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9.1 Types 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).

9.2 Molecular 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.

9.3 Bonding 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.

9.4 Free 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.

9.5 Band 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.

9.6 Semiconductors 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.

9.7 Semiconductor 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.

9.8 Superconductivity

  • 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.
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