1. Intermolecular Forces
Attractions between molecules

2. Overview Molecules may be weakly attracted to each other.
There are various categories of intermolecular attractions.
Intermolecular attractions affect macroscopic properties of compounds.

3. Intermolecular attractions
Also called “van der Waals forces” or “weak forces”
Generally weak
Depend on several factors
Molecule size
Molecule polarity
Number of bonding electrons
Affects molecular properties
Boiling point, evaporation time, melting point, viscosity

4. Ion-dipole attractions
Strongest of the weak forces
Ions attract polar molecules
Important role in dissolving ions in water
Enough of these can break apart a crystal lattice

5. Dipole dipole attractions
Dipoles arrange themselves to maximize attractions & minimize repulsions
Strength depends on the nature of the dipoles involved
Example: using a magnet to induce a dipole in a nail

6. Hydrogen bonding Subset of dipole-dipole interactions
Important in protein, DNA structure
Hydrogen bonding in water
Hydrogen bonding in DNA

7. Dipole-Induced Dipole Attractions
Presence of a permanent dipole can “induce” a teporary dipole in another molecule
Temporary effect
Explains why O2, CO2 can dissolve in water
Visuals on next slide

8. Dipole-induced dipole interactions+
Spherical nonpolar atom
Ion causes temporary uneven distribution of electrons +
Cation approaches from a distance

9. Induced dipole-induced dipole attractions
Also “London dispersion forces”
Weakest of the intermolecular attractions
Randomness in electron motion can result in uneven electron distribution for a moment
Transient effect
This can temporarily induce a dipole in another molecule

10. Induced dipole-induced dipole attractions

11. Induced dipole-induced dipole attractions
Most significant for larger atoms
Compare iodine and fluorine molecules
I2 larger atoms solid at room temperature
F2 smaller atoms gas at room temperature

12. Effects of intermolecular attractions
Lots of intermolecular attractions  difficult to separate molecules
High boiling point
May be solid or liquid at room temperature
Few intermolecular attractions  easy to separate molecules
Low boiling point
May be gases or liquids with low boiling points (“volatile”)