- Dipole–dipole: dipoles on average orient themselves so that the
positive end of one molecule is close to the negative one of
another, etc. - Dipole–Induced Dipole: a dipole always induces a slight uneven-
ness in the charge distribution—i.e., a dipole—in a nonpolar
molecule, thereby causing attraction. - Induced Dipole–Induced Dipole: even an atom is at any moment a
weak dipole, due to the oscillatory motion of its electrons,
although on average its dipole moment is zero. Fluctuating dipoles
arise and those of neighboring atoms or molecules affect each
other so that always a net attraction results. The resulting forces
are calledLondonordispersion forces, and they act always between
all atoms. The other two types need polar groups to be present,
which can only exist in molecules. In most pure compounds, the
dispersion forces are predominant, but not, for instance, in water,
with its strong dipole moment.
The van der Waals forces rapidly decay with distance between
molecules, the interaction energy being proportional tor^6. Since the forces
FIGURE3.2 Structure of water. (a) Model of a water molecule. The distance
between the nuclei of O and H isl& 0 :1 nm, the net chargesqare 0.24 times the
charge of an electron, and the bond anglesyare 109 8. After Israelachvilli (see
Bibliography). (b) Example of how water molecules form H-bonds with one another;
schematic and not to scale. (After O. R. Fennema. Food Chemistry, 3d ed. Marcel
Dekker, New York, 1996 (Chapter 2).