Intermolecular forces


Intermolecular forces: the forces between
(among) individual particles (atoms, ions,
molecules); they are weak relative to
intramolecular forces (i.e. covalent and ionic bonds
within a compound)
Intermolecular forces are increasingly significant
in this order:
 GAS < LIQUID < SOLID
Strength of Intermolecular forces
Ionic compounds
 Ion-ion interactions
Covalent/molecular compounds
 Dipole-dipole interaction

Hydrogen bonding



super strong type of dipole-dipole
occurs in molecules with O-H, N-H or F-H bonds
Dispersion (see text)
Main Types of Intermolecular
Attraction

Ion-ion interactions: the force of attraction between
2 oppositely charges ions, ionic bonding/attraction is
quite strong, so ionic compounds have high melting
points.
 containing highly-charged ions have higher melting
points than compounds containing univalent (1+ or 1-)
ions.
 Example: arrange the following ionic compounds
in the expected order increasing melting and
boiling pts:
+1 -1

Absolute diff. =
+2 -2 +2 -1
NaF, CaO, CaF2
2
4
3
NaF
CaF2
CaO
Increasing m.p./b.p.
Main Types of Intermolecular
Attraction
Dipole-dipole interactions: occur
between the + end of one polar
molecular and the - end of another.
Main Types of Intermolecular Attraction

Hydrogen bonding: occurs in polar molecules
that contain hydrogen that is bonded to one of
the very electronegative elements O, N, or F.

a + hydrogen atom is attracted to an unshared pair of
electrons on an O, N, or F atom on an adjacent
molecule:
Liquids and their Properties

surface tension: a measure of the inward
forces that must be overcome to expand the
surface of a liquid; molecules on the surface
are attracted only toward the interior
molecules.
Liquids & Their Properties


cohesive forces: the forces that hold a
liquid together.
adhesive forces: the forces between a
liquid and another surface.
example: Hg
cohesive forces
are stronger
than adhesive
forces
example: H2O
adhesive forces
are stronger
than cohesive
forces
Resonance

A molecule or polyatomic ion for
which 2 or more dot formulas with
the same arrangement of atoms can
be drawn is said to exhibit
RESONANCE.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3.
Resonance Example




CO32-
3 resonance structures can be drawn for CO32the relationship among them is indicated by
the double arrow.
the true structure is an average of the 3
Resonance Structures


Another way to represent this is by
delocalization of bonding electrons:
(the dashed lines indicate the 4
pairs of bonding electrons are
equally distributed among 3 C-O
bonds; unshared electron pairs are
not shown)

See p. 256
VSEPR
valence shell electron pair repulsion
Molecular Shape


Lewis structures (electron dot
structures) show the structure of
molecules…but only in 2 dimensions
(flat).
BUT, molecules are 3 dimensional!

for example, CH4 is:
Molecular Shape


Lewis structures (electron dot structures)
show the structure of molecules…but only in
2 dimensions (flat).
BUT, molecules are 3 dimensional!
 but in 3D it is:
a tetrahedron!
= coming out of page
= going into page
= flat on page
Why do molecules take on 3D shapes
instead of being flat?


Valence Shell Electron Pair
Repulsion theory
“because electron pairs repel one another,
molecules adjust their shapes so that the
valence electron pairs are as far apart from
another as possible.”
Why do molecules take on 3D shapes
instead of being flat?


Valence Shell Electron Pair
Repulsion theory
Remember: both shared and unshared
electron pairs will repel one another.
Non-Bonding
Pairs
H—N — H
H
Bonding
Pairs
5 Basic Molecule Shapes

Linear

Example: CO2
5 Basic Molecule Shapes



Bent or angular
Example: H2O
Notice electron pair repulsion
5 Basic Molecule Shapes

tetrahedral

example: CH4
5 Basic Molecule Shapes



Pyramidal
Example: NH3
(note: unshared pair of electron
repels, but is not considered part of
overall shape; no atom there to
contribute to the shape)
5 Basic Molecule Shapes

Trigonal planar or planar triangular

Example: BF3
Geometry and polarity


Three shapes will cancel out
polarity.
Shape One: Linear
Geometry and polarity


Three shapes will cancel out
polarity.
Planar triangles
120º
Geometry and polarity


Three shapes will cancel out
polarity.
Tetrahedral
Geometry and polarity


Others don’t cancel
Bent
Geometry and polarity


Others don’t cancel
Trigonal Pyramidal