Como identificar se uma molécula é polar ou apolar?

Do you know how to tell when a molecule is polar or nonpolar? In this video, we will learn to identify polar and nonpolar molecules using two basic pieces of information: the molecule's composition and its geometry. But first, do you know what polarity is?

In a chemical bond, polarity refers to how equal or unequal the sharing of electrons is in a covalent bond. In an apolar covalent bond, electrons are shared equally between the atoms involved in that bond, as is the case with the molecules Cl2 or N2. On the other hand, in a polar covalent bond, one of the atoms involved exerts a greater attraction for the electrons in the bond.

To estimate the polarity of a bond, we use electronegativity, which measures an atom's ability in a molecule to attract electrons towards itself. But as I mentioned, we will use composition and geometry to estimate a molecule's polarity. So, if you want to learn more about electronegativity, I'll leave an amazing video on the channel in the comments and description, and you can check it out later.

When atoms involved in a chemical bond are different, you can automatically consider it as polar. In this bond, a dipole is formed, with one of the atoms having a partial positive charge and the other having a partial negative charge. The quantitative measure of the magnitude of a dipole is called the dipole moment.

When a molecule is polar, the dipole moment is nonzero, meaning there is an inequality in how electrons are shared in that molecule. Likewise, if it is nonpolar, the dipole moment is zero, indicating an equal sharing of electrons. When a molecule has multiple chemical bonds, even if they are all polar, the geometry can lead to the cancellation of the dipoles formed.

The molecule will have a dipole moment equal to zero, and it is nonpolar. Check the video description, where I will also leave several videos about molecular geometry on the channel, and I'm sure you'll find them interesting. And don't forget to leave a like, okay?

So, let's start with a very traditional molecule when discussing polarity: the water molecule. Water has two polar bonds between oxygen and hydrogen, and it has an angular geometry. In this case, we know that oxygen is more electronegative than hydrogen, which causes the electrons to be pulled toward it.

This creates a dipole in each of the bonds, pointing toward the oxygen atom. Due to the bond angle, these dipoles add up to form a single dipole with a nonzero dipole moment. Therefore, the water molecule is polar, but this only happens due to its geometry.

Now, let's understand what happens with the carbon dioxide (CO2) molecule. Carbon dioxide has a linear geometry and two polar C-O bonds, with dipoles pointing towards the more electronegative oxygen atoms. Since these are two identical bonds, they have the same magnitude in their dipoles, but they point in opposite directions.

This results in one being virtually negative and the other being positive, and the resulting dipole moment is zero. In other words, the CO2 molecule is nonpolar, and this is directly related to its linear geometry. So, does this mean that all linear molecules are nonpolar?

Actually, no, because it also depends on the types of atoms involved in the molecule. For example, hydrogen cyanide (HCN), also known as hydrocyanic acid, has a linear geometry but has two different bonds: H-C and C-N, which will have dipole moments that do not cancel each other out. Therefore, HCN, despite being linear, is a polar molecule.

Now, let's take a look at the ammonia (NH3) molecule. Ammonia has a pyramidal geometry and three identical NH bonds. These three bonds have their dipoles pointing toward the nitrogen atom, which is the most electronegative.

They add up to create a nonzero dipole moment. Therefore, ammonia, like water, is a polar molecule. Finally, let's examine the geometry of the methane (CH4) molecule.

Carbon is more electronegative than hydrogen, and the four identical C-H bonds have their dipoles pointing toward the center of the molecule. They all have the same magnitude and converge to the same point. Since the bond angles are all equal, when the dipoles are summed, the result is a dipole moment equal to zero, and the CH4 molecule is nonpolar.

Now, let's put all four molecules together, but this time represented by their Lewis structures. What do you notice in common between the polar and nonpolar molecules? The polar molecules, H2O and NH3, have pairs of isolated electrons distributed asymmetrically in their respective molecules.

This creates a large region with a negative charge, resulting in a nonzero dipole moment. On the other hand, the nonpolar molecules shown here do not have isolated pairs of electrons. All electrons are bonded and evenly distributed in identical bonds, meaning they are symmetrically arranged in the molecular structure.

The dipole moment is zero in cases like these. So there you have it! I hope you enjoyed the video.

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