You’ve undoubtedly noticed that your teacher has been talking a lot about polarity and intermolecular forces lately. He or she has probably even made it sound as if they are important in understanding the properties of chemical compounds and how they may react. Your teacher happens to be right: They are really important.
You thought I was going to make some wiseass comment, didn’t you? Nope. Polarity and intermolecular forces are actually really important. No wiseass comment here.
What is polarity?
I’m glad you asked! Polarity occurs when there’s an unequal sharing of electrons. Bond polarity occurs when one atom in a bond is more electronegative than another, while molecular polarity occurs in asymmetric molecules where the atoms on one part of the molecule are more electronegative than the others.
Polarity only occurs in covalent compounds. After all, if you look at ionic compounds, there are no molecules and no real bonds. What you do see in ionic compounds is a bunch of cations and anions crammed together, but that’s not really what we’re talking about when we’re discussing polarity.
How can you tell if a bond is polar?
To tell if a bond is polar, look at the two atoms in the bond. If they are exactly the same atom, then the bond is nonpolar and there’s an equal sharing of electrons. If they’re different from each other, the bond is polar to some degree.
The problem with this definition is that polarity is meant to describe a significantly unequal sharing of electrons, which leads to unusual molecular behavior. Though carbon and hydrogen are different elements, their electronegativities are only very slightly different (2.5 and 2.1, respectively) so they pull on their shared electrons with almost exactly the same strength.¹ The molecule is technically polar, but for the purposes of classification, we’d say it’s a nonpolar bond.
To figure out if a bond is classified as polar or nonpolar, you need to know the electronegativities of both atoms. The difference between the two electronegativities will allow you to classify the bond according to the following table:²
Though the electronegativity difference is used to find out whether a bond is strictly nonpolar, polar-covalent, or an ionic interaction, in practice we usually just assume that if a metal bonds to a nonmetal, it’s an ionic compound, and most nonmetal-nonmetal bonds are polar covalent (unless they’re the same element). Metalloids are usually classified as nonmetals when doing this.
To show whether something is polar in a diagram, we use the following notation:
In this diagram, you’ll notice two odd things:
- The δ+ and δ- terms refer to where the electrons are pulled more strongly in the bond. δ+ means “partial positive charge” because the fluorine (which has the δ- term) is pulling electrons away from it. This causes oxygen to be a little more positive than usual and the fluorine to be a little more negative.
- The arrow at the bottom is called a dipole arrow and points toward the more electronegative atom, with the line on the back indicating that this atom has partial positive charge. Generally speaking, charts will show either the δ terms or the dipole arrow in a diagram, but not both.
How can you tell if a molecule is polar?
This is an easy one: Draw the molecule and check to see if all of the things connected to the central atom are the same. If they are, then the molecule is nonpolar (i.e. it has an equal sharing of electrons). If not, then it’s polar (unequal sharing).
Let’s draw a couple of diagrams:
In the left diagram, we can see that the central atom, oxygen, has two different types of things stuck to it: hydrogen atoms and lone pairs. Because a hydrogen atom is different than a lone pair, this molecule is polar. Because the oxygen atom is more electronegative than the hydrogen atoms, the dipole arrow points toward the oxygen atom and the little δ signs are located on the side of the molecule with partial positive or negative charge.
If you look at the diagram of water to the right, you’ll see something similar, with oxygen having two lone pairs an