(Polar molecules, Non-polar molecules, etc.)

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Montana James 4G
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Postby Montana James 4G » Mon Nov 18, 2019 11:15 pm

why are some molecules that are symmetrical (like CH2Cl2) polar? shouldn't the symmetry mean that the dipoles can cancel?

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Re: polarity

Postby nicolely2F » Mon Nov 18, 2019 11:17 pm

CH2Cl2 is a tetrahedral molecule, and tetrahedral molecules are only nonpolar if all bonded atoms are the same. The Cl pair is causing polarity to one direction and there isn't another opposing Cl pair.

Natalie Wang 1B
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Re: polarity

Postby Natalie Wang 1B » Mon Nov 18, 2019 11:20 pm

I was really confused about this too, but I highly recommend that you watch this Youtube video to contextualize the answer:
Start at the the 5 minute mark.

To give you a written answer, CH2Cl2 is polar because of the bond angles. Remember that a tetrahedral molecule has 109.5 degree bond angles. This means that every bond in CH2Cl2 is not directly across from each other. If the bonds were directly across from each other, the bond angles would be 180 degrees, but we know that's not the case. Since the bond angles are 109.5 degrees, there is no way for the two Cl atoms to be perfectly across from each other. They are always going to be angled in near each other. Because they are NOT DIRECTLY across from each other, the Cl bonds have dipole moments that do NOT cancel each other out. Look at the a picture of a tetrahedral molecule. You'll see that the bonds are all angles towards each other in some way.

Sam McNeill 1E
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Re: polarity

Postby Sam McNeill 1E » Tue Nov 19, 2019 9:51 am

For a tetrahedral shape, the only time it is non polar is when all 4 of the bonded atoms are the same.

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Re: polarity

Postby jisulee1C » Tue Nov 19, 2019 11:28 am

Even though a tetrahedral shape is non polar because there are different atoms bonded on each side of the central atom there will be dipoles. And because the dipoles are unequal they will not cancel each other out and therefore it will be a polar molecule.

Maya Beal Dis 1D
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Re: polarity

Postby Maya Beal Dis 1D » Tue Nov 19, 2019 2:51 pm

How do you know if they Cl atoms are across from each other or each across from a hydrogen atom when drawing the Lewis structure? Wouldn't this effect polarity?

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