Molecules and Condensed Matter

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1 Chapter 42 Molecules and Condensed Matter PowerPoint Lectures for University Physics, Thirteenth Edition Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson

2 Goals for Chapter 42 To understand the bonds holding atoms together To see how rotation and vibration of molecules affect spectra To learn how and why atoms form crystalline structures To apply the energy-band concept to solids To develop a model for the physical properties of metals To learn how impurities affect semiconductors and to see applications for semiconductors To investigate superconductivity

3 Introduction Why do CO 2 molecules raise the temperature of Venus so high? Why are they raising the earth s temperature? After looking at individual atoms in earlier chapters, we shall now look at molecules and condensed matter, formed when atoms combine. We ll see how energy bands help us understand solids and we ll look at semiconductors, which are used in calculators, computers, and much more.

4 Ionic bonds An ionic bond is an interaction between oppositely charged ionized atoms. Figure 37.1 (right) shows a graph of the potential energy of two oppositely charged ions. Follow Example 42.1.

5 Covalent bonds In a covalent bond, the wave functions are distorted and become more concentrated in certain places. Figure 42.2 (right) shows the hydrogen covalent bond, and Figure 42.3 (below) shows the methane molecule.

6 Q42.1 The difference between an ionic bond and a covalent bond is A. ionic bonds are only found in crystals such as sodium chloride (NaCl) where there are many atoms in close proximity. B. covalent bonds are only found in molecules with three or more atoms. C. ionic bonds are highly directional, while covalent bonds are not. D. ionic bonds involve the transfer of an electron from one atom to another, while covalent bonds involve electrons that spend much of their time between atoms.

7 A42.1 The difference between an ionic bond and a covalent bond is A. ionic bonds are only found in crystals such as sodium chloride (NaCl) where there are many atoms in close proximity. B. covalent bonds are only found in molecules with three or more atoms. C. ionic bonds are highly directional, while covalent bonds are not. D. ionic bonds involve the transfer of an electron from one atom to another, while covalent bonds involve electrons that spend much of their time between atoms.

8 Rotational energy levels Follow the text discussion of rotational energy levels for diatomic molecules. Figure 42.4 (left) shows a model of a diatomic molecule. Figure 42.5 (right) shows some rotational energy levels for a diatomic molecule. Follow Example 42.2.

9 Vibrational energy levels Follow the text discussion of vibrational energy levels of a diatomic molecule. Figure 42.6 (left) shows a model of a diatomic molecule. Figure 42.7 (right) shows some vibrational energy levels of a diatomic molecule.

10 Rotation and vibration combined Follow the text discussion. Figure 42.8 (right) shows an energy-level diagram for rotational and vibrational energy levels of a diatomic molecule. Figure 42.9 (below) shows a typical molecular band spectrum. Follow Example 42.3.

11 Q42.3 This diagram shows the vibrational and rotational energy levels of a diatomic molecule. Consider two possible transitions for this molecule: A. n = 2, l = 5 to n = 1, l = 4 B. n = 2, l = 1 to n = 1, l = 0 The energy change is A. greater for transition A. B. greater for transition B. C. the same for both transitions. D. any of the above, depending on circumstances.

12 A42.3 This diagram shows the vibrational and rotational energy levels of a diatomic molecule. Consider two possible transitions for this molecule: A. n = 2, l = 5 to n = 1, l = 4 B. n = 2, l = 1 to n = 1, l = 0 The energy change is A. greater for transition A. B. greater for transition B. C. the same for both transitions. D. any of the above, depending on circumstances.

13 Crystal lattices A crystal lattice is a repeating pattern of mathematical points. Figure (below) shows some common types of lattices.

14 Crystal lattices and structures Follow the text discussion of crystal lattices and structures using Figures (top) and (bottom). Figure (below) shows diamond. Follow Example 42.4.

15 Types of crystals Follow the text discussion of the types of crystals. Figure (left) shows a metallic solid, and Figure (right) shows an edge dislocation in two dimensions.

16 Energy bands Follow the text analysis, using Figures (right) and (below). Follow Example 42.5.

17 Q42.4 At absolute zero (T = 0 K), what is the difference between a semiconductor and an insulator? A. The conduction band is empty in a semiconductor but partially filled in an insulator. B. The conduction band is partially filled in a semiconductor but empty in an insulator. C. The energy gap between the valence and conduction bands is large in a semiconductor but small in an insulator. D. The energy gap between the valence and conduction bands is small in a semiconductor but large in an insulator.

18 A42.4 At absolute zero (T = 0 K), what is the difference between a semiconductor and an insulator? A. The conduction band is empty in a semiconductor but partially filled in an insulator. B. The conduction band is partially filled in a semiconductor but empty in an insulator. C. The energy gap between the valence and conduction bands is large in a semiconductor but small in an insulator. D. The energy gap between the valence and conduction bands is small in a semiconductor but large in an insulator.

19 Free-electron model of metals The free-electron model assumes that electrons are completely free inside the metal, but that there are infinite potential-energy barriers at the surface. The density of states, dn/de, is the number of states per unit energy range. Follow the text analysis using Figures (left) and (right).

20 Fermi-Dirac distribution The Fermi-Dirac distribution f(e) is the probably that a state with energy E is occupied by an electron. Follow the analysis of the Fermi-Dirac distribution in the text, using Figures (top) and (bottom). Follow Example 42.6.

21 Electron concentration and free-electron energy Follow the text analysis of electron concentration and Fermi energy. Follow Example 42.7 on the Fermi energy in copper. Follow the text analysis of the average freeelectron energy. Follow Example 42.8 which compares a freeelectron gas with an ideal gas.

22 Q42.5 If you increase the temperature of a block of copper from 300 K to 600 K, what happens to the average kinetic energy of the electrons in the conduction band? (Copper remains a solid at these temperatures.) A. The average kinetic energy increases by a factor of 4. B. The average kinetic energy increases by a factor of 2. C. The average kinetic energy increases by a factor of 2. D. The average kinetic energy changes by only a small factor.

23 A42.5 If you increase the temperature of a block of copper from 300 K to 600 K, what happens to the average kinetic energy of the electrons in the conduction band? (Copper remains a solid at these temperatures.) A. The average kinetic energy increases by a factor of 4. B. The average kinetic energy increases by a factor of 2. C. The average kinetic energy increases by a factor of 2. D. The average kinetic energy changes by only a small factor.

24 Semiconductors A semiconductor has an electrical resistivity that is intermediate between those of good conductors and good insulators. Follow Example 42.9 using Figure below.

25 Q42.6 How would you expect the electric conductivity of a semiconductor to vary with increasing temperature? A. It should increase, because more electrons are thermally excited from the valence band into the conduction band. B. It should increase, because more electrons are removed from their parent atoms and added to the valence band. C. It should decrease, because the added thermal energy breaks apart correlated electron pairs. D. It should decrease, because the atoms in the crystal will vibrate more and thus block the flow of electrons.

26 A42.6 How would you expect the electric conductivity of a semiconductor to vary with increasing temperature? A. It should increase, because more electrons are thermally excited from the valence band into the conduction band. B. It should increase, because more electrons are removed from their parent atoms and added to the valence band. C. It should decrease, because the added thermal energy breaks apart correlated electron pairs. D. It should decrease, because the atoms in the crystal will vibrate more and thus block the flow of electrons.

27 Holes A hole is a vacancy in a semiconductor. A hole in the valence band behaves like a positively charged particle. Figure at the right shows the motions of electrons in the conduction band and holes in the valence band with an applied electric field.

28 Impurities Doping is the deliberate addition of impurity elements. In an n-type semiconductor, the conductivity is due mostly to negative charge (electron) motion. In a p-type semiconductor, the conductivity is due mostly to positive charge (hole) motion. Follow the text analysis of impurities.

29 n-type and p-type semiconductors Figure (left) shows an n-type semiconductor, and Figure (right) shows a p-type semiconductor.

30 Photocell A photocell is a simple semiconductor device. See Figure below.

31 The p-n junction A p-n junction is the boundary in a semiconductor between a region containing p-type impurities and another region containing n-type impurities. Figure below shows the behavior of a semiconductor p-n junction in a circuit.

32 Currents through a p-n junction Follow the text analysis of currents through a p-n junction. Figure below shows a p-n junction in equilibrium.

33 Forward and reverse bias at a p-n junction Figure (left) shows a p-n junction under forwardbias conditions. Figure (right) shows a p-n junction under reversebias conditions.

34 Transistors Follow the analysis of transistors in the text. Figure (left) shows a p-n-p transistor in a circuit. Figure (right) shows a common-emitter circuit.

35 Integrated circuits Follow the text discussion of integrated circuits. Figure (left) shows a field-effect transistor. Figure (right) shows an actual integrated circuit chip.

36 Superconductivity Follow the text summary of BCS theory and superconductivity.

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