# Polarity of Molecules Michael J. Foster C.W. Baker High School Baldwinsville, NY.

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Polarity of Molecules Michael J. Foster C.W. Baker High School Baldwinsville, NY

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.

Let ’ s use the two steps to predict the polarity of CH 4 (methane) Step 1: Determine polarity of bonds Consulting the charts we find that the difference in electronegativity between C and H is 0.4. That means that the C-H bonds are non-polar. If the bonds making up a molecule are non-polar, than the molecule is non- polar. Therefore, CH 4 is a non-polar molecule.

Let ’ s try a couple of more examples: this time let ’ s look at carbon dioxide. Step 1: Determine polarity of bonds Consulting the charts we find that the difference in electronegativity between C and O is 0.9. That means that the C-O bonds are polar. If the bonds making up a molecule are polar, then 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 carbon atom is surrounded by two electron clouds (remember multiple bonds only count as one cloud), the shape of this molecule must be linear.

Step 2: Determine the shape of the molecule (continued) Once we know the shape of the molecule, we must analysize how the electrons are distributed to determine if there is an even distribution (non-polar) or uneven distribution (polar). In this case we know that the oxygen is more electronegative than the carbon and therefore should pull the electrons out away from the carbon.

If we look at the charge distribution in each bond, we get the following: Since the oxygen is more electronegative than the carbon, the electrons will be pulled toward the oxygen atoms and away from the carbon atoms. This will give the oxygen end of the bond with a partial negative charge, leaving the carbon end of each bond with a partial positive charge.

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.

This time let ’ s look at sulfur dioxide. Step 1: Determine polarity of bonds Consulting the charts we find that the difference in electronegativity between S and O is 0.9. That means that the S-O bonds are polar. 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 three electron domains/areas (remember multiple bonds only count as one area), the shape of this molecule must be bent 120°.

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 – SO 2 is a polar molecule.

Now that you have seen how to apply the two steps to determine the polarity of molecules, see if you can predict the polarity of the following: H 2 O PH 3 CCl 4 Ammonia (NH 3 ) SO 3 CH 3 Cl

H 2 O (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.

PH 3 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, PH 3 is a non-polar molecule.

NH 3 (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.

CCl 4 (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, CCl 4 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.

SO 3 (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, SO 3 is trigonal planar. Center of positive charge is on the sulfur, and center of negative charge is between the oxygen atoms (also on S). SO 3 a NON- POLAR molecule.

CH 3 Cl (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, CH 3 Cl 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.

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