2. Review
We know how to draw Lewis structures for simple
molecules and polyatomic ions.
We also know how to predict the 3-D geometry of
these molecules and ions, if we apply the VSEPR
Theory.
Electronegativity (EN) is an atom's tendency to
attract electrons in chemical bonds.
EN increases to the right and up on the periodic table,
excluding the noble gases.
3. Bond Polarity
When two nonmetal atoms bond, they share
electrons.
They may or may not share electrons evenly.
Consider the following molecules:
F2
HF
4. Bond Polarity
H vs. F
The EN of H is 2.2.
The EN of F is 4.0.
In F2, both atoms pull with equal strength on the
bonding e-.
The e- are shared evenly between them.
In HF, the F atom pulls harder than the H atom.
The e- are drawn more toward the F atom.
H
EN = 2.2
F
EN = 4.0
F F F H
5. Bond Polarity
EN is same for both
atoms.
e- density is spread
evenly around
molecule.
Highest e- density
occurs between atoms.
Bond is nonpolar.
EN is much higher for F
than for H.
e- density is drawn
toward F side.
F atom acquires partial
negative charge.
H atom acquires partial
positive charge.
Bond is polar.
F F F H
6. Bond Polarity
In general, a covalent bond is:
polar if it occurs between two different atoms.
nonpolar if it occurs between two identical atoms.
7. Dipole Moments
Dipole Moment - a measure of the polarity of a
bond.
Is often represented by a special arrow.
F
H
Arrow points toward
more EN atom.
8. Polarity of Diatomic Molecules
Diatomic Molecules - molecules made of only two
atoms.
If atoms are the same, molecule is nonpolar.
If atoms are diff., molecule is polar.
NOTE: Polar does not mean charged.
Is Cl2 polar or nonpolar?
Is CO polar or nonpolar?
9. Molecules With 3 or More Atoms
A molecule with 3 or more atoms is:
Polar if its central atom has lone pairs OR
If the outer atoms are not all the same.
Nonpolar if its central atom has no lone pairs AND
All the outer atoms are identical.
10. CO2 vs. H2O
Consider the Lewis structure of CO2:
This molecule is nonpolar.
11. CO2 vs. H2O
Consider the Lewis structure of H2O:
This molecule is polar.
12. CH4 vs. CH3Cl
Neither CH4 nor CH3Cl has any lone pairs on the
central carbon atom.
Is CH4 polar or nonpolar?
Is CH3Cl polar or nonpolar?
C C
H
H
H
H
Cl
H
H
H
13. “Like Dissolves Like”
Polar molecules mix with each other.
Nonpolar molecules mix with each other.
Polar and nonpolar molecules do not easily mix.
14. Amphipathic Molecules
Amphipathic - has a hydrophobic region and a
hydrophilic region.
Hydrophobic - “water-fearing”
Nonpolar.
Hydrophilic - “water-loving”
Polar or charged.
Dish detergents contain amphipathic molecules.
Why?
18. Intermolecular Forces
Intermolecular force - a force between two
molecules that does not result from chemical
bonding.
Dipole-dipole interaction.
Hydrogen bonding.
London force.
19. Dipole-Dipole Interactions
Dipole - polar molecule.
Like magnets, except poles are + and - instead of N
and S.
Polar molecules generally have higher melting and
boiling points than similar nonpolar molecules.
EXAMPLE: O2 (nonpolar) boils at -183ºC.
EXAMPLE: NO (polar) boils at -152ºC.
NO has a higher boiling point due to its polarity.
Still far below the boiling point of any ionic cmpd.
21. Hydrogen Bonding
Hydrogen bond - a stronger form of dipole-dipole
interaction.
Occurs in molecules that have H atoms bonded to O, N,
or F atoms.
The small size of the H atom allows these molecules
to get closer together.
Closer together = stronger forces.
EXAMPLE: H2O has a boiling point of 100ºC.
EXAMPLE: H2S has a boiling point of -60ºC.
The b.p. of H2O is higher b/c of hydrogen bonding.
22. Hydrogen Bonding
Boiling Points of Several Compounds
-250
-200
-150
-100
-50
0
50
100
150
H2O H2S H2Se H2Te
Compound
Boiling
Point
(ºC)
23. London Force (Dispersion)
London force - attraction between temporary
dipoles.
e- move randomly around molecules.
Nonpolar molecules become temporarily polar.
Allows for very weak attractions between nonpolar
molecules.
Named for Fritz London.
25. London Forces
The more e- a molecule has, the greater its London
forces are.
Large molecules tend to have higher melting/boiling
points than small molecules.
London forces apply to all molecules.
26. London Forces
Boiling Points of the Noble Gases
0
50
100
150
200
250
He Ne Ar Kr X
e Rn
Noble Gas
Boiling
Point
(Kelvins)
