Intermolecular
Forces of
Attraction
Learning Targets:
Determine the intermolecular
forces of a substance
Use intermolecular forces to
predict physical properties such as
viscosity, surface tension, melting
point, boiling point, and state of
matter at room temperature
Intermolecular
Forces of
Attraction
definition: a force of attraction
between molecules of the same
type
Intermolecular Forces
of Attraction
• Bonds are INTRAmolecular
forces because they
chemically combine one atom
to another atom
• INTERmolecular forces do not
chemically combine anything.
But they do help one molecule
“stick” to another
Intermolecular Forces
of Attraction
• Three intermolecular forces
– London Dispersion
– Dipole-Dipole
– Hydrogen Bonding
• One strong force (not
technically an IMF, but an
actual bond!)
– Ionic
London Dispersion
• Weakest of the weak
intermolecular forces
• Caused by random and
temporary movement of
electrons
• Short-lived, temporary force
• More electrons = greater
attractions
London Dispersion - A Short Visual
Molecule 1
e-
δ+
+
+
Molecule 2
e-
eδe-
ee-
δ+
+
e-
+
e-
e-
+
e-
Molecule 3
e-
+
e-
e-
+
e-
e-
+
e-
e-
+
e-
e-
+
e-
δδ+
+
δ+
+
e-
+
ee-
δ-
ee-
δ-
e-
δ+
δ+
+
+
ee-
δ-
ee-
δ-
• Three
Molecules
• When one
molecule
has a
temporary
imbalance
in electrons
• the other
molecules
respond to
the
imbalance
London Dispersion – Practice
• Which has stronger London
dispersion forces, F2 or Br2?
F
F
Br
Br
– Which molecule has more total
electrons?
• Notice these molecules have the same
number of valence electrons.
• Go to the periodic table and look how
many total electrons they have… F2 has
9+9=19 while Br2 has 35+35=70.
• Br2 has more total electrons, so its
London dispersion force is greater!
London Dispersion – Practice
• Which has stronger London
dispersion forces, CF2O or
F
SiF2O?
F
F
C
O
F
Si
O
– Which molecule has more total
electrons?
• Even though they have the same number
of valence electrons, SiF2O has more
total electrons, so its London dispersion
force is greater!
Dipole-Dipole
• Still a very weak force, but
stronger than London
dispersion
• Molecules must be
permanently polar with one
positive and one negative side
• The positive side of one
molecule attracts the negative
of another molecule
Dipole-Dipole - A Short Visual
δ+
δ-
δ+
• One end of a
molecule is
permanently
δpositive and
one end is
permanently
negative
• Opposites
attract
Dipole-Dipole - Another Short Visual
• A quick visual of what
this might look like…
• Notice there is not a
regular pattern of the
molecules - the
attraction is not
strong enough to hold
the molecules in a
fixed position.
Dipole-Dipole – Practice
• Which intermolecular force(s)
are in SO2?
– Draw the Lewis structure:
O
S
O
• What is the class, type, and shape?
– Class: 3, Type: CA2E, Shape: bent
• What is the polarity?
– Polar (the shape is polar, based on the CA2E
shape)
• What IMF’s will this molecule have?
– London dispersion AND Dipole
Dipole-Dipole – Practice
• Which intermolecular force(s)
are in SiS2?
– Draw the Lewis structure:
S
Si
S
• What is the class, type, and shape?
– Class: 2, Type: CA2, Shape: linear
• What is the polarity?
– Nonpolar (all attached atoms are the same)
• What IMF’s will this molecule have?
– London dispersion only!
Hydrogen Bonding
• A weak force of attraction, but
strongest compared to the
previous two (London dispersion
and dipole-dipole)
• Occurs when the most
electronegative elements – N, O,
and F – are directly bonded to H.
• High electronegative elements
attract electrons so strongly that
the molecule becomes very polar
Hydrogen Bonding
• The non-bonding pairs of electrons on
the N, O, or F are strongly attracted to
the highly positive H on another
molecule.
•
A “quarter bond” or “half bond” is formed
between the H and the pair of electrons,
resulting in a much stronger force than
the previous forces.
• Requirements (both are necessary!)
– H bonded to N, O, F
– Non-bonded pair of electrons on the
N, O, or F
Hydrogen Bonding – A Short Visual
δ-
• The very polar
molecule has a
very negative
end and a very
positive end.
O
H
δ+
H
δO
H
δ+
H
• The negative
electron pairs
are attracted to
the positive
hydrogen.
Hydrogen Bonding – Another Visual
• Here is an
example of
hydrogen
bonding at work!
• Notice how the
positive
hydrogens are
attracted to the
negative electron
pairs on the
oxygen!
• This is how ice
forms.
Hydrogen Bonding – Another Visual
• Examples:
– H2O
H
H
O
O
H
H
H
– CH3OH
H
H
H
O
C
O
C
H
H
H
H
H
– HNO
H
N
N
O
O
• The non-bonding pairs of electrons on
the N, O, or F are strongly attracted to
the H on another molecule.
Hydrogen Bonding – Practice
• Which intermolecular force(s)
H
are in NH3?
• Draw the Lewis Structure H N H
• What is the class, type, and shape?
– class: 4, type: CA3E, shape: pyramidal
• What is the polarity?
– Polar (since all attached atoms are the
same, the pyramidal shape is polar)
• Is H attached to N, O, or F?
– yes
• What IMF’s will this molecule have?
– London dispersion, Dipole, AND
Hydrogen bonding
Hydrogen Bonding – Practice
– Which intermolecular forces(s)
are in PH3?
H
• Draw the Lewis Structure
H P H
• What is the class, type, and shape?
– class: 4, type: CA3E, shape: pyramidal
• What is the polarity?
– Polar (since all attached atoms are the
same, the CA3E shape is polar)
• Is H attached to N, O, or F?
– No, so no hydrogen bonding exists
• What IMF’s will this molecule have?
– London dispersion and Dipole
Hydrogen Bonding – Practice
– Which intermolecular forces(s)
are in NH4+1?
H
[
]
• Draw the Lewis
H N H
Structure
H
• What is the class, type, and shape?
– class: 4, type: CA4, shape: tetrahedral
• What is the polarity?
– NONE!!! Ions have no polarity.
• What IMF’s will this molecule have?
– NONE!!! Why would a + molecule be
attracted to another + molecule?
+1
Ionic
• This is an intra molecular force,
NOT an intermolecular force of
attraction!
• Actual bonds are formed between
molecules forming a crystal lattice
(or network) of atoms.
• Attractive forces between positive
and negative ions are as strong
between molecules as within
molecules.
Ionic – A Short Visual
• Na+ and Cl- ions
are attracted to
each other
because opposite
charges attract!
• A network of
positive/negative
alternating
charges begins
to form.
Putting it all together!
• How do you determine which
intermolecular forces a
compound has?
– First, determine whether the
compound is made of ionic or
covalent bonds.
• Ionic compounds will have ionic
attractions.
• Covalent compounds will have
some sort of Intermolecular force.
Putting it all together!
• What forces does Li3N have?
– Is the compound made of ionic
or covalent bonds?
• Lithium is a metal
• Nitrogen is a nonmetal
• So the compound has ionic
attractions.
Putting it all together!
• All covalent molecules have
London dispersion forces.
– Every covalent molecule has
electrons moving around.
– London dispersion forces are
made when those electrons
move.
Putting it all together!
• To determine if the molecule
has Dipole-Dipole or
Hydrogen Bonding, draw the
Lewis dot structure.
– A polar molecule will have
Dipole-Dipole, in addition to its
London dispersion forces.
Putting it all together!
• If the molecule is polar and
has Dipole-Dipole forces,
determine whether there is a
bond between H–O, H–N,
and/or H–F.
• This will mean that the
molecule also has Hydrogen
Bonding.
Putting it all together!
• What forces does CH4 have?
– Is the compound made of ionic
or covalent bonds?
• Covalent
– Draw the Lewis Dot Structure
H
H C H
H
– Is the molecule polar or nonpolar?
• Nonpolar = London Dispersion only
Putting it all together!
• What forces does CH3OH have?
– Is the compound made of ionic
or covalent bonds?
• Covalent
– Draw the Lewis Dot Structure
H
H C O H
H
–
or nonpolar?
– Is
Sothe
thismolecule
moleculepolar
has all
three
• Polar = Londonforces:
Dispersion
and
intermolecular
London,
Dipole-Dipole
Dipole,
and Hydrogen
– Is there a H bonded to O, N, or F?
• Yes = Hydrogen Bonding
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– Viscosity
• The measure of a fluid’s
resistance to flow
• Honey is more viscous than water
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– Surface Tension
• The measure of how well
molecules can attract each other
at the surface of a liquid
• Water bugs can crawl across the
surface of ponds because water
has a high surface tension
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– Melting point
• The temperature at which a solid
turns into a liquid
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– Boiling point
• The temperature at which the
vapor pressure of a liquid equals
the pressure surrounding the
liquid – simply put, the
temperature at which a liquid
changes into a gas
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– State of matter
• Whether the compound exists as
a gas, liquid, or solid at room
temperature
State of Matter (cont.)
• From Weakest to Strongest:
– London Dispersion (tend to be
gases or sometimes liquids)
– Dipole (tend to be gases or
often as liquids)
– Hydrogen Bonding (tend to be
liquids or sometimes solids)
• Strongest Force (not an IMF):
– Ionic (are all solids)
Physical Properties
• Intermolecular forces help
determine many physical
properties of compounds.
– Higher intermolecular forces
cause greater viscosity, greater
surface tension, and higher
melting and boiling points