(Polar molecules, Non-polar molecules, etc.)
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2E.25 Draw the Lewis structure and predict whether each of the following molecules is polar or nonpolar: (a) CH2Cl2; (b) CCl4; (c) CS2; (d) SF4.
How can I tell if molecules are polar or nonpolar?
To determine whether a molecule is polar or nonpolar, you would determine its molecular shape based on the Lewis structure. Then using the electronegativity differences present in each bond, you would see if dipole moments are present in the molecule. If the dipole moments cancel out and the molecule is symmetric, the molecule is nonpolar with no net dipole moment. If the dipole forces DON'T cancel out and there is a net dipole moment pointing in one direction, then the molecule is polar.
For instance, for (a), C-Cl bonds are polar because the molecule has a non-symmetric tetrahedral shape that prevents the dipole moments from cancelling out, creating a net dipole moment. Therefore, CH2Cl2 is polar.
(b) CCl4 has a symmetric tetrahedral structure with dipole moments that cancel out, therefore it is nonpolar.
(c) CS2 has a linear structure that is symmetrical, so the dipole moments cancel out, resulting in no partial charges --> nonpolar molecule.
(d) SF4 has a lone pair on the sulfur atom and 4 F atoms bonded to it, so it has a see-saw molecular geometry. Because of the repulsive force exerted by the lone pair, the molecule is not symmetric and the dipole moments will not cancel out, making SF4 polar.
The best way to do this is to draw the lewis structures of each molecule. Once you draw the lewis structure and determine the shape you can figure out if its polar or not. Another option is drawing and canceling dipole moments on the lewis structures.
Visualizing it is the best way, so draw out the Lewis Structures as accurately as possible (based on valence electron count and formal charge like usual), you will see if dipole moments occur that are not symmetrical. What I mean is, if there is a tetrahedral like CCl4, there are dipole moments on each of the C-Cl bonds, BUT since it is equal pulling on each side, they cancel out and it is not a polar molecule. Nonpolar molecules can have two dipoles diagonal from each other too and still not be polar because they cancel out. A polar molecule would not have any canceling out of dipoles.
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