SM_chapter43

SM_chapter43 - 43 Molecules and Solids CHAPTER OUTLINE 43.1...

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43 Molecules and Solids 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 ANSWERS TO QUESTIONS Q43.1 Ionic bonds are ones between oppositely charged ions. A simple model of an ionic bond is the electrostatic attraction of a negatively charged latex balloon to a positively charged Mylar balloon. Covalent bonds are ones in which atoms share electrons. Classically, two children playing a short-range game of catch with a ball models a covalent bond. On a quantum scale, the two atoms are sharing a wave function, so perhaps a better model would be two children using a single hula hoop. Van der Waals bonds are weak electrostatic forces: the dipole-dipole force is analogous to the attraction between the opposite poles of two bar magnets, the dipole-induced dipole force is similar to a bar magnet attracting an iron nail or paper clip, and the dispersion force is analogous to an alternating-current electro- magnet attracting a paper clip. A hydrogen atom in a molecule is not ionized, but its electron can spend more time elsewhere than it does in the hydrogen atom. The hydrogen atom can be a location of net positive charge, and can weakly attract a zone of negative charge in another molecule. Q43.2 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 h 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. *Q43.3 If you start with a solid sample and raise its temperature, it will typically melt F rst, then start emitting lots of far infrared light, then emit light with a spectrum peaking in the near infrared, and later have its molecules dissociate into atoms. Rotation of a diatomic molecule involves less energy than vibration. Absorption and emission of microwave photons, of frequency ~H z , 10 11 accompany excitation and de-excitation of rotational motion, while infrared photons, of frequency z , 10 13 accompany changes in the vibration state of typical simple molecules. The ranking is then b > d > c > a. 519
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520 Chapter 43 Q43.4 From the rotational spectrum of a molecule, one can easily calculate the moment of inertia of the molecule using Equation 43.7 in the text. Note that with this method, only the spacing between adjacent energy levels needs to be measured. From the moment of inertia, the size of the molecule can be calculated, provided that the structure of the molecule is known.
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SM_chapter43 - 43 Molecules and Solids CHAPTER OUTLINE 43.1...

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