d two hydrogen atoms – this indicates a polar molecule. Unfortunately, it’s impossible to draw the dipole arrow for this molecule because the atoms are drawn symmetrically. What do we do?
Fortunately, we’re saved by one fact: Water isn’t a 2-D object. Though each of the diagrams shown portray it as being 2-dimensional, it’s actually a 3-D structure as shown in the cool picture below:
Because it’s a 3-D structure, there are different valid ways of drawing it depending on the observer’s viewpoint. Let me show you with a cool picture I took:
The cherries on the left show the orientation of water that’s shown on the left above. The cherries on the right are arranged in an identical way (getting them to balance like that was a pain in the butt), showing that from another angle they appear linear. They’re the same molecule but seem to have different shapes depending on how you look at it.
Going back to the diagram of water that looks like this:
we now understand that we drew the lone pairs and hydrogen atom in completely arbitrary positions. In order to proper show the polarity of the molecule, follow this rule:
If you have a polar molecule, you can switch around any combination of atoms and lone pairs if you want to. The correct arrangement for showing polarity is the one where the molecule looks uneven.
So, for this water molecule, if we switch a lone pair with a hydrogen, we end up with exactly the same thing as we got for the other:
Let’s look at carbon dioxide:
In this structure, we can see that carbon has two singly-bonded oxygen atoms stuck to it. Because these atoms are the same, the molecule is nonpolar and there’s no need to draw any of those δ things or an arrow.
However, what if we actually drew carbon dioxide like this:
This Lewis structure is completely valid and follows all of the rules. It also makes it look like the oxygen side of the molecule should have a partially negative charge and the carbon side should have a partial positive charge. Which, according to our method for finding polarity, we know it 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 