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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 non-polar. 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 atomThe 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 non-polar. 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 NON- POLAR molecule.
NH3 (Ammonia)
Step 1: Polarity of bondsBased on electronegativity difference between H and N, bond is polar
Step 2: Shape of moleculeBased 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 NON- POLAR 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.
