2E.25 (7th Ed.) Lewis structure and polarity

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

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Julia Lindner 1I
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Joined: Fri Sep 28, 2018 12:17 am

2E.25 (7th Ed.) Lewis structure and polarity

Postby Julia Lindner 1I » Mon Nov 19, 2018 11:37 pm

We learned that the polarity of a molecule can depend on the shape, since partial charges can cancel each other out. But for this problem, the answer says that CH2Cl2 is polar because the Lewis structure is drawn with the chlorine atoms on one side and the hydrogen atoms on the other. But wouldn't we be able to draw it with the chlorine atoms across from each other and hydrogen atoms across from each other, canceling out the partial charges? Is there some sort of rule saying that the same atoms should be grouped together around the central atom instead of placed across from each other?

ChathuriGunasekera1D
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Joined: Fri Sep 28, 2018 12:17 am

Re: 2E.25 (7th Ed.) Lewis structure and polarity

Postby ChathuriGunasekera1D » Tue Nov 20, 2018 8:21 am

So you're right, but when you think about its molecular geometry, it makes sense why it's at least a little polar. CH2Cl2 is tetrahedral, and when you arrange the atoms around C in the tetrahedral shape, you can never orient them so that the charges fully cancel. Nothing is directly across from each other (no 180 degree angles). Therefore, even though CH2Cl2 seems nonpolar if you switch the Cl and H on a 2D drawing, it might actually be polar no matter which way you orient the atoms around the central atom in the 3D shape.

But if you were to look at CH4, it's also tetrahedral. But because it's the same type of atom around the central atom, the molecule is nonpolar. I hope this kind of made sense! I could be wrong.

Mindy Kim 4C
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Joined: Fri Sep 28, 2018 12:25 am

Re: 2E.25 (7th Ed.) Lewis structure and polarity

Postby Mindy Kim 4C » Tue Nov 20, 2018 11:45 am

Shape is important when determining whether or not structures are polar or nonpolar. For example, if you look at the structure of H2O (bent/angular), the molecule would be nonpolar if the structure was linear (the two dipole moments cancel each other out). However, due to its bent structure, the dipole moments both point upwards and do not cancel each other out, making the molecule polar.


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