Lecture PowerPoint
Chapter 29
Physics: Principles with
Applications, 6 th edition
Giancoli
© 2005 Pearson Prentice Hall
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Units of Chapter 29
• Bonding in Molecules
• Potential-Energy Diagrams for Molecules
• Weak (van der Waals) Bonds
• Molecular Spectra
• Bonding in Solids
• Band Theory of Solids
• Semiconductors and Doping
• Semiconductor Diodes
• Transistors and Integrated Circuits
29.1 Bonding in Molecules
Molecule: two or more atoms strongly held together to function as a unit
This attachment is called a chemical bond
Two types of bond:
1. Covalent Electron orbit clouds overlap
2. Ionic Electrostatic attraction of ions
29.1 Bonding in Molecules
Hydrogen molecule, H
2
, is bound covalently.
If the atoms have their spins in the same direction, so S = 1 for the molecule, the atoms will not bond due to the exclusion principle.
29.1 Bonding in Molecules
The molecule will only form if S = 0. The two electrons are shared by both atoms:
The energy needed to separate the atoms is called the binding energy.
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29.1 Bonding in Molecules
An ionic bond is created by the attraction of ions.
For example, the outermost electron in the sodium atom spends most of its time around the chlorine atom in NaCl.
29.1 Bonding in Molecules
The reason this happens is that sodium has a single electron outside a closed shell, and it is not tightly bound. Conversely, the chlorine atom has an empty space; there is only one electron where two can be accommodated.
Sodium Chlorine
29.1 Bonding in Molecules
Pure covalent bonds are found in molecules consisting of only one type of atom. Otherwise, electrons are likely to spend more time around one type of atom than another, giving a partial ionic character. Water is one such molecule.
Water molecules are polar!
29.2 Potential-Energy Diagrams for
Molecules
Potential energy of two point charges:
29.2 Potential-Energy Diagrams for
Molecules
For the hydrogen molecule, H
2
, the force between the atoms is attractive at large distances. If the atoms are too close, the electrons are squeezed out; therefore there is a minimum in the potential.
Binding energy is typically a few eV.
29.2 Potential-Energy Diagrams for
Molecules
An activation energy may be required – often atoms must be separated from other molecules before they can combine to make new ones.
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29.2 Potential-Energy Diagrams for
Molecules
Sometimes the bond occurs in a configuration that is a local minimum of potential, but that takes energy to reach. This is important in living cells.
Energy storage: energy is released upon breaking the bond.
29.3 Weak (van der Waals) Bonds
Weak bonds are electrostatic bonds between molecules (and not between atoms within a molecule). The binding energy is much less than that of strong bonds, about 0.04 to 0.3 eV.
Weak bonds are usually the result of attraction between dipoles.
29.3 Weak (van der Waals) Bonds
Weak bonds become important in liquids and solids where strong bonds are absent.
DNA is double helix held together by weak bond.
29.3 Weak (van der Waals) Bonds
Close-up view: Cytosine and Guanine on separate strands.
29.3 Weak (van der Waals) Bonds
Weak bonds are easily broken.
Kinetic energy of particles at room temperature:
E
K
3
2 kT
0 .
04 eV
Same magnitude as weak binding energy.
DNA replication works by random molecular collisions.
Breaking-up of molecule requires enzymes.
29.4 Molecular Spectra
The overlap of orbits alters energy levels in molecules. Also, more types of energy levels are possible, due to rotations and vibrations.
The result is a band of closely spaced energy levels.
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29.4 Molecular Spectra
A diatomic molecule can rotate around a vertical axis. The rotational energy is quantized because angular momentum is quantized.
29.4 Molecular Spectra
These are some rotational energy levels and allowed transitions for a diatomic molecule.
Important: moment of inertia, I.
29.4 Example: rotational transition of CO
Angular momentum:
Rotational energy:
P
ω
=
I
ω =
L
(
L
+
1
) h
2
π
E
ω
=
1
2
I
ω
2 =
L
(
L
+
1
)
2 I h
2
4
π 2
Transition L=1 to L=0:
∆
E
ω
= h
2
4
π 2
I
Measure
λλλλ
=2.6 mm: E
ω
= r hc
λ
≅
I
=
1 .
5
0 .
1 nm
⋅
10
−
46 kg m 2
29.4 Molecular Spectra
Small-amplitude vibrations of a diatomic molecule will be simple harmonic. Again, the energy is quantized.
29.4 Molecular Spectra
Here are some vibrational energy levels in a diatomic molecule, and allowed transitions.
f depends on molecule.
Lowest level is not zero!
29.5 Bonding in Solids
Some solids are amorphous, but many are crystalline, having their molecules arranged in a regular lattice.
Here are three cubic crystal lattices:
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29.5 Bonding in Solids
Here is what it salt (NaCl) looks like, with the atoms in their actual “packed” configuration.
29.5 Bonding in Solids
Metallic bonds, where electrons are shared by all atoms in the metal, are neither ionic or covalent. The binding energy of metallic bonds is slightly weaker than that of ionic or covalent bonds – about 1 to 3 eV – but they are still strong bonds.
29.6 Band Theory of Solids
The more atoms are bound together with overlapping wave functions, the more continuous the energy bands will become. Here is what happens with two, six, and many atoms:
29.6 Band Theory of Solids
A good conductor has its highest energy band only partially filled, as in the figure.
An insulator has its highest energy band completely filled, with a substantial gap separating it from the next level.
29.6 Band Theory of Solids
A semiconductor also has its highest band filled, but the gap to the next level is small.
29.7 Semiconductors and Doping
The most common semiconductors in use are silicon and germanium.
A tiny amount of impurity gives the semiconductor useful properties – this is called doping.
The doped semiconductor becomes slightly conducting; the conductivity can be controlled with great precision.
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29.7 Semiconductors and Doping
Arsenic-doped silicon is an n-type semiconductor, as the current is carried by negative charges.
29.7 Semiconductors and Doping
Gallium-doped silicon is a p-type semiconductor
– the current is carried by “holes,” or spots that are missing electrons.
29.8 Semiconductor Diodes
When an n-type and a p-type semiconductor are joined, the result is a pn junction diode.
This diode will conduct electricity in one direction but not the other.
29.8 Semiconductor Diodes
A graph of the current vs. voltage shows this effect clearly. If the potential difference is large enough, current will flow in the reverse direction as well.
29.8 Semiconductor Diodes
A diode can serve as a rectifier – a device that changes ac into dc.
The simplest circuit is a half-wave rectifier:
29.9 Transistors and Integrated Circuits
A junction transistor is one type of semiconductor sandwiched between layers of another – npn or pnp. These layers are called the collector, base, and emitter.
The voltage at the base determines the resistance between emitter and collector.
Small signals can be amplified!
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Summary of Chapter 29
• Molecules form either covalent or ionic bonds
• Electron wave functions overlap
• Weak (van der Waals) bonds are dipole attractions between molecules
• Energy levels in molecules are altered
• Additional energy levels are possible, corresponding to rotational and vibrational states
• Energy levels become closely-spaced bands
• Rotational energy levels are quantized
Summary of Chapter 29
• Vibrational energy levels are quantized too
• Solids can be bound by ionic, covalent, or metallic bonds
• Electron energy levels in crystals are bands, with gaps in between
• In conductors, the highest band is partially full
• In insulators, the highest band is completely full, and there is a large gap to the next band
Summary of Chapter 29
• In semiconductors, the highest band is completely full but the energy gap is much smaller
• In doped semiconductors, small amounts of impurities allow current to be very precisely controlled
• Doped semiconductors can be either p-type or
n-type
• A diode is a pn junction
• A transistor is a pnp or npn junction
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