INTERMOLECULAR FORCES

2. A molecule is
formed when two
or more atoms
join together
chemically.
The smallest bit
of each of these
substances.
Diatomic molecules
are made of two
atoms of the same
element.
Examples
• Hydrogen – H2
• Oxygen – O2
• Water- H2O

3. Intermolecular forces are the attractive forces
between molecules, between ions, or between
ions and molecules.
Question: What is the difference between
Intramolecular and Intermolecular Forces?
Intramolecular
Force that hold atoms
together in a
molecule
Intermolecular
are attractive
forces between
molecules

4. “Measure” of intermolecular force
• boiling point
• melting point
• Hvap
• Hfus
• Hsub

5. has no separation of
charge, so no positive or negative poles are
formed.

6. Attractive forces between polar
molecules.
Polar molecules have permanent dipoles that interactwith
the permanent dipoles of neighboring molecules. The
positive end of one permanent dipole is attracted to the
negative end of another permanent dipole.
Polar molecules have a higher melting and boilingpoints
than nonpolar molecules of similar molar mass.

7. Orientation of Polar Molecules in a Solid

9. The relative magnitude of these forces can also be used to explain
trends in melting points and boiling points.
It must be remembered that both melting point and boiling point
tend to increase with increasing molar mass, all other factors being
equal.
Influence of dipole-dipole forces is seen in the boiling points of simple
molecules.
Compd Mol. Wt. Boil Point
N2
CO
Br2
ICl
28
28
160
162
-196 oC
-192 oC
59 oC
97 oC

10. Hydrogen bonding is a special case of dipole - dipole
forces, and only exists between hydrogen atoms bondedto
F, N, or O, and F, N, and O atoms bonded to hydrogen
atoms.

11. H-bonding is especially strong in biological
systems — such as DNA.
DNA — helical chains of phosphate groups and
sugar molecules. Chains are helical because of
tetrahedral geometry of P, C, and O.
Chains bind to one another by specific
hydrogen bonding between pairs of Lewis
bases.
—adenine with thymine
—guanine with cytosine

13. Attractive forces between an ionand
a polar molecule.
• a negative ion attracts the positive dipole of another
molecule
• a positive ion attracts the negative dipole of another
molecule

14. Ion-Dipole Interaction

15. • The strongest force is the ion - ion force
and is considered later in the section on
ionic solids.
• These forces (ion-ion) increase as the size
of the ion decreases and as the magnitude
of the charge increases.
• Remember that anions are larger than the
atoms they are derived from and cations
are smaller than the atoms they are derived
from.

16. Na+ — Cl- insalt.
These are the
strongest forces.
Lead to solids with
high melting
temperatures.
NaCl, mp = 800 oC

17. • Also known as the Dispersion Forces.
• Interactions involving Induced Dipoles.
Nonpolar molecules have no permanent dipole
moment, but transient dipoles exist due to therandom
motion of the electrons about the positive charge
center.
Induced dipole forces result when an ion or adipole
induces a dipole in an atom or a molecule with no
dipole. These are weak forces.

20. Molecules that have induced dipoles may also induce neighboring
molecules to have dipole moments, so a large network of induced
dipole-induced dipole interactions may exist.