1. Electronegativity and Polarity

2. Nonpolar Covalent Bond
When electrons are shared between 2 atoms, a covalent bond is formed.
If the atoms are identical, e.g. Cl2, the electrons are shared equally (nonpolar)

3. Polar Covalent Bond
If the electrons are shared between 2 different atoms, e.g. HBr, the sharing is unequal
The bonding electrons spend more time near the more electronegative atom H Br

4. Electronegativity Differences
The absolute value of the difference in electronegativities of two bonded atoms provides a measure of polarity of a bond.
The greater the difference, the more polar the bond.
0 to 0.4
Nonpolar covalent
0.41 to 1.69
Polar covalent
> 1.7
Ionic
Electronegativity Difference

5. Polar Molecules
Note: Not all molecules with polar bonds are polar molecules

6. Practice
P. 253 # 1-3
P. 255 # 6-11

7. Intermolecular Forces

8. 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)

9. 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

10. van der Waals Forces
Dipole-dipole
London Dispersion

11. 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

12. 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

13. 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

14. 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

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

16. Hydrogen Bonding
Strong dipole-dipole force between the positive H atom and the lone pair of electrons

17. Properties of Gases and Liquids
Very weak intermolecular forces acting on gases
In liquids, intermolecular forces account for some special properties.

18. Surface Tension
Molecules in a liquid are attracted by molecules on all sides
But, molecules at the surface are only attracted down or sideways.
Penny drop, alka seltzer

19. Cohesion and Adhesion Cohesion: The attraction between like molecules
Adhesion: The attraction between unlike molecules.
Adhesion between water and glass is higher than cohesion between water molecules.
Cohesion between mercury is higher than adhesion between mercury and glass

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