¿Cómo determinar la POLARIDAD de las moléculas?

Hello everyone, in the last video we learned that molecules are three-dimensional structures and that is why we could determine their molecular geometry.

And today we will learn why it is important to determine this geometry. And in itself there are several aspects, especially when one studies organic chemistry, but today we will focus on how geometry affects the polarity of a molecule. We already talked a little about polarity when we saw the covalent link. We said that when there is a polar covalent link, which is when two non-metals join by sharing a pair of electrons in an unequal way, because an element is much more electronegative and attracts electrons stronger.

Well, that produced the polarity in a link, that is, that there is a positive partial charge at one end and a negative partial charge at the other. In addition, we can represent this with a diagram that shows the electron density of the molecule. Remember that with electron density we refer to the zone of the molecule where the electrons are concentrated, that is, the high density zone is the negative pole and the low density zone is the positive pole.

Ok, if you have not had problems with the terms I am using so far, I think we can continue. If not, I recommend checking the video of covalent links to review those concepts and we will return. That said, in the covalent link we used the difference of electronegativity to know if a molecule had poles. Today we are going to use an even more special term for that, the dipolar moment. The definition of dipolar moment as it is is somewhat complex and the truth is that it is not very useful because it is a calculation that is almost not done.

In reality, the dipolar moment is determined experimentally and with the data obtained, which we can consult in books or on the internet, we can analyze and better understand a molecule. So, to keep this easy, for us the dipolar moment will represent a value that tells us how polar or non-polar a molecule is. Basically it serves as a polarity scale.

For example, if we have a high dipolar moment, it means that a molecule is very polar, that is, that the positive and negative partial loads of that molecule are large. On the other hand, a low dipolar moment means that the partial loads are small. In fact, if the dipolar moment is equal to zero, we are talking about a neutral molecule, an apolar molecule.

What you should keep in mind is that the dipole moment of a complex molecule will depend on two factors mainly: the link dipoles and the molecular geometry. First let's see what is that link dipole. Imagine that we have the hydrochloric acid, the chlorine, joined to the hydrogen.

We know that chlorine is more electronegative than hydrogen, either by placing them in the periodic table or we could look for the exact value in any book. Anyway, we can be sure that there is a difference in electronegativity between them. And by the simple fact of knowing that there is that difference, we know that this link presents a dipole.

is usually represented as an arrow pointing towards the most electronegative atom, let's say towards the negative side of the link. Now, in this case, as it is a simple molecule, there is no other link that we have to analyze, this same link dipole represents the dipolar moment, which is sometimes also known as the net dipole. In the following example you will be able to see the difference between these concepts. For now, let's know what these arrows represent.

But anyway, with this information we can assure that this molecule is polar, that is, the dipolar moment is greater than zero, simply because of that difference in electronegativity. With it we know that chlorine concentrates more the electronic density of the molecule, and we can see that in the color diagram, there we distinguish the positive pole and the negative pole.

That's a simple case, now let's compare what happens with the acid oceanidric, that is a more complex molecule, so first we have to check its molecular geometry, which would be linear.

In addition, in this case we have to check two link dipoles, one that would be between nitrogen and carbon and the other between hydrogen and carbon. First we have to check who is more electronegative and again using the table or looking for the data we would know that nitrogen is the most electronegative, then it would follow carbon and hydrogen is the least electronegative.

Then, once we make sure that there is a difference in electronegativity, we can draw the link dipole between the carbon and the nitrogen, which would be pointing towards the nitrogen, remember, towards the negative side. But we also have to draw a link dipole from the hydrogen to the carbon, because we know that there is a small difference in electronegativity between them. Now the question is, what is the dipolar moment or the net dipole of this molecule?

For that you should know that the dipole moments and the link dipole behave as vector magnitudes. That means that if two dipoles go in the same direction, they add up. Or if they go in opposite directions, they cancel. For example, here the two dipoles go in the same direction. So the dipole moment of the whole molecule, one is quite large because it is the sum of the other two,

and 2 would also go there therefore this molecule the dipolar moment is greater than zero that is, it is a polar molecule that is why we also call it net dipole because it refers to taking into account all the dipoles that are there that in this case are added but it is not always like that for example let's change to carbon dioxide this is a molecule that also has linear geometry that we have to analyze two links first with a periodic table we can see that oxygen is more electronegative than carbon

Then we can draw a link dipole that goes from carbon to oxygen and also another from the same carbon to the other oxygen. Remember, we have to analyze each link. And now the question is: Is there a dipolar moment or a net dipole? And the answer is no.

In this case, unlike the previous one, the link dipoles go in totally opposite directions, so they cancel each other. Imagine it like this: it's like two friends were holding me by the arms and they were pulling me. If both are equally strong, well, I'm not going anywhere, I'm going to stay in the same place. That's what happens here. The linear geometry of the carbon dioxide makes each oxygen pull the electronic density with the same force to opposite sides. That's why we say that the dipoles cancel each other.

Therefore, in this case, the carbon dioxide has a dipolar moment equal to zero. That is, despite having covalent polar links, it is an apolar molecule. That is why you should look at these two aspects to determine if a molecule has a dipolar moment, that there is a difference in electronegativity and also that the molecular geometry allows it, that the link dipoles are not canceled. For example, let's quickly check other cases with different geometries.

Let's see two compounds with a flat trigonal geometry, the boron trifluoride and the formaldehyde. In the case of boron trifluoride, the fluor is much more electronegative, so we would draw the dipole towards the fluor. It would clearly be the same with each link. So, here, as all the dipoles go in opposite directions, they cancel each other, there is no dipolar moment.

Remember that the flat trigonometric geometry is like this: as the atoms are in totally opposite directions, they can cancel each other. And it is merely visual to know if the dipoles can be added or canceled, which could also be done with calculations, but it would just be getting into trouble. Then it shows. But anyway, the boron trifluoride would be apolar for this same reason. Now, in the case of the formaldehyde, the carbon is more electronegative than the hydrogen.

then both dipoles would be pointing towards the carbon and we already know that oxygen is much more electronegative so it would be another dipole even larger upwards therefore here we do have a dipolar moment let's say that the net dipole would go upwards for me it seems as if the hydrogen were pushing the electronic density from below and the oxygen still pulls it up more so that is why the dipolar moment results in that same direction and gives a very polar molecule

Changing the geometry, in the case of the tetrahedral structure something similar happens. For example, the carbon tetrachloride compound is like this. In itself, the chlorine is more electronegative than the carbon. Then the link dipoles seem to go from the center outwards. And just like the boron trifluoride, here all those dipoles point in totally opposite directions. They are also of the same magnitude, so they cancel and result in an apolar molecule.

On the other hand, in the case of methane, the carbon is more electronegative, so all the dipoles would be pointing towards it, but in itself they also go in opposite directions, so here they are also canceled. In itself, the only way for a dipolar moment to form in the tetrahedral geometry is that there are different atoms. For example, imagine that we change a hydrogen to a chlorine to methane.

This is a compound called methyl chloride, and here it is very different because the chlorine is more electronegative, so the direction of that dipole would be to the other side. Therefore, this molecule does have a net dipole that would go practically in that direction. In fact, methyl chloride is a very polar molecule because all those dipoles are being added.

Finally, let's check an important detail: free electrons also affect the polarity of a molecule. For example, check the water molecule, which would have angular geometry.

Since oxygen is more electronegative than hydrogen, we could draw the link dipoles pointing towards oxygen, which would already give a dipolar moment to this molecule. But we know that water is a very polar molecule, and that is thanks to the fact that it has free electrons. Remember that the electrons are moving in their orbitals, which in this case would be on this side, contrary to the hydrogen. So practically all the electron density is joining on this side.

If you want to see it in a simple way, it's like if we put another arrow towards the free electrons, which would go from the central atom to the opposite side. That's why it's a very marked dipolar moment towards where all the arrows point.

A similar case would be ammonia. Although it has a different molecular geometry, here it is a pyramidal triangle, both the dipole of hydrogen and the pair of free electrons of nitrogen would be pointing in the same direction. That is why ammonia is also a very polar molecule.

And so far today's topic, I hope it has been very clear. In the next video we will see how it affects us that a molecule is polar or not polar, especially affects the physical properties. But anyway, there we see, thank you very much. And do not forget to take a look at our social networks, we have summaries, more examples, more exercises, scientific dissemination and much more.