We just discussed examples where the dipole moments were enforced by one another according to their vector sum. So what about molecules when the dipole moments are in the opposite direction?
There are two scenarios here. If the dipole moments have the same magnitude at o , then the molecule is nonpolar sine the diple moments are canceled.
So, to generalize, remember that symmetrical molecules have no dipole moment regardless of how polar the covalent bonds are because the overall dipole moment of the molecule depends on the magnitude and direction of individual dipoles. If the dipole moments are of different magnitudes but in opposite directions, then the net dipole moment will be the vector sum subtraction of these dipole moments. See the practice question below.
There may be a lot of cases where the molecule is not drawn or the geometry is not given , and you are asked to determine whether the given molecule is polar or not. To answer this question, you need to first determine the geometry of the molecule following the rules of VSEPR theory.
These are summarized in the following table, but are also covered in detail here , so feel free to check them and the Lewis structures before we go over the next example:. Determine if the molecule is polar or not by showing the corresponding dipole moment s.
For organic molecules containing a hydrocarbon chain with few polar bonds, the molecular dipole is determined by the sum of these dipole moments. For example, diethyl ether possesses a net dipole moment which is the vector sum of two C-O polar bonds :. For larger molecules with lots of sigma bonds , it is more difficult to determine the net dipole moment because of the free rotation about these single bonds. However, the presence of double bonds that restrict the constant change of the dipole moment direction makes it possible to determine the molecular dipole moment.
This can be illustrated by comparing the boiling points of the cis — and trans -1,2-Dichloroethene :. The dipole moments of the C-Cl bonds in cis -1,2-Dichloroethene reinforce and the molecule exhibits a net dipole. In contrast, the trans isomer has no molecular dipole since the C-Cl bonds are at the opposite direction and their dipole effects are canceled:. As a result, the trans isomer is a non-polar molecule with a lower boiling point 48 o vs 60 o C than the cis -dichloroethene because of the intermolecular dipole-dipole interactions in the latter.
There is an excellent handout of the various shapes of molecules that shows which molecules are polar and which are nonpolar despite having polar bonds. How do polar bonds contribute to the polarity of a molecule? Ernest Z. Dec 21, A molecule like H-F has two different atoms attached to each other by a covalent bond. The F atom has a greater tendency to attract the shared electrons toward itself.
It will have a partial negative charge, and the H atom will have a partial positive charge. The molecule as a whole will be polar. Since the centres of positive and negative charge do not coincide, a water molecule is polar. Related questions How do I determine the molecular shape of a molecule?
VSEPR for 5 electron clouds part 1. VSEPR for 5 electron clouds part 2. VSEPR for 6 electron clouds. Molecular polarity. Next lesson.
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