(This lecture is meant to be an approx. 10 min review of intermolecular forces for AP Chemistry students in their first week of class.)

*Today's Objective*
You will be able to describe the causes and strengths of intermolecular forces.

*Elicit prior knowledge discussion*
What do you see in this picture? Where have you seen something like this before? What's causing the water to behave this way?

*Lecture*
Intermolecular forces (IMFs) are attractions between particles like molecules. They are caused by attraction of positive and negative charges caused by the movement of electrons. They are different from bonds, which are INTRAmolecular forces (forces inside molecules).

We will explain this picture later.

Image CC BY-NC-ND 2.0. Erwin, N. Spring rain [Image]. Retrieved from https://flic.kr/p/U5io6o

You're familiar with ionic bonds, which form between a metal and nonmetal. An ionic bond is made when electrons are transferred between atoms. The resulting ions are very strongly attracted because of the + and - charges they have. Ionic bonds are a very strong kind of intermolecular force because we can imagine the ions as separate particles.

Image adapted from OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3A207_Ionic_Bonding-01.jpg

Metals conduct electricity. That means electrons can flow by them. The valence electrons from metals can move around a collection of atoms. This is called the sea of electrons. Because the atom cores are positive, they attract to the sea. Metallic bonding is strong because the positive cores attract to the negative sea of electrons. Metals are solid because of the strong attraction. The atoms can also usually move around each other, explaining why many metals are bendable.

Image from Haiku Deck search.

Water is strongly attracted to other water molecules because of hydrogen bonding. This is a special kind of strong IMF that's caused by H attracting to a N, O, or F on another molecule.

Water has charges because of the electronegativity difference between O and H. Because of water's bent shape, the O end of the molecule is partially - and the H end is partially +. This makes a dipole and the molecule is called polar. Dipole-dipole attractions are weaker attractions than ion-ion; therefore, water is a liquid at room temperature instead of a solid.

The hydrogen bonding is even smaller than dipole-dipole attractions, but weaker than ionic and metallic bonding or ion-dipole attractions.

Image from Haiku Deck search.

Van der Waals forces is the name given to attractions between partially charged particles.

The picture shows two kinds of particles - red water molecules and a green chlorine molecule. They are attracted to each other because of the partial charges in the molecules. We already discussed why water is polar.

Cl2 has no electronegativity difference between the atoms in it. One end of the molecule is near a partially + H and the other end is near partially - O. Some electrons in its cloud move to be near the positive H. As a result, the chlorine gets an induced dipole with weak partial charges. The top end is partially - and the bottom end is partially +. Cl2 attracts to other Cl2 very weakly. This is why Cl2 is a gas instead of a liquid or sold.

Water is more attracted to itself than it is to chlorine. Dipole-dipole attractions between H2O and H2O are stronger than dipole-induced dipole attractions like when Cl2 is dissolved in H2O. Only small numbers of chlorine molecules could attract to water molecules.

Image By Riccardo Rovinetti (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3AForze_di_Debye.png

Induced dipoles are also called dispersion forces. If the particle speed slows down enough at low temperatures, even weak attractions can cause the molecules to stick together. Nitrogen is normally a gas, but at very low temperatures it can attract to form a liquid.

Image CC BY-NC-SA 2.5. Naturally On The Edge at Amherst College. Retrieved from http://www3.amherst.edu/~note/hdd-destruction/media/nitrogen/nitro07.jpg

Wrapping up, think back to the image at the beginning. Water forms droplets because the water molecules are attracted to each other. They form dipoles with partial charges. The leaf itself must not be polar, so it doesn't attract to the water as much as the water attracts to itself.

Image from Haiku Deck search.