1. Unit 6: Covalent Bonding Intermolecular Forces

2. Intra- versus Inter- molecular Forces Intra (means “within”) and refers to the forces that hold atoms together in a molecule. –Covalent –Ionic –Metallic Inter (means “between”) and refers to the forces that hold billions of molecules together –A glass of H 2 O

3. Definition of IMF Attractive forces between molecules. Much weaker than chemical bonds within molecules.

4. Intermolecular Forces (IMF) The connections that are broken when a solid melts to a liquid and a liquid turns into a gas. They are much weaker than intramolecular bonds. There are 3 main types of IMF’s and all depend on the polarity of the molecule. The more polar a molecule is the “stickier” it is, and thus it is more attracted to the molecules near it.

5. Dipole Dipole Forces Force that holds polar molecules together. The positive end of a molecule is attracted to the negative end of another molecule. These forces have ~10% the strength of an ionic bond. +-+-+-+-+-

6. Dipole Interactions    

7. Hydrogen Bonding A special type of dipole dipole force that occurs between Hydrogen in one molecule and a Nitrogen, Oxygen or Fluorine atom in another molecule. H-bonding is the strongest type of intermolecular force and gives water an extremely high boiling point.

8. Van Der Waals (Dispersion) Forces These forces hold non-polar molecules together. At any given moment electrons distribution within a molecule can be uneven causing a momentary dipole. -- ++ Creates temporary dipole

9. London Dispersion Forces/Van Der Waals Present in all atoms and molecules Major type of IMF for non-polar molecules View animation online.animation

10. Van Der Waals (Dispersion) Forces They are the weakest forces These forces account for the lower boiling and melting points for non-polar covalent molecules because they are so weak. Molecular weight is also a factor for non- polar molecules Low MW – less dispersion  Gas! Higher MW – more dispersion  Liquid!

11. Determining IMF – Depends on Polarity NCl 3 –polar = dipole-dipole = dipole-dipole CH 4 –Nonpolar = dispersion HF Polar – with hydrogen attached to small electronegative atom = hydrogen bonding

13. IMF tutorial link IMF tutorial link