Polarity and Intermolecular Forces
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.
Bond Polarity
 When two nonmetal atoms bond, they share electrons.
 They may or may not share electrons evenly.
 Consider the following molecules:
 F2
 HF
Bond Polarity
 H vs. F
 The EN of H is 2.2.
H
F
EN = 2.2
EN = 4.0
 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.
F
F
F
H
Bond Polarity
F
F
 EN is same for both atoms.
 e- density is spread evenly
around molecule.
 Highest e- density occurs
between atoms.
 Bond is nonpolar.
F H
 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.
Bond Polarity
 In general, a covalent bond is:
 polar if it occurs between two different atoms.
 nonpolar if it occurs between two identical atoms.
Dipole Moments
 Dipole Moment - a measure of the polarity of a bond.
 Is often represented by a special arrow.
H
F
Arrow points toward
more EN atom.
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?
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.
CO2 vs. H2O
 Consider the Lewis structure of CO2:
This molecule is nonpolar.
CO2 vs. H2O
 Consider the Lewis structure of H2O:
This molecule is polar.
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?
H
HCH
H
H
H C Cl
H
“Like Dissolves Like”
 Polar molecules mix with each other.
 Nonpolar molecules mix with each other.
 Polar and nonpolar molecules do not easily mix.
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?
Grease and Water Don't Mix!
Amphipathic Molecules
Hydrophilic head
Hydrophobic tails
Amphipathic Molecules
Intermolecular Forces
 Intermolecular force - a force between two molecules that
does not result from chemical bonding.
 Dipole-dipole interaction.
 Hydrogen bonding.
 London force.
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.
Dipole-Dipole Interactions
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.
Hydrogen Bonding
Boiling Points of Several Compounds
150
Boiling Point (ºC)
100
50
0
-50
-100
-150
-200
-250
H2O
H2S
H2Se
Compound
H2Te
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.
London Forces
+
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.
London Forces
Boiling Points of the Noble Gases
Boiling Point (Kelvins)
250
200
150
100
50
0
He
Ne
Ar
Kr
Noble Gas
Xe
Rn