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Question: I'm getting a little confused with determining whether or not a molecule is polar. I understand that if the electric dipoles cancel each other out, then the molecule is polar but I'm having trouble visualizing this for a tetrahedral molecule. Why is polar but nonpolar?
Answer: In the 4 dipoles cancel each other out making the molecule nonpolar. In , the 2 C-Cl bonds create a dipole towards the Cl since Cl is highly electronegative. This makes the molecule negative towards the Cl and away from the 2 H. It is a similar idea to how water is polar with the negative towards the oxygen.
The shape for ch2cl2 is tetrahedral. The lewis structure drawing is really misleading because it makes you think that the chlorine atoms are directly opposite from each other on a 1D surface. In reality though, the angles are 109.5 degrees so wherever you place the chlorine atoms, they will always be next to each other at 109.5 degrees. Therefore, you get dipole moments going towards chlorine in two different directions that do not cancel. Hope this clears things up!
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