In this video we are going to go over a way that we can quickly distinguish a polar molecule from a nonpolar molecule. Now it's easier if you know what to look for when a molecule is nonpolar and before you find out what to look for when it's polar. The first category of nonpolar molecules are those that are made up of one element.
For example, if you have a monoatomic atom or a diatomic atom, That's completely positive one element, it's nonpolar. Examples include, for example, N2, that's the diatomic molecule, O2, Cl2, F2, H2. These are all nonpolar.
And some of the monatomic gases like helium, neon, argon, xenon, the normal gases, those are also nonpolar. The next category to look for is a molecule that contains only carbon and hydrogen. Those molecules are non-polar. So for example, methane, your alkanes, ethane, C2H6, ethene, CH2, double bond CH2.
These are composed only of carbon and hydrogen and therefore they are non-polar. The carbon-hydrogen bond is non-polar. The third area to look for is if the molecule has symmetry. So, for example, carbon tetrafluoride. Notice that all of the outer elements are the same.
This molecule has symmetry. This is nonpolar. Sulfur hexabromide. All of the outer elements are identical. This molecule is also nonpolar.
Carbon dioxide, another non-polar molecule. The two outer elements, the oxygen's are the same. Phosphorus trichloride, that's another non-polar molecule. Each of these molecules, they have symmetry as you can see.
BH3, boron trihydride, also non-polar. Now, let's say if the molecule lacks symmetry. Is it always going to be polar? Well, it all depends on the EN difference. If the electronegativity difference, if it's less than 0.5, then it's going to be nonpolar.
If it's more polar, let me give you an example of that. So, iodine monobromide. Iodine. has an electronegativity value of 2.5 and for bromine it's about 2.8.
Because the EN difference is like 0.3, this bond is relatively nonpolar. Another example, if there's no symmetry, is bromine monochloride. Chlorine has an electronegativity value of 3.0. And here, once again, the EN difference is less than 0.5, so that molecule is nonpolar. So those are some quick ways that you can use to identify if a molecule is non-polar.
If it doesn't meet those characteristics, generally it's going to be polar. So what are some quick ways to identify polar molecules? The first thing is, if a molecule has hydrogen bonding, it's going to be polar.
So if hydrogen is directly attached to nitrogen, oxygen, or fluorine, it's polar. Examples include water, NH3. H.F. CH3OH, even though the carbon-hydrogen bond is nonpolar, the presence of the alcohol means that you have hydrogen bond in, and therefore it's going to be polar.
CH3NH2, because of the NH2, it's polar. CH3COOH, once again, because of the OH, it's polar. Now also, if the molecule lacks symmetry, and if the bond is polar, then it's going to be a polar molecule. Here's another example. Here the oxygens, here these options are identical and it has symmetry which makes it non-polar.
Here this molecule, it does not have symmetry like this one did. Carbon has an electronegativity value of 2.5, oxygen is 3.5, so this bond is polar. Here for sulfur, It's also 2.5, so that carbon-sulfur bond is non-polar. But because of the carbon-oxygen bond, it's polar.
In the case on the right, when we have carbon dioxide, even though the carbon-oxygen bond is polar, because there's two of them, because of the symmetry, those dipole moments cancel out, and therefore CO2 is non-polar. But if you replace one of the oxygens with a sulfur molecule, then it becomes polar. Let's look at another example, CH3F.
Notice that it's similar to this molecule right here. The carbon-fluorine bond is very polar. Carbon has an electronegativity value of 2.5, and for fluorine it's 4.0. It's well above that 0.5 difference. And so because of the carbon fluorine bond, it's polar.
The carbon hydrogen bond is relatively nonpolar, so the dipole moments are very small for that. But in this case, over here, these dipole moments, they cancel. And that's why, because if you have symmetry, it makes it nonpolar. But because this molecule lacks symmetry, it's going to be polar.
Another example is sulfur dioxide. Because sometimes the shape of the molecule can affect the polarity. See, carbon dioxide, which looks very similar to sulfur dioxide, in the fact that there's two oxygens and one atom in the middle. Because carbon dioxide has a linear shape, the dipole moments, they cancel.
But because sulfur dioxide has a bent shape, the arrows, the dipole moments, they don't cancel. And you can see, because of the lone pair, this molecule lacks symmetry. So that also makes it polar.
The last example, if you look at NH3, even though it has hydrogen bonded, which makes it polar, the geometry also makes it polar as well. If you draw the arrows, they all point towards the partially negative fluorine atom. And they point in one direction, they don't cancel. And that's another reason why this molecule is polar.
So, just to review. Molecules that are nonpolar tend to be diatomic molecules that are composed of one element, or monatomic atoms like noble gases. They can be pure hydrocarbons, or they can be molecules that have symmetry, where all of the outer elements are the same.
If you see that, 99% of the time it's going to be nonpolar. If you don't see it, for the most part it's probably going to be polar, but... Just to be on the safe side, look at the electronegativity values.
If it's greater than 0.5, draw the arrow from the partially positive atom to the partially negative atom. And if the arrows don't cancel, then your molecule is going to be polar. So hopefully this video will help you to quickly distinguish polar and nonpolar molecules.
And I wish you well in your general chemistry course.