Van der Waal Forces of Attraction In covalent compounds the bonds within a molecule are quite strong but the attraction between different molecules is relatively weak. These weak forces of attraction between individual molecules are known as intermolecular forces (IMFs) or Van der Waal Forces. There are three kinds of Intermolecular forces – London Dispersal Forces, Dipole-Dipole Interactions, and Hydrogen Bonds.

London Dispersal Forces London Dispersal Forces are the weakest of the intermolecular forces and are caused by the temporary shifts of electrons in the electron clouds. These occur when nonpolar molecules are attracted to other nonpolar molecules.

Dipole-Dipole Interactions are attractions between oppositely-charged regions of polar molecules which allows one polar molecule to “stick” to another polar molecule.

Hydrogen Bonding is a special type of dipole-dipole interaction between a hydrogen on one polar molecule and an nitrogen, oxygen, or fluorine on an other polar molecule.

Hydrogen Bonding examples The best example of hydrogen bonding is water The strong IMFs create LARGE amounts of surface tension Explains why you can float a paper clip on the surface of water, but it will sink if it’s already under it. Aquatic bugs use this to walk on water Also explains why water is the only substance where the solid form (ice) floats on top of the liquid form.

Diagram/Summary of IMFs Hydrogen bonding – H, and N or O or F Dipole-Dipole forces – all polar compounds Polar Molecules London Dispersion forces – all compounds

IMFs and Phase Changes IMFs determine if the compound would rather be a solid, liquid, or gas at room temperature Nonpolar molecules can’t form dipole-dipole interactions or H-bonding, so they are most often found as gases Extremely polar molecules are more likely to be found as liquids because they are held more tightly together due to dipole-dipole interactions Molecules that have H-bonding (and therefore all the rest) are almost always a liquid because of how close together the molecules naturally align

IMFs and Phase Changes Also determine melting and boiling pts of the compound More kinds of IMFs means harder to change into a looser or more spread-out phase Also play a role in viscosity, surface tension, and capillarity. Let’s watch and see some of the cool stuff water can do with all of its IMFs!