When a molecule has a dipole we call it a polar molecule. In order for a molecule to have a dipole there are two key criteria. First, the molecule must have some polar bonds. Second, the dipoles created by these bonds must not cancel out as a result of the symmetry of the molecule. This is one of our key interests in the shape of the molecule. We would like to know if certain regions within the molecule have a higher electron density than other regions. This will strongly affect both how the molecule interacts with other molecules as well as the molecule's chemistry.
Let's look at a couple of key examples to demonstrate this idea. One of the most important chemical substances in the world is water. The properties of water derive in many ways from the means in which the electrons are distributed in the molecule. Water consists of a central oxygen atom covalently bonded to two hydrogen atoms. Using the VSEPR model we would deduce that water has a bent structure. To examine if water is polar, we need to examine the bonds within the molecule. We would classify the O-H bonds in water as polar covalent bonds since oxygen is significantly more electronegative than hydrogen. This means that each of these bonds has a dipole that arises from the partial positive charge on the hydrogen and the partial negative charge on the oxygen. To see if the overall molecule has a dipole, we need to look at the geometrical arrangement of these dipoles. Because water is bent, when we add up the dipoles of the bonds, we find that there is a net dipole. That is, essentially water has a positive side (where the hydrogens are) and a negative side (the oxygen end). The dipole is symmetric in the "middle" of the molecule since each "side" with a hydrogen is symmetric. This is most easily seen in the diagram of the dipole for water. The net (or molecular dipole) is shown in green.
In contrast to this example, carbon dioxide does not have a molecular dipole despite the fact that it has polar covalent bonds. This is a direct result of the geometry of the molecule. The oxygens in the molecules have a greater electron density and thus have a partial negative charge. This makes the carbon oxygen double bonds polar. However, since the molecule is linear the two dipoles from the two bonds in the molecule are pointing in exactly opposite directions. As a result, the dipoles cancel and the overall molecule has no net dipole.
