Intermolecular Forces: Review
The electron density of a polar bond accumulates towards one end of the bond, causing that end to carry a slight negative charge and the other end a slight positive charge. Likewise molecules in which there is an accumulation of electron density at one end of the molecule, giving that end a partial negative charge and the other a partial positive charge, are called polar molecules.
There is a series of steps you can take to determine if a molecule is polar or not. As you work through these steps you will see that molecules with polar bonds are not necessarily polar molecules.
Step 1: Indicate polar bonds in molecule or ion.
Remember that a polar bond is one in which the electrons are unevenly distributed. This occurs because of a difference in electronegativity of the two atoms that share the electrons.
The Lewis structures for H2S, BF3, and CCl2H2 are shown below. Click on the bonds that are polar.
Good! Are there any other polar bonds in any of these molecules?
While carbon is more electronegative than hydrogen, the difference in electronegativity values is so slight that carbon hydrogen bonds are usually considered nonpolar.
Good! You have found all of the polar bonds in each of these three molecules.
Step 2: Using the molecular geometry, determine if any of the dipole moments will cancel. Any resulting dipole indicates a polar molecule.
The bond dipole moments will cancel if:
Let's look at two examples: carbon dioxide and water. Carbon dioxide is a linear molecule with two polar bonds. Water is a bent molecule with two polar bonds. Carbon dioxide will not be polar because both dipole moments are equal in magnitude (since they are both carbon oxygen bonds) and arranges symetrically about the central atom in a linear geometry. Water will be polar. Even though the two dipole moments are equal in magnitude (since they are both oxygen hydrogen bonds), they are not arranged symetrically about the central atom and thus will not cancel out.
What is the molecular geometry of the following molecules?
Good! The molecular geometries with the bond dipoles are shown below:
Using the molecular geometries, click on the molecules that will have a molecular dipole moment.
Good! H2S is a polar molecule with a net dipole moment as shown on the left.
While the boron fluoride bonds are polar, the dipole moments cancel out. Therefore, BF3 is a nonpolar molecule.
Good! CCl2H2 is a polar molecule with a net dipole moment as shown on the left.
Are there any other polar molecules?