Determining Polarity of Molecules

As we will see in future chapters, the charge distribution (known as polarity) of a molecule is one of the most important factors in understanding many of the physical properties of the substance.  Determining whether a molecule is polar or nonpolar requires a multi-step analysis, as outlined below.  We will use water (H2O), carbon dioxide (CO2), and formaldehyde (H2CO) as examples for this tutorial.

1.  Draw the Lewis Structure
2.  Determine the Shape
3.  Determine the Polarity

1.  Draw the Lewis Structure

The first step in the process is to determine the Lewis Structure of the molecule (see the Lewis Dot Tutorial for details of this process).  If the molecule has more than 2 atoms, it is important to distinguish between "central" and "outer" atoms in the molecule.

Lewis structures for water, carbon dioxide, and formaldehyde

2.  Determine the Shape

The next step is to determine the shape of the molecule.  If the molecule only has 2 atoms (e.g., HCl), then this is trivial.  In order to determine the shape of molecules with 3 or more atoms,
  1. First, count the number of substituents around the central atom, remembering that a non-bonding pair (lone pair) of electrons counts as a single substituent, as does any type of bonded atom.

Counting number of substituents:  4 around O in H2O, 2 around C in CO2, 3 around C in H2CO

  1. Determine the geometry according to the VSEPR model (use Table 6.2 as a guide).

Geometries: H2O is tetrahedral, CO2 is linear, H2CO is triangular planar.

  1. Decide on the shape of the molecule by placing substituents (atoms or lone pairs) into the geometry, and considering that the lone pairs are "invisible".  Note that in linear, triangular planar, and tetrahedral geometries, all of the substituent locations are equivalent.

Shapes:  H2O is bent, CO2 is linear, H2CO is triangular planar

3.  Determine the Molecular Polarity

The final step is to use the shape of the molecule, together with an analysis of the bond polarities, to determine the polarity of the molecule.
  1. First, look up the electronegativity value for each atom in the molecule (see Fig 6. ).  Remember that this value gives a relative measure of how strongly a particular kind of atom pulls on the electrons in a bond.

Electronegativities of atoms in molecules: H is 2.2, O is 3.44, C is 2.55

  1. Next, take the difference between each pair of electronegativity values in order to determine the polarity of each individual bond in the molecule.  The difference reflects how much harder one atom pulls on the electrons than the other.  If the difference is very small, then the electrons will be equally balanced between the two atoms (i.e., a nonpolar bond).  If the difference is large, then the electrons will be much closer to the atom with the higher electronegativity, and therefore that end of the bond will be negatively charged.

Polarities of bonds and differences in electronegativities.

  1. Finally, try to assess how all of the bond polarities in the molecule will "add up," while trying to visualize the 3-dimensional shape of the molecule.  Sometimes the polarities of the bonds may cancel each other altogether, yielding a nonpolar molecule (e.g., CO2).  Other times, polarities will partially cancel, but will still result in a net polarity (e.g., H2O and H2CO).  Note that in H2CO, the C-H bonds have very little polarity (and much of that cancels out), but that the polar C=O bond causes the molecule to be polar.

Net polarities of molecules:  in water, O is negative; in CO2, polarities cancel; in H2CO, O is negative

If you have trouble visualizing how the individual bond polarity arrows add up to give the net polarity, try the following technique:  starting at a designated point (the red dot in the pictures below), draw all of the bond polarities (blue arrows) sequentially in a tip-to-tail fashion (order doesn't matter), while preserving the orientation of each arrow.  Then draw an arrow (purple) from the starting point to the tip of the last arrow; this will give the net polarity.

Tip-to-tail addition of polarity arrows

Now that we have determined whether each molecule is polar or nonpolar, we can predict many of the properties of these substances.  This is the focus of Chapter 7.

Schematic representations of H2O, CO2, and H2CO molecules