relatively few electrons in the ground state, only a relatively few number of photons that correspond to wavelengths in the Lyman series will be absorbed. Most of the "Lyman photons" will remain in the radiation; therefore, the exiting radiation will not contain absorption lines that correspond to wavelengths in the Lyman series. Although the absorption lines that correspond to transitions in the Lyman series are not present, there will be more absorption lines in the exiting radiation compared to the situation when the electrons are in the ground state, because absorption lines corresponding to both the Balmer and Paschen series will be present.
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This note was uploaded on 05/09/2008 for the course PHYS 25 taught by Professor Holland during the Spring '08 term at Pacific.

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ch30 - CHAPTER 30 THE NATURE OF THE ATOM CONCEPTUAL...

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