1. Properties of Covalent (Molecular) Substances

2. Properties
depend on strength of IMF between “particles” or separate units
covalent substances:
units are molecules

3. Intermolecular Forces
Dispersion forces occur between nonpolar molecules (Van der Waals)
Dipole-dipole forces occur between polar molecules
Hydrogen bonding occurs between molecules with an H-F, H-O, or H-N
Remember: IMF determine phase!

4. Dispersion forces ↑ as size molecule ↑
weakest IMF = dispersion forces
- occur between nonpolar molecules
monatomic molecules:
diatomics of same element:
symmetric molecules:
hydrocarbon molecules:
He, Ne, Ar, Kr
O2, H2, N2
CO2, CCl4, CF4
CH4, C4 H10
Reminder:
Dispersion forces ↑ as size molecule ↑

5. Dipole-dipole forces occur between:
- polar molecules (permanent charge separation)
HCl and HBr are examples of polar diatomic molecules

6. hydrogen bonds occur between one molecule containing an H with a neighboring molecule containing an F, O or N
 H-bonds: strongest IMF

7. Properties: Covalent (Molecular) Substances
poor conductors: heat & electricity
no charged particles!
low mp & low bp:
easy to pull molecules apart from each other
low Hf and Hv compared to ionic & metallic substances
high VP compared to ionic & metallic substances
majority of solids are soft

8. mp, bp, Hf and Hv & VP depend on how hard it is to separate particles from each other
strong IMF – difficult to separate particles
weak IMF – easy to separate particles

9. Which substance has the strongest IMF?
The weakest?
How know which is which?
Water
Ether