MOLECULAR POLARITY

Whether or not a molecule is polar has profound effects on the physical properties of the substance like solubility, boilng point and melting point.  Molecular polarity determines the strength and types of intermolecular forces of attraction at work in a sample of the substance.  The magnitude of these forces is directly proportional to boiling and melting points.  In addition, molecular polarity affects solubility in that polar molecules are best solvated by polar solvent molecules and nonpolar molecules are best solvated by nonpolar solvent molecules;  i.e., "like dissolves like".  How do we figure out if a molecule is polar or which of two molecules is more polar?  Answering this question can be broken down into three steps:  draw the Lewis structure, determine the geometry of the molecule, identify the bond polarities and then add them together.  Adding several bond polarities need not be too painful.  It is usually easiest to think of adding two bonds at a time and then adding these subtotal sums to get the sum for the whole molecule. If the sum of the bond polarities is zero, the molecule is nonpolar which means the substance will not be very soluble in polar sovents like water and will be more soluble in nonpolar solvents like hexane and it will have relatively low boiling and melting points.   If, however, the sum of the bond polarities is not zero, the molecule is polar.  The greater the sum, the more polar the molecule and the greater its solubility in polar solvents like water and the higher its expected boiling and melting points.
Let's first look at carbon dioxide, CO2.
After you draw the Lewis structure, you must determine the molecule's geometry.  We use the Valence Shell Electron Pair Repulsion Theory (VSEPR Theory) to do this.  The Theory says that the molecule will assume a geometry which allows the sets of electrons around the central atom to be as far from each other as possible. In the Lewis structure of carbon dioxide, there are two sets of electrons (note that the word "sets" is used--a set of electrons is the group of electrons between two atoms) around the central carbon atom.  The geometry which allows these two sets to be as far apart as possible is a linear geometry. Thus we find that the molecular geometry will be linear.  Each of the carbon-oxygen double bonds is polarized toward the more electronegative oxygen.  Since the bond dipole moments are of the same magnitude and aligned tail-to-tail they exactly balance each other and their sum is zero.  Hence, carbon dioxide is a nonpolar molecule.
After you draw the Lewis structure, you must determine the molecule's geometry. Again, using the VSEPR Theory, we want the four sets of electrons (note that here, the bonding electrons and the unshared electrons are counted as sets) around the central sulfur atom to be as far apart as possible.  The geometry which allows this is a tetrahedral geometry in which the four sets of electrons are directed toward the corners of a tetrahedron making them 109.5° apart.  Each of the hydrogen-sulfur bonds is polarized toward the more electronegative sulfur atom.  The lone-pair electrons on the sulfur also have dipole moments directed away from the sulfur atom.  When these four forces are added together, they do not cancel each other out;  i.e., there is a net molecular dipole moment and the molecule is polar.  (Remember, it is usually easiest to think of adding two bonds at a time and then adding the subtotal sums.  Here, think of adding the two sulfur-hydrogen bond dipoles together.  This subtotal sum would be a dipole halfway between the two bonds directed toward the sulfur atom.  Now, think of adding the impact of the two unshared pairs of electrons.  This subtotal sum would be halfway between the two pairs directed toward the unshared electrons.  Now, add the two subtotal sums.  Since they are both directed in the same direction, they will definitely not cancel each other out so the molecule has a net dipole moment.)
Finally, consider AlCl3.
After you draw the Lewis structure, you must determine the molecule's geometry. Hopefully, you remember that the elements in Group 3A (boron and aluminum are the most common) may violate the octet rule and form molecules in which the central atom (B or Al) have only a total of 6 electrons around them.  This gives a Lewis structure in which there are three sets of electrons around the central aluminum atom.  The geometry which allows three sets to be as far apart as possible is a planar triangular geometry.  The three bond dipole moments are then 120° apart and of equal magnitude all directed at the more electronegative hydrogen atoms.  The sum of these three forces will be zero and the molecule will be nonpolar.

QUIZ

1.    What are the molecular geometries of CCl4, CHCl3, and CH2Cl2?
a)  linear            b)  square planar        c)  tetrahedral        d)  triangular planar

2.    The C-H bond(s) is(are) polarized toward the _____ atom and the C-Cl bond(s) is(are) polarized toward the _____atom.
a)  carbon, carbon        b)  carbon, chlorine        c)  hydrogen, chlorine        d)  hydrogen, carbon

3.    Which of the three molecules in Question #1 are polar?
a)  CCl4  and CHCl3           b)  CHCl3 and CH2Cl2                c)  CCl4 and CH2Cl2            d)  all of the molecules are polar

4.    Which of the following molecules is(are) polar?
Cl2, CH4, CH2O, CH3Cl
a)  CH4,CH2O, CH3Cl        b)  CH2O, CH3Cl        c)  CH4, CH3Cl        d)  Cl2, CH2O

5.    Of the polar molecules in Question #4, which is more polar?
a)  Cl2        b)  CH4        c)  CH2O        d)  CH3Cl