Molecular Shape and Polarity

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Many physical properties, like melting point and boiling point, are affected by molecular polarity. The higher the molecular polarity, the more polar molecules attract each other. The attractions require extra energy to break during phase changes from solid to liquid (melting) and liquid to gas (boiling). The strength of the attraction depends on the amount of charge on either end. In other words, it depends on dipole moment.
Some molecules have polar bonds but are not polar molecules. Molecular structure plays a role in this.
In the HCl molecule, which is polar, the H and the Cl differ in electronegativity. Electrons tend to spend more time near the Cl and creates a negative pole there. Therefore the molecule is polar.
For molecules that contain more than two atoms, the combined effects of all the polar bonds must be considered. For example lets take the basic shapes--linear (with 3 atoms), planar triangular, and tetrahedral. A linear molecule, like CO2 has electronegativity differences between the C and the O. Electrons tend to stay longer with the O, but the molecule is not polar. This is because the bond dipols point in opposite directions and work against each other. 
  O = C = O
   <-   ->
polar tendency
  working in
   opposite
  directions
The same holds true for the planar triangular and tetrahedral structures. The tendency either works toward opposite directions or toward the center. That is if the surrounding atoms are all the same.
For example, if we had chloroform (CHCl3), a tetrahedral molecule with the C in the center and the H and Cl as the verticies, the tendency would go toward the Cl and away from the H, making the H side positive and the Cl side negative.
Also, on molecules with lone pairs, polarity usually occurs because there is no opposite vertex to cancel and tendancies.
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