1. Objectives:
Forces of attraction between particles are important in determining the macroscopic properties of a substance, including how the observable physical state changes with temperature.

2. Zooming Out to the Macroscopic World …
Coulomb’s Law and its effects continue to be seen as we change perspectives.
Atomic Structure

3. Zooming Out to the Macroscopic World …
Coulomb’s Law and its effects continue to be seen as we change perspectives.
Ionic Bonding
Covalent Bonding
Intramolecular Forces – Attractive forces that occur between atoms, creating new substances called compounds (chemical bonds)

4. Zooming Out to the Macroscopic World …
Coulomb’s Law and its effects continue to be seen as we change perspectives.
Intermolecular Forces – Attractive forces that occur between separate molecules (weak compared to chemical bonds)

5. IM Forces between polar molecules:
When one polar molecule is near another polar molecule, they are attracted to one another
see hydrogen fluoride, HF
This is called a dipole-dipole interaction
Properties of substances that experience dipole-dipole forces:
State of matter?
Melting point?
Boiling point?

6. IM Forces between polar molecules:
1. Dipole-dipole interaction (polar molecules)
1a. Hydrogen Bond (very polar molecules)
A special type of dipole-dipole interaction that occurs when H is covalently bonded to O, N or F
Electronegativities:
(2.1)
(3.5)
(3.0)
(4.0)

7. IM Forces between polar molecules:
1. Dipole-dipole interaction (polar molecules)
1a. Hydrogen Bond (very polar molecules)
A special type of dipole-dipole interaction that occurs when H is covalently bonded to O, N or F
So not all dipole-dipole interactions are created equal.
The more polar the molecule is, the stronger the dipole-dipole interaction.

8. H – F H – F H – Cl H – Cl H – Br H – Br
IM Forces between polar molecules:
Which dipole-dipole attraction is strongest? Which, if any, is a hydrogen bond?
H – F
H – F
H – Cl
H – Cl
H – Br
H – Br
The more polar the molecule is, the stronger the dipole-dipole interaction.

9. IM Forces: 1. Dipole-dipole interaction (polar molecules)
1a. Hydrogen Bond (very polar molecules)
2. London Dispersion interactions (all molecules)
AKA induced dipole-induced dipole
Explains why Br2 is a liquid and I2 is a solid at room T
All molecules will experience LD forces, but …
it is usually too weak to make a difference for polar molecules.

10. IM Forces: 1. Dipole-dipole interaction (polar molecules)
1a. Hydrogen Bond (very polar molecules)
2. London Dispersion interactions (all molecules)
The strength of LD forces depends greatly on the size of the electron cloud around each species
The larger the electron cloud, the more polarizable the molecule, the stronger the LD Force.

11. F – F F – F Cl – Cl Cl – Cl Br – Br Br – Br IM Forces:
Which London dispersion force is strongest?
F – F
F – F
Cl – Cl
Cl – Cl
Br – Br
Br – Br
The larger the electron cloud, the more polarizable the molecule, the stronger the LD Force.

12. What type of IM force will be most important in each of the following substances:
Carbon tetrachloride, CCl4 Methanol, CH3OH
Fluorine gas, F2 Carbon dioxide gas
Formaldehyde, CH2O Sulfur hexafluoride, SF6
Sulfur dioxide gas, SO2 Liquid nitrogen, N2