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Dipole moments for particular bonds are given below:
Notice that the largest dipole
moments are correlated with species
having the largest ionic character in
the bonds. The dipole moment is a
very important property of a chemical
bond because it really tells us “what
the electrons are doing “ in a bond.
Dipole moments are vector quantities.
The H2O molecule provides an
especially good example of this idea.
The fact that the H2O molecule has a
net dipole moment proves that the
molecule is nonlinear.
Note that the dipole moment points from positive to negative.
The vector addition of bond dipoles is illustrated in the examples below:
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CO2, SO3, CCl4 have no net dipole moment, although the bonds themselves are polar.
The Electronegativity Triangle
We saw in the previous lecture that we can correlate ionic character with the
electronegativity difference of the elements bound together. But, we can do better than
that if we plot The average value of the electronegativity vs. the difference in
electronegativity.
•
F2O
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When you do that, you realize that the difference in electronegativity is not a unique way
to characterize bonds.
As an example, we see that the covalent species F2O (indicated with an arrow) has a
difference in electronegativity that is ~0.2 units, the same as CuAl2 (also indicated with
an arrow). But, the first of these species is clearly covalent, while the second is clearly
metallic. We see that the metallic species are in the lower left corner of the plot, covalent
species are in the lower right part of the plot, and ionic solids are in the top corner of the
plot. Not a bad way to characterize species. The properties of the three types of bonding
are summarized on the vertices of the diagram. (Diagram courtesy of Prof. George
Bodner, Purdue University)
Properties of Bonds
A practicing chemist often finds it useful to remember specific bond energies. Here are
some trends with single bonds:
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Bonds with hydrogen: with the halogens, the H-F bond is the strongest at 565 kJ/mol.
D(O-H) >D(C-H) > D(N-H)
Bonds to carbon: C-F is the strongest bond. (485)
For other bonds to carbon, D(O-C) > D(C-C) > D(N-C)
Note that bond length and bond strength are inversely related. Long bonds are weaker
bonds.