Topic outline

  • Dipole-Dipole Attraction

    Many molecules contain bonds that fall between the extremes of ionic and covalent bonds. The difference between the electronegativities of the atoms in these molecules is large enough that the electrons aren’t shared equally, and yet small enough that the electrons aren’t drawn exclusively to one of the atoms to form positive and negative ions. The bonds in these molecules are said to be polar, because they have positive and negative ends, or poles, and the molecules are often said to have a dipole moment.

    HCl molecules, for example, have a dipole moment because the hydrogen atom has a slight positive charge and the chlorine atom has a slight negative charge. Because of the force of attraction between oppositely charged particles, there is a small dipole-dipole force of attraction between adjacent HCl molecules.

    Hydrogen Bonding

    The hydrogen bond is a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine. In other words – the hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.  In other words, hydrogen bonds only occur when hydrogen is directly bonded to FON (fluorine, oxygen and nitrogen)

    In the graphic below, the hydrogen is partially positive and attracted to the partially negative charge on the oxygen or nitrogen. Because oxygen has two lone pairs, two different hydrogen bonds can be made to each oxygen. This is a very specific bond as indicated. Some combinations that are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon.

    Hydrogen Bonding in Water


    Dipole-Induced Dipole

    What would happen if we mixed HCl with argon, which has no dipole moment? The electrons on an argon atom are distributed homogeneously around the nucleus of the atom. But these electrons are in constant motion. When an argon atom comes close to a polar HCl molecule, the electrons can shift to one side of the nucleus to produce a very small dipole moment that lasts for only an instant.

    By distorting the distribution of electrons around the argon atom, the polar HCl molecule induces a small dipole moment on this atom, which creates a weak dipole-induced dipole force of attraction between the HCl molecule and the Ar atom.

    Induced-dipole induced-dipole (London Dispersion Forces)

    As the electrons move about the nucleus, a momentary non-symmetrical electron distribution can develop that produces a temporary dipolar arrangement of charge. The formation of this temporary dipole can, in turn, affect the electron distribution of a neighboring atom. That is, this instantaneous dipole that occurs accidentally in a given atom can then induce a similar dipole in a neighbouring atom. This phenomenon leads to an interatomic attraction that is relatively weak and short-lived but that can be very significant especially for large atoms.

    Now go through this animation to see how  London Dispersion, Dipole and Hydrogen Bonding looks in an animation

    Note, that London Dispersion forces = LD 

    Bonding Animation

    Watch the following video by Oldsite Vanden Bout:


    Or this more thorough explanation by Bozeman Science: