http://www.ck12.org Chapter 9. Covalent Bonding
FIGURE 9.19
Interactions of ions with polar water
molecules.Dispersion Forces
Somewhat more challenging to visualize aredispersion forces. These interactions are defined as attractive forces
that arise as a result of temporary dipoles induced in atoms or molecules. They are often referred to as London
dispersion forces due to the work done by the German-American physicist Fritz London (1900-1954).
Two things need to be kept in mind with regard to these forces: they are relatively weak, and they do not require
any permanent polarity. Induced dipoles are caused by local and temporary changes in the environment immediately
around a molecule. Brief distortions in the electron cloud cause temporary dipoles to come and go, and these provide
a way for even completely nonpolar molecules to attract one another. Although these may seem almost insignificant
compared to the stronger forces discussed above, nonpolar substances would have no way to form solids or liquids
without them. Note that all molecular substances experience dispersion forces, but for small polar molecules, dipole-
dipole interactions will be the dominant attractive force.
The Hydrogen Bond
A “special case” of dipole-dipole interactions is referred to as thehydrogen bond. Hydrogen bonding occurs only
in molecules where hydrogen is covalently bonded to one of three elements: fluorine, oxygen, or nitrogen. These
three elements are so electronegative that they withdraw the majority of the electron density from the covalent bond
with hydrogen, leaving the H atom very electron-deficient. Because the hydrogen atom does not have any electrons
other than the ones in the covalent bond, its positively charged nucleus is almost completely exposed, allowing
strong attractions to other nearby lone pairs. These lone pairs are generally on atoms with partial negative charges in
adjacent molecules, although hydrogen bonds within a single molecule can also occur if the structure of the molecule
is appropriate.
A particularly important example of hydrogen bonding occurs between water molecules. Because water has two
O-H bonds and two lone pairs on each oxygen atom, extensive networks of hydrogen bonds can form, allowing ice
and liquid water to exist.