t doesn’t.
Rest easy! This structure may be technically right, but it doesn’t follow VSEPR, which says that all of the things stuck to an atom want to spread themselves out as far as possible. In this case, we can clearly see that the O=C=O bond isn’t shown as spread out as it is in the first diagram. This makes the second portrayal technically correct, but misleading. And still nonpolar.
Another example: Difluoromethane (CH₂F₂):
Difluoromethane looks like this:
As with water, both of these diagrams are identical – just shown from different angles. If we want to figure out whether the molecule is polar or not, just look at the central carbon – you’ll see that it has two fluorine atoms and two hydrogen atoms stuck to it. Given that fluorine and hydrogen are different from each other, this indicates that it’s a polar molecule.
Now, where to draw the dipole? If we look at the first drawing, we can see that everything looks pretty symmetrical, making it impossible to show the electrons more on one side of the molecule than the other. However, the drawing on the right switches one of the hydrogen atoms with a fluorine, giving us a structure where we can show the dipole:
And this is your final answer.
Except… What would happen if you had instead switched two different atoms with each other than the one I showed? For example, let’s say we got this diagram instead:
It’s a valid Lewis structure and it’s shown asymmetrically. As a result, we can go ahead and draw the dipole arrow:
Even though it’s a little different than the other answer I said was correct, you can see that they’re basically the same thing, with the dipole pointing toward the side of the molecule with the fluorine atoms and the hydrogen atoms having partial positive charge. It’s the same thing, only viewed from a different angle.
And with that, you’re a polarity pro!
- It’s a little more complicated than this in practice due to something called the inductive effect. The idea behind the inductive effect is that the “pull” of atoms in a bond may be somewhat different depending on the other atoms stuck to it. For example, a C-C bond will be nonpolar if both carbons have the same atoms attached, but if one of the carbon atoms has three fluorine atoms, the highly electronegative fluorine atoms will cause that carbon to pull strongly on the C-C bonding electrons. This is fairly important in organic chemistry, but you’re not likely to see it in high school.
- These numbers are more-or-less arbitrary, and you may see different values cited by different sources. This takes place because polarity is a continuum where some things can be a little polar while others can be a lot polar. The cutoff between a little and a lot is arbitrary, so when people cite different numbers they’re both basically right.
- Fish: Image courtesy of digitalart at FreeDigitalPhotos.net.
- Rotating water molecule: By Darekk2 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons.
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