Chapter 43 - 43 Molecules and Solids CHAPTER OUTLINE 43.1...

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43 CHAPTER OUTLINE 43.1 Molecular Bonds 43.2 Energy States and Spectra of Molecules 43.3 Bonding in Solids 43.4 Free-Electron Theory of Metals 43.5 Band Theory of Solids 43.6 Electrical Conduction in Metals, Insulators, and Semiconductors 43.7 Semiconductor Devices 43.8 Superconductivity Molecules and Solids ANSWERS TO QUESTIONS Q43.1 Rotational, vibrational and electronic (as discussed in Chapter 42) are the three major forms of excitation. Rotational energy for a diatomic molecule is on the order of = 2 2 I , where I is the moment of inertia of the molecule. A typical value for a small molecule is on the order of 11 0 3 meV eV = . Vibrational energy is on the order of hf , where f is the vibration frequency of the molecule. A typical value is on the order of 0.1 eV. Electronic energy depends on the state of an electron in the molecule and is on the order of a few eV. The rotational energy can be zero, but neither the vibrational nor the electronic energy can be zero. Q43.2 The Pauli exclusion principle limits the number of electrons in the valence band of a metal, as no two electrons can occupy the same state. If the valence band is full, additional electrons must be in the conduction band, and the material can be a good conductor. For further discussion, see Q43.3. Q43.3 The conductive properties of a material depend on the electron population of the conduction band of the material. If the conduction band is empty and a full valence band lies below the conduction band by an energy gap of a few eV, then the material will be an insulator. Electrons will be unable to move easily through the material in response to an applied electric field. If the conduction band is partly full, states are accessible to electrons accelerated by an electric field, and the material is a good conductor. If the energy gap between a full valence band and an empty conduction band is comparable to the thermal energy kT B , the material is a semiconductor. Q43.4 Thermal excitation increases the vibrational energy of the molecules. It makes the crystal lattice less orderly. We can expect it to increase the width of both the valence band and the conduction band, to decrease the gap between them. 545
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546 Molecules and Solids Q43.5 First consider electric conduction in a metal. The number of conduction electrons is essentially fixed. They conduct electricity by having drift motion in an applied electric field superposed on their random thermal motion. At higher temperature, the ion cores vibrate more and scatter more efficiently the conduction electrons flying among them. The mean time between collisions is reduced. The electrons have time to develop only a lower drift speed. The electric current is reduced, so we see the resistivity increasing with temperature. Now consider an intrinsic semiconductor. At absolute zero its valence band is full and its conduction band is empty. It is an insulator, with very high resistivity. As the temperature increases, more electrons are promoted to the conduction band, leaving holes in the valence band. Then both electrons and holes move in response to an applied electric field. Thus we see the resistivity
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This homework help was uploaded on 04/13/2008 for the course PHYS 211 taught by Professor Shannon during the Spring '08 term at MSU Bozeman.

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Chapter 43 - 43 Molecules and Solids CHAPTER OUTLINE 43.1...

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