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 non-polar molecules (Van der Waals)
dipole-dipole forces occur between polar molecules
hydrogen bonding forces occur between molecules with H-F, H-O, or H-N
Remember: IMF determine phase!

4. dispersion forces ↑ as size molecule ↑
weakest IMF = dispersion forces
- occur between non-polar 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. If a covalent molecule doesn’t meet the requirements for a non-polar substance
than it is polar
and
will have dipole-dipole IMF or H-bonding forces

6. 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 with ionic & metallic substances
high VP:
majority of solids are soft

7. mp, bp, Hf and Hv & VP depend on how difficult it is to separate particles from each other
strong IMF – difficult to separate particles (more energy)
weak IMF – easy to separate particles (less energy)

8. Which substance has the strongest IMF?
The weakest?
How know which is which?
water
ether