COVALENT
BONDING
– POLARITY &
INTERMOLECULAR
FORCES
Chem-To-Go
Lesson 18
Unit 4
PHYSICAL PROPERTIES
The Lewis dot structure of a compound
gives you hints about some of the
compound’s physical properties.
PHYSICAL PROPERTIES
Melting: process by which the
particles of a substance begin
to move more quickly and
further apart
Boiling: process by which the
particles of a substance begin
to move EVEN more quickly
and EVEN further apart
POLARITY OF THE MOLECULE
The first step in predicting simple physical properties
like melting point and boiling point is to determine the
polarity of a molecule.
Polar Molecules
• Have a lone pair of electrons on the
central atom in the Lewis dot
structure
Nonpolar Molecules
• Have no lone pair of electrons on
the central atom
AND
OR
• Have one terminal atom that
differs from the other terminal
atoms (asymmetrical)
• All terminal atoms are similar
(symmetrical)
EXAMPLE #1: CH 4
• Does the molecule
have a lone pair of
electrons on the
central atom?
OR
• Have one terminal
atom that differs
from the other
terminal atoms?
(asymmetrical)
EXAMPLE #2: NH 3
• Does the molecule
have a lone pair of
electrons on the
central atom?
OR
• Have one terminal
atom that differs
from the other
terminal atoms?
(asymmetrical)
INTERMOLECULAR FORCES
Water molecules
INTER = BETWEEN
• Polar molecule due to a lone
pair of electrons on the
central O atom
• Since O has more electrons
surrounding it, the O end of
the bond is partially
negative.
• This leaves the H end of the
bond partially positive.
• Opposites attract: The
partially positive H end of
one molecule is attracted to
the partially negative O end
of another molecule.
T YPES OF INTERMOLECULAR FORCES
1) London dispersion forces (LDF) –
•
•
•
•
Weakest
Temporary
Between nonpolar molecules
Result in low melting & boiling points (easy to melt &
boil)
2) Dipole-dipole forces –
• Forces of attraction/repulsion that exist between POLAR
molecules as a result of the partial charges
• Result in high melting & boiling points (difficult to melt &
boil)
3) Hydrogen bonding – SPECIAL T YPE OF DIPOLE FORCE
• STRONGEST intermolecular force
• Occurs when H is bonded to FON creating REALLY strong
partial charges
• Results in highest melting & boiling points (most difficult
to melt & boil)
EXAMPLE #3: Br 2
Is the molecule
polar?
London dispersion forces
• Nonpolar only
Dipole-dipole forces
• Polar only
Hydrogen bonding
• Polar AND H bonded
to F, O, or N
EXAMPLE #4: HCl
Is the molecule
polar?
London dispersion forces
• Nonpolar only
Dipole-dipole forces
• Polar only
Hydrogen bonding
• Polar AND H bonded
to F, O, or N
EXAMPLE #5: CO 2
Is the molecule
polar?
London dispersion forces
• Nonpolar only
Dipole-dipole forces
• Polar only
Hydrogen bonding
• Polar AND H bonded
to F, O, or N
EXAMPLE #6: H 2O
Is the molecule
polar?
London dispersion forces
• Nonpolar only
Dipole-dipole forces
• Polar only
Hydrogen bonding
• Polar AND H bonded
to F, O, or N