1. Intermolecular Forces of Attraction (IMFs)

2. Definitions
IMF: a force of attraction between molecules of the same type
Bonds are INTRAmolecular forces because they chemically combine one atom to another atom
•INTERmolecular forces do not chemically combine anything. But they do help one molecule “stick” to another

3. Types of IMFs London Dispersion Ion-dipole Dipole-Dipole
Hydrogen Bonding
• One strong force (not technically an IMF, but an actual bond!)  Ionic

4. London Dispersion Forces
Weakest of the weak intermolecular forces
Caused by random and temporary movement of electrons
Short-lived, temporary force
More electrons = greater attractions

5. Ion-dipole Forces
Occur when an ion interacts with a polar substance.

6. Dipole-Dipole
Still a very weak force, but stronger than London dispersion
Molecules must be permanently polar with one positive and one negative side
The positive side of one molecule attracts the negative of another molecule

7. Dipole-Dipole Continued
One end of a molecule is permanently positive and one end is permanently negative
Opposites attract

8. Hydrogen Bonding
A weak force of attraction, but strongest compared to the previous two (London dispersion and dipole-dipole)
Occurs when the most electronegative elements – N, O, and F – are directly bonded to H. (H-bonding is FON!!!)
High electronegative elements attract electrons so strongly that the molecule becomes very polar

9. H-Bonding Continued
The non-bonding pairs of electrons on the N, O, or F are strongly attracted to the highly positive H on another molecule.
A “quarter bond” or “half bond” is formed between the H and the pair of electrons, resulting in a much stronger force than the previous forces.
Requirements (both are necessary!) H bonded to N, O, F
Non-bonded pair of electrons on the N, O, or F

10. Ionic
This is an intra molecular force, NOT an intermolecular force of attraction!
Actual bonds are formed between molecules forming a crystal lattice (or network) of atoms.
Attractive forces between positive and negative ions are as strong between molecules as within molecules.

11. So Now What? – Let’s put it all together!
How do you determine which intermolecular forces a compound has?
First, determine whether the compound is made of ionic or covalent bonds.
Ionic compounds will have only ionic attractions.
Covalent compounds will have some sort of Intermolecular force.

12. All covalent molecules have London dispersion forces
All covalent molecules have London dispersion forces. Every covalent molecule has electrons moving around.
London dispersion forces are made when those electrons move.
You need to draw an accurate LEWIS structure to verify if the molecule is POLAR or NON-POLAR

13. If the molecule is polar it has Dipole-Dipole forces, but if it also contains a bond between H–O, H–N, and/or H–F, then it also has Hydrogen Bonding.
If the molecule is non-polar, it only has London Dispersion forces

14. Physical properties depend on these forces
Physical properties depend on these forces. The stronger the forces between the particles,
(a) the higher the melting point.
(b) the higher the boiling point
(c) the lower the vapor pressure (partial pressure of vapor in equilibrium with liquid or solid in a closed container at a fixed temperature).
(d) the higher the viscosity (resistance to flow).
(e) the greater the surface tension (resistance to an increase in surface area)

15. Effect of Intermolecular Forces on the Melting/Boiling Points of Covalent Compounds
Melting/Boiling results from a weakening of the attractive forces between covalent molecules. The stronger the forces, the higher the melting/boiling points.
If the molecule is non-polar, the more electrons it has the higher its melting/point due to only having London dispersion forces acting on it.
If the molecule is polar – but does not contain H-bonding to F, O or N, it’s melting point will be higher than a non-polar substance, but lower that one that has H-bonding or is ionic.