Solubility:
Molecular Polarity
How do molecules stay
together?
Two main types of forces that hold molecules
together exist:
1) Intramolecular forces: (Intra = “within”).
Bonds found inside molecules.
Ex: Covalent and ionic bonds
Two main types of forces that hold molecules
together exist:
1) Intramolecular forces: (Intra = “within”).
2) Intermolecular forces: (Inter = “between”).
Bonds found between molecules. Also called
van der Waals forces.
Ex: London forces, Dipole-dipole forces
The covalent
intramolecular
bond in I2 is
very strong.
I
I
I
I
I
I
I
I
I
I
I
I
There are weaker
intermolecular
forces which hold
covalent molecules
together as a solid
or liquid.
Types of Intramolecular Bonds
• Ionic – transfer of e- from one atom to another
• Covalent - sharing of e- between atoms
a) nonpolar covalent – equal sharing of eb) polar covalent – unequal sharing of e- Polar bonds happen when the
two atoms attached to the bond
have very different
electronegativities.
Determining Polarity
Electronegativity
Difference
0-0.3
Type of Bond
0.4-1.9
Polar covalent
2.0 or greater
Ionic
Nonpolar covalent
Electronegativity of
Oxygen = 3.44
Electronegativity of
hydrogen = 2.22
Difference = 1.22
Electronegativity
Difference
Type of Bond
0-0.3
Nonpolar covalent
0.4-1.9
Polar covalent
2.0 or greater
Ionic
Electronegativity of
Oxygen = 3.44
Electronegativity of
hydrogen = 2.22
Difference = 1.22
Polar covalent bonds
between O and H.
Oxygen has a higher EN
(stronger attraction for
the e-). Electrons are
still shared, but spend
more time around the
oxygen.
Polar covalent bonds
One end of the water
molecule will be
partially negative, while
the other end will be
partially positive. This
separation in charge is
called a dipole.
EN of C = 2.5
EN of F = 4.0
Difference = 1.5
POLAR COVALENT
BOND
Nonpolar Bond (no dipole) vs. Polar
Bond (dipole)
+
+
-
Showing Polarity of a Bond
Dipole moment
Determining Polarity Without Knowing
the exact EN
• Polarity depends on the shape of the molecule
• Ex. CO2 (nonpolar) and CH2O (polar)
Steps:
1) Draw the Lewis dot structure.
2) Analyze each bond to see which atom the electrons
in the bond are more attracted to (more EN).
3) Draw your dipole moments.
4) If the dipole moments cancel out = nonpolar
If they do not = polar.
• Carbon dioxide has polar
bonds, but because they
cancel out, it is a non-polar
molecule.
• CH2O has two non-polar
bonds, but it has a net
dipole moment, so it is a
polar molecule.
Try: NH3
Net dipole
moment
This is a polar molecule (has a dipole).
Try: Carbon tetrafluoride
CF4
All moments
cancel each
other out – no
net moment.
This is a non-polar molecule (has no
dipole).
Easy Tips:
• If there are lone pairs on the central molecule – the
molecule is polar.
• If the molecule is asymmetrical – then it is polar.
• If the molecule is symmetrical, and does not have
lone pairs – it is non-polar.
Symmetrical – no
lone pairs, therefore,
non-polar.
You try: are the following molecules, polar, or nonpolar? Draw them out and show the dipole moments.
If polar, show which atom is partially negative, and
which is partially positive.
•
•
•
•
•
•
CHCl3
O2
SO3
CH4
AsI3
C2H6
Homework:
Are the following molecules, polar, or non-polar?
Draw them out and show the dipole moments. If polar,
show which atom is partially negative, and which is
partially positive.
1) Sulfur dioxide
2) Nitrogen dioxide (-1 charge)
3) 4) PH3
5) HCN
POLAR Bear
How do intermolecular (Van der
Waals) forces form?
They form when dipoles exist.
They determine whether a
compound is solid, liquid or gas at a
given temperature (affects the
melting and boiling points of a
substance)
There are three types of intermolecular
forces:
- Dipole-dipole forces
- Hydrogen bonds
- London Dispersion Forces
There are two types of dipoles:
- Temporary dipoles
- Permanent dipoles
1) Permanent dipoles
- When 2 atoms in a molecule have
very different electronegativities, a
permanent dipole is present.
Dipole-dipole force
Dipole-dipole forces form between
molecules with permanent dipoles.
Another type of intermolecular force
that forms between permanent dipoles:
hydrogen bonds.
Hydrogen bonds form between
molecules containing a
H-N, H-O, or H-F bond.
- It is the strongest type of
intermolecular force.
Hydrogen bond
What type of intermolecular force is
found between these polar molecules?
PH3
NH3
OF3
OH-
Temporary dipoles (induced dipoles):
Electrons are always moving around the
atom, so the electron distribution in an atom
may become unsymmetrical, causing a
dipole.
e+2
d+
eHe
d-
The random dipole created in one atom, can affect the
electron distribution in a nearby atoms.
Polarization occurs, and a temporary dipole forms.
e-
e-
+2
+2
e-
e-
He
d+
d-
d+
He
d-
The weak attractive force between the (+) side
of one molecule and the (-) side of another
molecule is called London Dispersion Forces
London Forces are short lived and temporary,
because the electrons are always moving.
e-
e-
+2
+2
e-
e-
He
d+
d-
d+
He
Polarization = the process of repelling a
nearby atom’s electrons.
d-
London Forces:
- They are present in every molecule.
Including covalent and ionic molecules.
- They are very weak
- The greater the number of electrons, the
stronger the London Force. So the
greater the atomic number, the stronger
the London Force.
Questions…
1) What happens to the strength of the London
forces between two identical atoms going:
a) Down a group?
b) left to right across a period?
What type of intermolecular force?
Strongest bond  Weakest bond
Strongest
Covalent bond
Ionic bond
Hydrogen bond
Dipole-dipole force
Weakest
London dispersion
force
Melting point: The temperature when a
solid becomes a liquid.
Boiling point: The temperature when a
liquid becomes a gas.
The stronger the intermolecular bonds,
the higher the melting and boiling
points.
Which has a higher boiling point?
HCl or F2?
Homework:
P181: #77
P202: #12
P203: #13, 14, 16