are additive, however, the decay between larger bodies may be much
weaker. This is discussed in Section 12.2.1.
Hydrogen Bonds. These bonds form between a covalently bound
hydrogen atom and an electronegative group like¼Oor:N. In a water
molecule, for instance, as shown in Figure 3.2a, the two H atoms each
provide a local positive region (since its only electron is exclusively in the
orbital with the oxygen) and the O atom provides two negative ones. Hence
a net attraction between H and O of different molecules results, if their
mutual orientation allows this. The so formed hydrogen bond is to some
extent like a covalent one, especially in the sense that it is direction
dependent: a small deviation from the optimum orientation results in
appreciable weakening of the bond. An H bond is clearly weaker than a
covalent one, and it will often be short-lived. On the other hand, H bonds
are much stronger than van der Waals interactions.
Water. Extensive hydrogen bonding occurs in water, about as
depicted in Figure 3.2b; the resulting configuration is somewhat comparable
to that in ice. The molecules try to make as many H bonds as possible,
without losing too much entropy. This results in a fluctuating network of
bonded molecules: although there are many H bonds at any one time, the
bonds continually break and form again, though often in a different
configuration. This implies that water, though a liquid, has some ordering,
i.e., a structure.
The extensive hydrogen bonding gives water some of its specific
properties. Compared to other compounds consisting of small molecules, it
has high melting and boiling temperatures; the enthalpies of fusion and of
vaporization are high; also the surface tension is high. Moreover, the
temperature dependence of several properties is exceptional, such as the
well-known maximum in density at 4 8 C. Some values of water properties are
in the Appendices 8 and 9.
Several other molecules can make H bonds with one another, but if
they are dissolved in water, H bonds between solute and water are
preferentially formed in most cases; the various H bonds are of about the
same strength, and association of solute molecules would lead to loss of
entropy. This implies that strong hydrogen bonding especially occurs in an
apolar solvent, for instance between the carboxyl groups of fatty acids in oil,
as discussed in Section 2.2.5, point 4.
For sake of completeness, it may be added thatfrictionalforces can act
between molecules or between molecules and particles. These arise owing to
externalforces that cause flow, sedimentation, electrophoresis, etc.