low vapor pressures, high heats of vaporization, and high melting points. In order
for vaporization or melting to take place, molecules must be separated. Energy
must be expended to break hydrogen bonds and thus break down the larger
clusters of molecules into separate molecules. As with the boiling point, the
melting point of H 2 O is abnormally high when compared with the melting points
of the hydrogen compounds of the other elements having six valence electrons,
which are chemically similar but which have no apparent hydrogen bonding. The
hydrogen bonding effect in water is discussed in Chapter 7.
DOUBLE AND TRIPLE BONDS
To achieve the octet structure, which is an outer energy level resembling the
noble gas configuration of eight electrons, it is necessary for some atoms to share
two or even three pairs of electrons. Sharing two pairs of electrons produces a
double bond. An example:
In the line formula, only the shared pair of electrons is indicated by a bond
(—). The sharing of three electron pairs results in a triple bond. An example:
It can be assumed from these structures that there is a greater electron density
between the nuclei involved and hence a greater attractive force between the
nuclei and the shared electrons. Experimental data verify that greater energy is
required to break double bonds than single bonds, and triple bonds than double
bonds. Also, since these stronger bonds tend to pull atoms closer together, the
atoms joined by double and triple bonds have smaller interatomic distances and
greater bond strengths, respectively.
TIP
Resonance structure is a hybrid of the possible drawings because no one Lewis structure can
represent the situation.RESONANCE STRUCTURES
It is not always possible to represent the bonding structure of a molecule by either
the Lewis dot structure or the line drawing because data about the bonding