STRUCTURE AND BONDING 51on wave-mechanics. In this theory, it is supposed that the true
structure of the molecule is a resonance hybrid of two or more
structures which can be written in a conventional way (i.e. as H—Cl
or H "*" Cl~). We can say that just as a hybrid plant is better than the
individual true-breeding plants from which it was produced, so a
resonance hybrid is a 'better' molecule than any of the structures
that we can write for it. It must be realised that for example, hydro-
gen chloride does not consist of a mixture of the forms I and II
nor does the molecule of hydrogen chloride exist for part of the time
in form I and for part in form II. Forms I and II are purely imaginary
structures which contribute to structure III.
The resonance concept is of great value in organic chemistry. For
example, the carbon-carbon bond lengths found in benzene are all
0.139 nm in length. This compares with a carbon-carbon single
bond length of 0.154 nm and a carbon-carbon double bond length
of 0.134 nm. The heat of formation of benzene is found to be greater
than that calculated and the chemical properties indicate the
absence of a normal carbon-carbon double bond. Resonance theory
explains these facts by suggesting a number of structures, each con-
tributing to the true structure in which all six carbon atoms are
equivalent, and all the carbon-carbon bonds are of equal length.
Dipole moments
The unequal distribution of charge produced when elements of
different electronegativities combine causes a polarity of the covalent
bond joining them and, unless this polarity is balanced by an equal
and opposite polarity, the molecule will be a dipole and have a
dipole moment (for example, a hydrogen halide). Carbon tetra-
chloride is one of a relatively few examples in which a strong
polarity does not result in a molecular dipole. It has a tetrahedral
configuration
ClcK I ^ci
Cl
and the effect of each chlorine is exactly balanced by the others
so that there is no residual dipole. However, chloromethane
(methyl chloride, CH 3 C1) has a pronounced dipole moment although
the shape of the molecule is also tetrahedral. Because of the dipole,
chloromethane molecules are attracted to each other by dipole-
dipole forces—the negative end of one molecule attracting the