Section 5.5—Intermolecular
Forces
Intra- versus Inter-molecular Forces
So far this chapter has been discussing
intramolecular forces
Intramolecular forces = forces within the
molecule (chemical bonds)
Now let’s talk about intermolecular forces
Intermolecular forces = forces between
separate molecules
Breaking Intramolecular forces
Breaking of intramolecular forces (within
the molecule) is a chemical change
2 H2 + O2  2 H2O
Bonds are broken within the molecules and new
bonds are formed to form new molecules
Breaking Intermolecular forces
Breaking of intermolecular forces
(between separate molecules) is a
physical change
Breaking glass is breaking the intermolecular
connections between the glass molecules to
separate it into multiple pieces.
Boiling water is breaking the intermolecular
forces in liquid water to allow the molecules to
separate and be individual gas molecules.
London Dispersion Forces
All molecules
have
electrons.
+
Positively charged nucleus
+
-
-
Negatively charged electron
-
-
This lop-sidedness of
electrons creates a
partial negative
charge in one area
and a partial
positive charge in
another.
Electrons move
around the
nuclei. They
could
momentarily
all “gang up”
on one side
Electrons are
fairly evenly
dispersed.
-
-
+
+
As electrons
move, they
“gang up” on one
side.
London Dispersion Forces
Once the electrons have “ganged up” and
created a partial separation of charges,
the molecule is now temporarily polar.
The positive area of one temporarily polar
molecule can be attracted to the negative
area of another molecule.
+
-
+
-
Strength of London Dispersion Forces
Electrons can
gang-up and
cause a nonpolar molecule
to be
temporarily
polar
The electrons
will move
again,
returning the
molecule back
to non-polar
The polarity was
temporary,
therefore the
molecule
cannot always
form LDF.
London Dispersion Forces are the weakest of the intermolecular
forces because molecules can’t form it all the time.
Strength of London Dispersion Forces
All molecules have electrons…all molecules can have London
Dispersion Forces
Larger molecules
have more
electrons
The more
electrons that
gang-up, the
larger the
partial negative
charge.
The larger the
molecule, the
stronger the
London
Dispersion
Forces
Larger molecules have stronger London Dispersion Forces than
smaller molecules.
Dipole Forces
Polar molecules have permanent partial
separation of charge.
The positive area of one polar molecule
can be attracted to the negative area of
another molecule.
+
-
+
-
Strength of Dipole Forces
Polar molecules
always have a
partial
separation of
charge.
Polar molecules
always have
the ability to
form
attractions
with opposite
charges
Dipole forces are
stronger than
London
Dispersion
Forces
Hydrogen Bonding
 Hydrogen has 1 proton and 1 electron.
There are no “inner” electrons. It bonds with the only
one it has.
 When that electron is shared unevenly (a polar
bond) with another atom, the electron is farther
from the hydrogen proton than usual.
This happens when Hydrogen bonds with Nitrogen,
Oxygen or Fluorine
 This creates a very strong dipole (separation of
charges) since there’s no other electrons around
the hydrogen proton to counter-act the proton’s
positive charge.
Strength of Hydrogen Bond
Hydrogen has no
inner
electrons to
counter-act
the proton’s
charge
It’s an extreme
example of
polar bonding
with the
hydrogen
having a large
positive
charge.
This very positivelycharged hydrogen
is highly attracted
to a lone pair of
electrons on
another atom.
This is the strongest of all the intermolecular forces.
Hydrogen Bond
N
H
H
Hydrogen bond
N
H
H