Intermolecular
Forces
Learning Goal 3: Part A
Intermolecular Forces
Forces of attraction between
different molecules rather than
bonding forces within the same
molecule.
Dipole-dipole attraction
Hydrogen bonds
Dispersion forces
Forces and Phases of Matter
o Substances with very little intermolecular
attraction exist as
gases
o Substances with strong intermolecular
attraction exist as
liquids
o Substances with very strong
intermolecular (or ionic) attraction exist
as
solids
1. Hydrogen Bond (strongest)
Bonding between hydrogen and more electronegative
neighboring atoms such as oxygen and nitrogen.
IT IS NOT A BOND.
A
H… B
or
A
H… A
A & B are N, O, or F
11.2
Polarity
A molecule, such as HF, that has a
center of positive charge and a
center of negative charge is said
to be polar, or to have a dipole
moment.
H F
+
-
- water is a polar molecule because oxygen is more
electronegative than hydrogen, and therefore electrons
are pulled closer to oxygen.
2. Ion-Dipole Forces
Attractive forces between an ion and
a polar molecule
Ion-Dipole Interaction
11.2
3. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
11.2
4. Dispersion Forces – van der Walls
forces/London forces (weakest)
Attractive forces that arise as a
result of temporary dipoles
induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
11.2
Summary: Intermolecular Forces
Intermolecular
Forces
Formed by the
Examples
attraction between …
Ion-dipole
an ion and a polar
molecule
Na+ and H2O
Hydrogen bond
molecules which
have H on N, O, or F
atoms
H2O and H2O; H2O
and CH3CH2OH
Dipole - Dipole
two polar molecules
CH3Br and ICl;
CH3Br and H2O
Ion - Induced dipole an ion and a
nonpolar
Fe2+ and O2
Dipole - Induced
dipole
a polar molecule
and a
nonpolar molecule
HCl and Cl2
London (dispersion)
forces
two nonpolar
molecules
CH4 and CH4; F2 and
F2; CH4 and F2
MOLECULAR
GEOMETRY
Learning Goal #3
Part B
MOLECULAR GEOMETRY
VSEPR
• Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion between
electron pairs.
Molecule
adopts the
shape that
minimizes the
electron pair
repulsions.
Some Common Geometries
Linear
Trigonal Planar
Tetrahedral
VSEPR charts
• Use the Lewis structure to determine the
geometry of the molecule
• Electron arrangement establishes the bond
angles
• Molecule takes the shape of that portion of
the electron arrangement
• Charts look at the CENTRAL atom for all
data!
Structure Determination
by VSEPR
Water, H2O
The electron pair
geometry is
TETRAHEDRAL
2 bond pairs
2 lone pairs
The molecular
geometry is BENT.
1. Two pairs of electron on a central atom in a
molecule are always placed 180o apart
(linear).
2. Three pairs of electrons on a central atom in
a molecule are always placed 120o apart on
the same plane as the central atom (trigonal
Planar).
3. Four pairs of electrons on a central atom in a
molecule are always placed 109.5o apart
(tetrahedral).
4. When every pair of electrons on the central
atom is shared with another atom, the
molecular structure has the same name as
the arrangement of the electron pairs.
Number of Pairs
Name of Arrangement
2
Linear
3
Trigonal Planar
4
tetrahedral
5. When one or more of the electron
pairs around a central atom are
lone pairs, the name for the
molecular structure is different from
the arrangement of electron pairs.
Determining polarity
of a molecule based
on the symmetry of
the molecule.