Hydrogen Bonding, Dipole-Dipole & Ion-Dipole Forces: Strong Intermolecular Forces

  • 0:05 Intermolecular Forces
  • 0:53 Dipole-Dipole Forces
  • 1:35 Ion-Dipole Force
  • 2:20 Hydrogen Bond
  • 4:13 Effects of…
  • 5:23 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn about intermolecular vs. intramolecular forces. Learn the different intermolecular bonds (including hydrogen bonding, dipole-dipole and ion-dipole forces), their strengths, and their effects on properties, such as boiling and melting points, solubility, and evaporation.

Intermolecular Forces

Intermolecular forces are forces between molecules and intramolecular forces act within molecules
Intermolecular Forces

Intermolecular forces are the forces that exist between molecules. Don't confuse these with intramolecular forces, which are the strong forces that keep a molecule together. 'Intra' means inside, so these are the inside forces in a molecule. 'Inter' means between, so these are the forces between molecules. To remember the difference between inter and intra, I always think of 'interstate.' In every state there is an interstate road that goes across more than one state, like Interstate 80 that runs from the Pacific Ocean in California to the Atlantic Ocean in New York. Intermolecular forces are weaker than intramolecular forces but still very important. The two intermolecular forces we are talking about in this video include dipole-dipole and ion-dipole.

Dipole-Dipole Forces

A dipole is a molecule that has both positive and negative regions. Although we talk as though electrons distribute their time evenly among all atoms in a molecule, some elements have more affinity for the electrons than others, and they hang out around that atom more. We also talk about these molecules being polar. A polar molecule is a molecule with a slightly positive side and a slightly negative side. I always think about the North and South Poles of the earth to help me remember what a polar molecule is. A dipole-dipole force is when the positive side of a polar molecule attracts the negative side of another polar molecule. In order for this kind of bond to work, the molecules need to be very close to each other, like they are in a liquid.

Ion-Dipole Forces

Cations are atoms that have lost a valence electron, while anions have gained an electron
Anions and Cations

An ion-dipole force is just what its name says. It is a force between an ion and a dipole molecule. Remember that an ion is an atom that has gained or lost one or more electrons and therefore has a negative or positive charge. A cation is an atom that has lost a valence electron and therefore has more positive protons than negative electrons, so it is positively charged. An anion is an atom that has gained a valence electron and is negatively charged.

So knowing that a dipole molecule has a slight charge on either side and that ions have charges, either negative or positive, you can understand how these would bond together. Cations would be attracted to the negative side of a polar molecule and anions would be attracted to the positive side.

Hydrogen Bond

Simply put, a hydrogen bond is an attraction between a slightly positive hydrogen on one molecule and a slightly negative atom on another molecule. Hydrogen bonds are dipole-dipole forces. The large electronegativity difference between hydrogen atoms and several other atoms, such as fluorine, oxygen and nitrogen, causes the bond between them to be polar. The other atoms have more affinity for the shared electrons, so they become slightly negatively charged and hydrogen becomes slightly positively charged. Hydrogen atoms are small, so they can cozy up close to other atoms. This allows them to come very close to the slightly negatively charged unshared electron pair of a nearby atom and create a bond with it. A hydrogen bond is usually represented as a dotted line between the hydrogen and the unshared electron pair of the other electronegative atom. It looks like this.

Hydrogen bonds are the strongest of all intermolecular forces
Hydrogen Bond

Hydrogen bonds are the strongest of all intermolecular forces. They are extremely important in affecting the properties of water and biological molecules, such as protein. Water is a great example of hydrogen bonding. Water is a polar molecule composed of two hydrogens and one oxygen. The oxygen is slightly negative while the hydrogens are slightly positive. The hydrogens form hydrogen bonds with the oxygen of adjacent molecules. Then those molecules form more bonds with other adjacent water molecules. It looks like this.

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