Intra- and Intermolecular Forces
Whether a particular group of bonded molecules takes the
form of a solid, liquid, or gas depends not only on the bonds that
exist within each individual molecule, but also on the presence
and type of bonds between molecules. Hark back to the different
types of bonds we reviewed in the last chapter: ionic, covalent,
and metallic. Of these, ionic bonds tend to be the strongest, and
this means that substances that contain ionic bonds are solids at
room temperature. Substances that are primarily made up of covalent
bonds, which are weaker, can be solid or liquid, and their state
will depend on the presence and type of intermolecular forces.
The two main types of intermolecular forces that exist
between molecules are dipole-dipole forces (including hydrogen bonds)
and London dispersion forces.
Dipole-dipole attractions take place when two or more
neutral, polar molecules are oriented such that their positive (+)
and negative (-) ends are close to each other.
Because of the attraction between unlike charges, this
is a fairly strong type of intermolecular force, and molecules held
together by dipole-dipole forces tend to be in the solid or liquid
state. Also, for molecules that are about the same size and weight,
the strength of the dipole-dipole forces increases as the degree
of polarity increases. In other words, the more polar a molecule
is, the stronger the dipole-dipole forces it will form with itself
and other molecules.
One very important and unique case of the dipole-dipole
attraction is known as hydrogen bonding. Hydrogen bonds
are not true bonds: they’re just strong attractive forces between
the hydrogen on one molecule and a highly electronegative atom on
a nearby molecule.
Hydrogen bonds most commonly form between hydrogen atoms
and fluorine, oxygen, or nitrogen. This type of intermolecular force
is responsible for water’s unique characteristics, such as its high
specific heat and boiling point temperature—but more about that
London Dispersion Forces—Weak Intermolecular Forces
London forces are relatively weak forces of attraction
that exist between nonpolar molecules and noble gas atoms, like
argon (a noble gas) and octane (a hydrocarbon; C8H18). These
types of attractive forces are caused by a phenomenon known as instantaneous dipole
formation. In this process, electron distribution in the
individual molecules suddenly becomes asymmetrical, and the newly
formed dipoles are now attracted to one another.
The ease with which the electron cloud of an atom can
be distorted to become asymmetrical is called the molecule’s polarizability.
Think of this as a probability issue. The greater the number of
electrons an electron has, the farther they will be from the nucleus,
and the greater the chance for them to shift positions within the
molecule. This means that larger nonpolar molecules tend to have
stronger London dispersion forces. This is evident when you look
at the diatomic elements in group 7, the halogens. All of these
diatomic elements are nonpolar, covalently bonded molecules. Now,
going down the group, fluorine and chlorine are gases, bromine is
a liquid, and iodine is a solid! For nonpolar molecules, the farther
you go down the group, the stronger the London dispersion forces.