Chapter 29 Molecules and Solids

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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

Chapter 29

Molecules and Solids

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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