Fluctuating Induced Dipole Bonds
A dipole may be created or induced in an atom or molecule that is normally electrically
symmetric; that is, the overall spatial distribution of the electrons is symmetric
with respect to the positively charged nucleus, as shown in Figure 2.13a. All
atoms are experiencing constant vibrational motion that can cause instantaneous
and short-lived distortions of this electrical symmetry for some of the atoms or molecules,
and the creation of small electric dipoles, as represented in Figure 2.13b. One
of these dipoles can in turn produce a displacement of the electron distribution of
an adjacent molecule or atom, which induces the second one also to become a dipole
that is then weakly attracted or bonded to the first; this is one type of van der
Waals bonding. These attractive forces may exist between large numbers of atoms
or molecules, which forces are temporary and fluctuate with time.
The liquefaction and, in some cases, the solidification of the inert gases and
other electrically neutral and symmetric molecules such as and are realized
because of this type of bonding. Melting and boiling temperatures are extremely
low in materials for which induced dipole bonding predominates; of all possible
intermolecular bonds, these are the weakest. Bonding energies and melting temperatures
for argon and chlorine are also tabulated in Table 2.3.
Polar Molecule-Induced Dipole Bonds
Permanent dipole moments exist in some molecules by virtue of an asymmetrical
arrangement of positively and negatively charged regions; such molecules are
H2 C