There are three types of van der Waals forces. The first type, called the dipole–dipole force, is an interaction between two molecules each having a permanent electric dipole moment. For example, polar molecules such as HCl have permanent electric dipole moments and attract other polar molecules.
DNA molecules are held together by hydrogen bonds
In general, a molecule vibrates and rotates simultaneously. To a first approximation, these motions are independent of each other, so the total energy of the molecule for these motions is
Therefore, the molecular absorption spectrum consists of two groups of lines:
This minimum energy U0 is called the ionic cohesive energy of the solid, and its absolute value represents the energy required to separate the solid into a collection of isolated positive and negative ions.
The atomic cohesive energy of NaCl is
f(E) is called the Fermi–Dirac distribution function and EF is called the Fermi energy.
Statistical physics can be applied to a collection of particles in an effort to relate microscopic properties to macroscopic properties. In the case of electrons, it is necessary to use quantum statistics, with the requirement that each state of the system can be occupied by only two electrons (one with spin up and the other with spin down) as a consequence of the exclusion principle.
Band theory allows us to build simple models to understand the behavior of conductors, insulators, and semiconductors as well as that of semiconductor devices, as we shall discuss in the following sections.
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