by Prof. Brent Iverson, University of Texas
For the model on the left, the white atom is hydrogen and the green atom is fluorine. The
surface on the right uses color to indicate where the electrons are located in the H-F
molecule. Here, the red color represents a PARTIAL negative charge (on fluorine atom),
while the blue color represents PARTIAL positive charge (on hydrogen atom). There are
large differences in electronegativity between hydrogen and fluorine, so that the majority
of electron density in the hydrogen-fluorine bond ends up on the much more electronegative
fluorine. Bonds such as this in which the electrons are not shared evenly are referred to
as polar covalent bonds. Polar covalent bonds have a bond dipole moment.
For the molecular model shown on the left, the green atoms are fluorine, the light blue
atom is carbon and the white atom is hydrogen. Difluoromethane (CH2F2)
has two polar covalent C-F bonds as shown. For the surface on the right that indicates
where their electrons are, the red color represents PARTIAL negative charge, while the
blue color represents PARTIAL positive charge. As you can see, the entire molecule has a
molecular dipole moment resulting from the vector sum of the two C-F bond dipole moments.
For the molecular model shown on the left, the light blue atom is carbon and the red atoms
are oxygen. Carbon dioxide has two polar covalent C-O bonds. However, the bond dipole
moments exactly cancel each other since they are pointing in exactly opposite directions.
Thus, CO2 has no molecular dipole moment. Please note that the cancellation of
bond dipole moments does not change the existence or placement of electron density. Each
individual C-O bond has the normal bond dipole moment even though the molecular dipole
moment is zero. By the way, you should be able to identify the carbon atom in CO2
as being sp hybridized.
For the molecular model shown on the left, the white atoms are hydrogen and the red
atom is oxygen. Water has two very polar covalent bonds between oxygen and hydrogen.
Because water is bent (the oxygen atom is sp3 hybridized), the two bond dipole
moments add up to give water a relatively large molecular dipole moment. This molecular
dipole moment combined with the individual bond dipole moments give water its unique
properties that have allowed life to evolve as it has. In particular, later in the
semester, we will see how these dipole moments explain water's remarkably high boiling
point, as well as its ability to dissolve charged species such as the salt in the ocean.
CF4 has four polar covalent bonds. Because they are arranged in a symmetrical tetrahedral array, all of the bond dipole moment vectors exactly cancel, leaving NO MOLECULAR DIPOLE MOMENT for CF4. Hopefully, you can appreciate that deducing whether a given molecule has a molecular dipole moment is a favorite question of mine, because it forces you to synthesize everything we have learned thus far (molecular shape based on hybridization state and/or VSEPR, electronegativities) into the answer to a single question. Best of all, the prediction of molecular dipole moments will allow you to predict the properties of different molecules. You will encounter many examples of this as the semester unfolds.