1. Intermolecular Forces

2. Intramolecular Forces:
The attractive forces between atoms and ions within a molecule
e.g ionic, covalent, metallic
Strong
Intermolecular Forces:
The attractive forces between molecules
E.g. Van der Waals forces (London dispersion forces, dipole-dipole forces), hydrogen bonds
Weak (in comparison to intramolecular forces)
I.e. much less energy to melt H2O (inter) than for it to decompose into H2 and O2 (intra)

3. Intermolecular Forces
If covalent bonds were the only forces at work, most molecular compounds would be gases as there would be no attraction between molecules strong enough to group the molecules as liquids or solids

4. van der Waals Forces
Dipole-dipole
London Dispersion

5. Dipole-dipole
Forces of attraction between oppositely charged ends of polar molecules.
The strength of the dipole-dipole force is dependent on the polarity of the molecule

6. London Dispersion
Attractive forces between all molecules, including nonpolar molecules
Result of temporary displacements of the electron cloud around atoms in a molecule (extremely short-lived dipoles)
Strength depends on number of electrons in a molecule
Therefore weaker than dipole-dipole

7. London Dispersion Forces
Strength depends on
number of electrons, more electrons more chances for instantaneous dipoles
molecular shape
bp = 36˚C ˚C ˚C
Tetris Analogy

8. Intermolecular Forces and Boiling Point
The stronger the intermolecular forces the higher the boiling point.
The more polar the molecule, the higher the boiling point
More electrons = more LDF = higher boiling point

9. Boiling Points
Unexpectedly high boiling points of hydrogen compounds containing nitrogen, oxygen, and fluorine
Additional forces must exist

10. Hydrogen Bonding
Strong dipole-dipole force between the positive H atom and the lone pair of electrons
H has no electrons other than the bonding electrons, it is a “bare-naked” nucleus

11. Structure Dictates Melting/Boiling Point Explain
Compound
Molar Mass (g/mol)
Number of Electrons
Melting Point (ºC)
Boiling Point (ºC)
water
ammonia
-77.7
-33.3
methane
-182.5
-161.6

12. Practice
P. 260 # 1-4
P. 264 # 9, 11, 12
P. 266 #1-7