April 2nd - The Different Types of Molecular Bonds

Today, we learned all about intermolecular and intramolecular bonds.

As we know, bonds can exist within a molecule. Ionic bonds, metallic bonds, and covalent bonds are all types of bonds that hold ions together within a compound. We can call the bonds that hold ions together intramolecular bonds.

Intermolecular bonds, on the other hand, exist between separate molecules. The stronger the intermolecular bonds, the higher the boiling point or melting point of the substance. Two types of intermolecular bonds are Van der Walls bonds and hydrogen bonds, which we will cover below.

Mr. Van der Waals, in all his glory.

Van der Waals bonds are based on electron distribution. Van der Waals bonds can either be dipole-dipole bonds or weak bonds caused by the London Dispersion Force. 

1. Dipole-dipole bonds occur because of the charge separation in a molecule. The positive end of a polar molecule attracts the negative end of another polar molecule, creating a bond. This type of bond is very strong.
2. London Dispersion Forces are bonds that can occur in all molecules. However, it creates the weakest bond. If a substance is non-polar, dipole-dipole forces cannot exist. Instead, electrons are free to move around and, sooner or later, will find themselves bunched on one side. This causes the side with the electrons to become negatively charged, and the other end positive. Like dipole-dipole bonds, the positive and negative ends are attracted to the positive and negative ends of other molecules.

Observe the LDF in action.

It is also important to note that the greater the number of electrons in a molecule, the stronger the London Dispersion Force will be. 

Ex.) Which of the following compounds will have the greatest LDF? CH₄ or C₂H₆? The answer is C₂H₈ because it has more electrons. It has 18 electrons vs. methane’s 10 electrons.

Hydrogen bonding is the last type we learned about. If hydrogen is bonded to certain elements (specifically fluorine, oxygen, or nitrogen), the bond is highly polar. This forms a very strong intermolecular bond.

Ex.) Which molecule has the highest boiling point? C₂F₄ or C₂Cl₆? Remember, high boiling points occur because there are strong bonds between molecules. Both of these molecules are polar, so we have to look at the LDF of each.

Of course, C2Cl6 has more electrons, so it has stronger bonds (and therefore, a higher boiling point.)

Ex.) Which molecule has the highest boiling point? H₂O or H₂S?

Both of these are hydrogen bonds. However, water is a special case because hydrogen is bonded to oxygen, and there is a large charge separation. Therefore, it has the highest boiling point of the two.

What fun! As always, the video:

Posted by Michael.