Dipole moment of a molecule characterizes electric properties of the molecule as a system of charged particles. Dipole moment is equal to zero in the absence of external electric field, if the molecule is non-polar. in polar molecules their constant (inherent) dipole moment is equal to the product of distances between the centres r of the positive and negative charges by their value q and is directed (conventionally) from the negative charge to the positive one:

p = qr.

Dipole moment of a molecule or an atomic group expresses asymmetry of electric charge distribution in an electrically neutral system. Dipole moment, which is induced by an electric field with intensity E, is equal to

p = aE,where

a - is polarizability, which characterizes the ability of the electron cloud of a molecule or a group of atoms to distort in an electric field.