University Physics Volume 3

# Summary

### 9.1Types of Molecular Bonds

• Molecules form by two main types of bonds: the ionic bond and the covalent bond. An ionic bond transfers an electron from one atom to another, and a covalent bond shares the electrons.
• The energy change associated with ionic bonding depends on three main processes: the ionization of an electron from one atom, the acceptance of the electron by the second atom, and the Coulomb attraction of the resulting ions.
• Covalent bonds involve space-symmetric wave functions.
• Atoms use a linear combination of wave functions in bonding with other molecules (hybridization).

### 9.2Molecular Spectra

• Molecules possess vibrational and rotational energy.
• Energy differences between adjacent vibrational energy levels are larger than those between rotational energy levels.
• Separation between peaks in an absorption spectrum is inversely related to the moment of inertia.
• Transitions between vibrational and rotational energy levels follow selection rules.

### 9.3Bonding in Crystalline Solids

• Packing structures of common ionic salts include FCC and BCC.
• The density of a crystal is inversely related to the equilibrium constant.
• The dissociation energy of a salt is large when the equilibrium separation distance is small.
• The densities and equilibrium radii for common salts (FCC) are nearly the same.

### 9.4Free Electron Model of Metals

• Metals conduct electricity, and electricity is composed of large numbers of randomly colliding and approximately free electrons.
• The allowed energy states of an electron are quantized. This quantization appears in the form of very large electron energies, even at $T=0KT=0K$.
• The allowed energies of free electrons in a metal depend on electron mass and on the electron number density of the metal.
• The density of states of an electron in a metal increases with energy, because there are more ways for an electron to fill a high-energy state than a low-energy state.
• Pauli’s exclusion principle states that only two electrons (spin up and spin down) can occupy the same energy level. Therefore, in filling these energy levels (lowest to highest at $T=0K),T=0K),$ the last and largest energy level to be occupied is called the Fermi energy.

### 9.5Band Theory of Solids

• The energy levels of an electron in a crystal can be determined by solving Schrödinger’s equation for a periodic potential and by studying changes to the electron energy structure as atoms are pushed together from a distance.
• The energy structure of a crystal is characterized by continuous energy bands and energy gaps.
• The ability of a solid to conduct electricity relies on the energy structure of the solid.

### 9.6Semiconductors and Doping

• The energy structure of a semiconductor can be altered by substituting one type of atom with another (doping).
• Semiconductor n-type doping creates and fills new energy levels just below the conduction band.
• Semiconductor p-type doping creates new energy levels just above the valence band.
• The Hall effect can be used to determine charge, drift velocity, and charge carrier number density of a semiconductor.

### 9.7Semiconductor Devices

• A diode is produced by an n-p junction. A diode allows current to move in just one direction. In forward biased configuration of a diode, the current increases exponentially with the voltage.
• A transistor is produced by an n-p-n junction. A transistor is an electric valve that controls the current in a circuit.
• A transistor is a critical component in audio amplifiers, computers, and many other devices.

### 9.8Superconductivity

• A superconductor is characterized by two features: the conduction of electrons with zero electrical resistance and the repelling of magnetic field lines.
• A minimum temperature is required for superconductivity to occur.
• A strong magnetic field destroys superconductivity.
• Superconductivity can be explain in terms of Cooper pairs.