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 bond
Intermolecular forces determine phase!

4. Dispersion forces ↑ as size molecule ↑
weakest IMF = dispersion forces
- occur between nonpolar molecules
Monatomic molecules: He, Ne, Ar, Kr, etc.
Diatomics of same element: O2, H2, N2
Symmetric molecules: C4H10
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 bonding occurs between molecules containing an H-F, H-O, or H-N bond
H-bonding: strongest IMF

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

8. mp, bp, Hf and Hv and vapor pressure depend on how hard it is to pull particles apart
Weak IMF – easy to pull particles apart
Strong IMF – more difficult to pull apart

9. Which substance has the strongest intermolecular forces?
The weakest?
Water
Ether