Polarity of Molecules
11/18/14
Polar Molecules are molecules which have an uneven
distribution of charge. One side of the molecule is
negative while one side of the molecule is slightly
positive.
Non-polar molecules are molecules in which there is no
net separation of charge. The electrons are evenly
distributed. There is no net separation of charge.
There are two steps in determining
the polarity of a molecule:
Step 1: Use electronegativity values to determine
the type of bonds (polar or non-polar) making up
the molecule.
Step 2: If the molecule contains polar bonds,
determine if there is a net separation of charge
by looking at the shape of the molecule.
Polarity ranges
Nonpolar: 0-0.5
Polar: 0.5-1.8
Ionic: 1.8+
Let’s use the two steps to predict the
polarity of CH4 (methane)
Step 1: Determine polarity of bonds
If the bonds making up a molecule are
non-polar, then the molecule is nonpolar. Therefore, CH4 is a non-polar
molecule.
Carbon dioxide.
Step 1: Determine polarity of bonds
If the bonds making up a molecule are
polar, than the molecule may be polar or
non-polar, depending on its shape.
Step 2: Determine the shape of the molecule
To determine the molecular shape of a molecule we
must first determine its Lewis dot diagram.
According to VSEPR theory, the shape of this
molecule must be linear.
Use electronegativity to show partial charges
Next, locate the center of the positive and negative
charges.
The center of the positive charges in located
on the carbon atom
The center of the negative charge is also
located on the carbon atom.
Since the center of both the positive and negative
charge are located in the same spot in the molecule,
there is no net (or overall) separation of charge and
the molecule is non-polar.
Sulfur dioxide
Step 1: Determine polarity of bonds
If the bonds making up a molecule are
polar, than the molecule may be polar or
non-polar, depending on its shape.
Step 2: Determine the shape of the molecule
To determine the molecular shape of a molecule we
must first determine its Lewis dot diagram.
According to VSEPR theory, since the sulfur atom is
surrounded by two bonds and one electron pair, the
shape of this molecule must be bent.
If we look at the charge distribution in each bond, we
get the following:
In this case, the center of positive charge is on the
sulfur atom.
While the center of negative charge is located ½
way between the two oxygen atoms.
Since the polarity of the bonds and the shape of the
molecule result in an uneven distribution of charge
– SO2 is a polar molecule.
Predict the polarity of the following:
H2O
PH3
CCl4
NH3
SO3
CH3Cl
H2O (Water)
Step 1: Polarity of bonds
Based on electronegativity difference
between H and O, bond is polar
Step 2: Shape of molecule
Based on VSEPR theory,
water is bent. Center of
positive charge is between
the two hydrogen, and
center of negative charge
on oxygen. WATER is a
POLAR molecule.
PH3
Step 1: Polarity of bonds
Based on electronegativity difference
between H and P, bonds are non – polar.
If the bonds making up a molecule are
non-polar, than the molecule is nonpolar. Therefore, PH3 is a non-polar
molecule.
CCl4 (carbon tetrachloride)
Step 1: Polarity of bonds
Based on electronegativity difference
between C and Cl, bonds are polar
Step 2: Shape of molecule
Based on VSEPR theory,
CCl4 has a tetrahedral
shape. Center of positive
charge is on carbon, and
center of negative is also on
the carbon. No separation
of charge. Carbon
tetrachloride is a NONPOLAR molecule.
NH3 (Ammonia)
Step 1: Polarity of bonds
Based on electronegativity difference
between H and N, bond is polar
Step 2: Shape of molecule
Based on VSEPR theory, ammonia has a
trigonal pyramidal shape. Center of
positive charge is between hydrogen
atoms, and center of negative charge on
oxygen. AMMONIA is a POLAR
molecule.
SO3 (Sulfur trioxide)
Step 1: Polarity of bonds
Based on electronegativity difference
between S and O, bond is polar
Step 2: Shape of molecule
Based on VSEPR theory,
SO3 is trigonal planar.
Center of positive charge
is on the sulfur, and center
of negative charge is
between the oxygen atoms
(also on S). SO3 a NONPOLAR molecule.
CH3Cl (Chloromethane)
Step 1: Polarity of bonds
C-H bonds are non-polar, C-Cl bon is polar
Step 2: Shape of molecule
Based on VSEPR theory,
CH3Cl is tetrahedral. Cl
end of bond is negative ,
while C end of bond is
positive. There is a net
separation of charge so
molecule is POLAR.
Intermolecular forces
• The polarity of a molecule determines how
it interacts with other molecules. The
attraction between molecules is called
intermolecular forces.
• These are different than the forces within
molecules (intramolecular forces) such as
the bonds.
IMFs
• Nonpolar molecules have a weak type of
IMF called London Dispersion Forces.
• This is caused by random motion of
electrons within the molecule sometimes
more of the electrons are in one area than
another.
• This force is bigger for bigger sized
molecules.
IMFs
• Polar molecules have a much stronger IMF
called dipole-dipole interactions. This
means that the partial negative in one
molecule is attracted to another molecules
partial positive.
IMFs
• The strongest type of IMF is called
hydrogen bonding. This is a special type
of dipole interaction that occurs when H is
bonded to one of the highly electronegative
elements F, O, or N.