Similar
calculations can be carried out for H20
and NH3
, but they are complicated by the fact that two or
three individually polar bonds are pointing in different directions.
What is measured for the whole molecule is their vector sum. In calculating
the dipole moment of an entire molecule, one can add the individual
dipole moments of the polar bonds as if they were small vectors, of
length proportional to the dipole moments, and pointing from the negative
ends of the bonds to the positive ends, as shown at the right. When
the geometry of the molecule is taken into account correctly, the
conclusion is that the H-F bond in HF is 41% ionic, the O-H bond in
water is 33% ionic, and the N-H bond in ammonia is 27 % ionic. This
agrees with what one would expect from the decrease in electronegativities
found in going from F to O to N. We can obtain no information from
dipole moments about the polarity of the C-H bond in a methane molecule,
because the four tetrahedrally arranged C-H bonds would add up to
a net molecular dipole moment of zero, even if the individual bonds
had a large dipole moment.