 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 and two hydrogen atoms – this indicates a polar molecule. Unfortunately, it’s impossible to draw the dipole arrow for this molecule because the atoms are drawn symmetrically. What do we do?
Fortunately, we’re saved by one fact: Water isn’t a 2-D object. Though each of the diagrams shown portray it as being 2-dimensional, it’s actually a 3-D structure as shown in the cool picture below:
Because it’s a 3-D structure, there are different valid ways of drawing it depending on the observer’s viewpoint. Let me show you with a cool picture I took:
The cherries on the left show the orientation of water that’s shown on the left above. The cherries on the right are arranged in an identical way (getting them to balance like that was a pain in the butt), showing that from another angle they appear linear. They’re the same molecule but seem to have different shapes depending on how you look at it.
Going back to the diagram of water that looks like this:
we now understand that we drew the lone pairs and hydrogen atom in completely arbitrary positions. In order to proper show the polarity of the molecule, follow this rule:
If you have a polar molecule, you can switch around any combination of atoms and lone pairs if you want to. The correct arrangement for showing polarity is the one where the molecule looks uneven.
So, for this water molecule, if we switch a lone pair with a hydrogen, we end up with exactly the same thing as we got for the other:
Let’s look at carbon dioxide:
In this structure, we can see that carbon has two singly-bonded oxygen atoms stuck to it. Because these atoms are the same, the molecule is nonpolar and there’s no need to draw any of those δ things or an arrow.
However, what if we actually drew carbon dioxide like this:
This Lewis structure is completely valid and follows all of the rules. It also makes it look like the oxygen side of the molecule should have a partially negative charge and the carbon side should have a partial positive charge. Which, according to our method for finding polarity, we know i