dipole moment

Examples of dipole moment in the following topics:

• Dipole Moments

  • The electric dipole moment is a measure of polarity in a system.
  • There are many different types of dipole moments, including electric dipole moments, magnetic dipole moments, and topological dipole moments.
  • Among the subset of electric dipole moments are transition dipole moments, molecular dipole moments , bond dipole moments, and electron electric dipole moments.
  • For the purposes of this atom we will focus on a broad overview of electric dipole moment in static situations.
  • Relate the electric dipole moment to the polarity in a system

• Dipole Moment

  • The bond dipole moment uses the idea of the electric dipole moment to measure a chemical bond's polarity within a molecule.
  • Bond dipole moments are commonly measured in debyes, represented by the symbol D.
  • Molecules with only two atoms contain only one (single or multiple) bond, so the bond dipole moment is the molecular dipole moment.
  • At one extreme, a symmetrical molecule such as chlorine, Cl2, has 0 dipole moment.
  • Symmetry is another factor in determining if a molecule has a dipole moment.

• Bond Polarity

  • The Debye unit, D, is commonly used to express dipole moments.
  • In molecules containing more than one polar bond, the molecular dipole moment is just the vector addition of the individual bond dipole moments.
  • The linear shape of the CO2 molecule results in the canceling of the dipole moments of the two polar C=O bonds.
  • The net, molecular dipole moment of CO2 is therefore zero, and the molecule is nonpolar.
  • Apply knowledge of bond polarity and molecular geometry to identify the dipole moment of molecules

• A Physical Aside: Multipole Radiation

  • It is possible to calculate the radiation field to higher order in $L/(c\tau)$.This is necessary if the dipole moment vanishes, for example.
  • where $k\equiv\omega/c$$n=0$ gives the dipole radiation, $n=1$ gives the quadrupole radiation and so on.

• Ferromagnetism

  • Ferromagnetism arises from the fundamental property of an electron; it also carries charge to have a dipole moment.
  • This dipole moment comes from the more fundamental property of the electron—its quantum mechanical spin.
  • However, in materials with a filled electron shell, the total dipole moment of the electrons is zero, as the spins are in up/down pairs.
  • Only atoms with partially filled shells (i.e., unpaired spins) can have a net magnetic moment.
  • (According to Hund's rules, the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment. )

• Dipole-Dipole Force

  • Dipole-dipole interactions are intermolecular attractions that result from two permanent dipoles interacting.
  • Dipole-dipole forces: electrostatic interactions of permanent dipoles in molecules; includes hydrogen bonding.
  • For example, a water molecule (H2O) has a large permanent electric dipole moment.
  • Molecules often contain polar bonds because of electronegativity differences but have no overall dipole moment if they are symmetrical.
  • Dipoles may form associations with other dipoles, induced dipoles or ions.

• Polarization

  • This separation creates a dipole moment, as shown in .
  • On the molecular level, polarization can occur with both dipoles and ions.
  • One example of a dipole molecule is water, (H2O), which has a bent shape (the H-O-H angle is 104.45°) and in which the oxygen pulls electron density away from the H atoms, leaving the H relatively positive and the O relatively negative, as shown in .
  • Water is an example of a dipole molecule, which has a bent shape (the H-O-H angle is 104.45°) and in which the oxygen pulls electron density away from the H atoms, leaving the H relatively positive and the O relatively negative.
  • The atom's dipole moment is represented by M.

• Total Polarization

  • The physical mechanism for this can be qualitatively understood from the manner in which electric dipoles in the media respond to p-polarized light (whose electric field is polarized in the same plane as the incident ray and the surface normal).
  • One can imagine that light incident on the surface is absorbed, and then re-radiated by oscillating electric dipoles at the interface between the two media.
  • The refracted light is emitted perpendicular to the direction of the dipole moment; no energy can be radiated in the direction of the dipole moment.
  • Thus, if the angle of reflection θ1 (angle of reflection) is equal to the alignment of the dipoles (90 - θ2), where θ2 is angle of refraction, no light is reflected.

• The Shape of Molecules

  • One way in which the shapes of molecules manifest themselves experimentally is through molecular dipole moments.
  • A molecule which has one or more polar covalent bonds may have a dipole moment as a result of the accumulated bond dipoles.
  • The bond dipoles are colored magenta and the resulting molecular dipole is colored blue.
  • In the linear configuration (bond angle 180º) the bond dipoles cancel, and the molecular dipole is zero.
  • In a similar manner the configurations of methane (CH4) and carbon dioxide (CO2) may be deduced from their zero molecular dipole moments.

• Ion-Dipole Force

  • Ion-dipole and ion-induced dipole forces operate much like dipole-dipole and induced dipole-dipole interactions.
  • However, ion-dipole forces involve ions instead of solely polar molecules.
  • Ion-dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole; the strength of the ion-dipole force is proportionate to ion charge.
  • Ion-dipole bonding is also stronger than hydrogen bonding.
  • Like a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud in the non-polar molecule, causing a temporary partial charge.