BNG_Polar Molecules Lesson

Polar Molecules

We have already learned that there are both polar and nonpolar covalent bonds. Now, let's look at polarity on a larger scale by studying polar molecules. Pay close attention to the words polar bond versus polar molecule. They are not the same thing! In order for a molecule to be polar, two qualifications must be met:

1. The molecule must contain at least one polar bond.
2. The molecule must be shaped in a way that makes it possible to have one end of the molecule partially negative and one end partially positive.

The extent of a molecule's polarity is often referred to as its dipole moment which is determined by the product of the amount of charge on either end of the molecule and the distance between the charges.   (Think about resultant vectors from physics or maybe your math class.)

Steps to determining if a molecule has a dipole moment

1. Draw the molecule (with the correct shape).
2. Determine the polarity of each bond. For all polar bonds, label each with the symbols, δ- δ+, where the element with the higher electronegativity receives the negative charge.

This simply means to decide if the bond is polar or nonpolar. Recall that this can be done by finding the electronegativity difference. On the AP exam however, you will not be given electronegativity tables. Therefore you must learn to predict the extreme types of bonds by the periodic trends, not by the actual difference in electronegativities. So, remember that electronegativity increases up and to the right on the periodic table. So, if there is a significant distance between the positions of two nonmetals on the periodic table, the bond can be considered polar. Until you get comfortable using only the periodic table to predict electronegativity, you may refer to their specific values.

3. If there is a net dipole moment (an overall positive side and an overall negative side), then the molecule is polar. Look at the examples below to get a visual understanding of this step.

Watch the following video to see examples of using the steps above to determine the polarity of molecules.

Here are some hints to help determine the polarity of a molecule:

If the molecule is symmetrical, with no lone pairs of electrons, and all the atoms attached to the central atom are the same, the molecule is nonpolar because the dipoles (individual polar bonds) will cancel each other. An example is methane, CH₄, pictured to the right.

If all the atoms attached to the central atom are not the same, then the molecule most likely will be polar. (It is still possible for the dipoles to cancel, causing it to be nonpolar, but polar is the best choice here.) An example is chloroform, CHCl_3. You can see that the shape is the same as methane. But, because the carbon is bound to different atoms, there is no cancellation in the dipole moment. So, this molecule is polar.

Reasons for Knowing Polarity

Why is it important to know if a molecule is polar or not? Why go to all this trouble? Well, knowing the polarity of a molecule tells you a lot about the physical properties of the molecule. Melting point, boiling point, and solubility are all are determined by polarity. In a liquid or solid the molecules line up so that the positive end of one molecule is pointing toward the negative end of another molecule. Therefore, the attraction between molecules (think of Coulomb's Law) increases as the dipole moment of the molecule increases. This means that polar molecules have higher melting and boiling points and that polar solvents dissolve ionic and polar molecules more efficiently than non-polar solvents.   

Review these basic concepts about polar molecules before you move on to the next page.

Remember to work on the module practice problems as you complete each section of content.

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