Molecules and Condensed Matter -...

Copyright © 2012 Pearson Education Inc.

PowerPoint® Lectures for

University Physics, Thirteenth Edition

– Hugh D. Young and Roger A. Freedman

Lectures by Wayne Anderson

Chapter 42

Molecules and

Condensed Matter


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


Introduction

• Why do CO2 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.


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.


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.


The difference between an ionic bond and a covalent bond is

Q42.1

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.


The difference between an ionic bond and a covalent bond is

A42.1

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.


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.



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.


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.



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

Q42.3

A. greater for transition A.

B. greater for transition B.

C. the same for both transitions.

D. any of the above, depending on circumstances.


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

A42.3

A. greater for transition A.

B. greater for transition B.

C. the same for both transitions.

D. any of the above, depending on circumstances.


Crystal lattices

• A crystal lattice is a repeating pattern of

mathematical points. Figure 42.11 (below)

shows some common types of lattices.


Crystal lattices and structures

• Follow the text discussion of

crystal lattices and structures

using Figures 42.12 (top) and

42.13 (bottom). Figure 42.14

(below) shows diamond.



Types of crystals

• Follow the text discussion of the types of crystals.

• Figure 42.15 (left) shows a metallic solid, and Figure 42.16 (right) shows an edge dislocation in two dimensions.


Energy bands

• Follow the text analysis, using Figures 42.18 (right) and 42.19 (below).



At absolute zero (T = 0 K), what is the difference between

a semiconductor and an insulator?

Q42.4

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.


At absolute zero (T = 0 K), what is the difference between

a semiconductor and an insulator?

A42.4

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.


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 42.20 (left) and 42.21 (right).


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 42.22 (top) and 42.23 (bottom).



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 free-electron energy.

• Follow Example 42.8 which compares a free-electron gas with an ideal gas.


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.)

Q42.5

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 .

D. The average kinetic energy changes by only a small factor.

2


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.)

A42.5

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 .

D. The average kinetic energy changes by only a small factor.

2


Semiconductors

• A semiconductor has an electrical resistivity that is intermediate between those of good conductors and good insulators.

• Follow Example 42.9 using Figure 42.24 below.


How would you expect the electric conductivity of a

semiconductor to vary with increasing temperature?

Q42.6

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.


How would you expect the electric conductivity of a

semiconductor to vary with increasing temperature?

A42.6

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.


Holes

• A hole is a vacancy in a semiconductor.

• A hole in the valence band behaves like a positively charged particle.

• Figure 42.25 at the right shows the motions of electrons in the conduction band and holes in the valence band with an applied electric field.


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.


n-type and p-type semiconductors

• Figure 42.26 (left) shows an n-type semiconductor, and Figure 42.27 (right) shows a p-type semiconductor.


Photocell

• A photocell is a simple semiconductor device.

• See Figure 42.28 below.


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 42.29 below shows the behavior of a semiconductor p-n junction in a circuit.


Currents through a p-n junction

• Follow the text analysis of currents through a p-n junction.

• Figure 42.30 below shows a p-n junction in equilibrium.


Forward and reverse bias at a p-n junction

• Figure 42.31 (left) shows a p-n junction under forward-bias conditions.

• Figure 42.32 (right) shows a p-n junction under reverse-bias conditions.


Transistors

• Follow the analysis of transistors in the text.

• Figure 42.33 (left) shows a p-n-p transistor in a circuit.

• Figure 42.34 (right) shows a common-emitter circuit.


Integrated circuits

• Follow the text discussion of integrated circuits.

• Figure 42.35 (left) shows a field-effect transistor.

• Figure 42.36 (right) shows an actual integrated circuit chip.


Superconductivity

• Follow the text summary of BCS theory and superconductivity.

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