Shape Up!
VSEPR and Molecular Geometry
When you draw a Lewis structure for a molecule on paper, you are making a two dimensional
representation of the atoms. In reality however, molecules are not flat—they are three-dimensional.
The true shape of a molecule is important because it determines many physical and chemical
properties for the substance. Let’s practice.
1. Start by drawing the Lewis Structure for each molecule and identify the hybridization of each
atom. Show your drawings to your teacher to receive a molecular modeling kit to build the
molecules and complete the rest of the chart.
Molecule
Lewis Structure
Hybridization
3D Drawing
Shape Name
HCN
CH2Cl2
CH2O
CO2
H3O+
HF
O2
OCl2
SO2
2. These molecules are a bit trickier, but you can do it.
Molecule
Lewis Structure
Hybridization
3D Drawing
Shape Name
CH3OH
H2O2
HCOOH
C6H6
3. A student does not “waste” his time drawing a Lewis structure before determining the shapes of
PF3 and SO3. The student thinks that both molecules must be trigonal planar because there are
three atoms attached to the central atom.
a. Draw the Lewis structure for PF3 and SO3.
b. Was the student’s answer for the shape of the molecules correct? Explain.
c. Why is it important to draw the Lewis structure for a molecule before identifying the
shape of the molecule?
4. Ozone, O3, is not a linear molecule. Actually it is bent with an angle that is a little less than 120°.
a. Draw the Lewis structure of ozone, O3.
b. Describe why ozone has a bent shape instead of a linear shape.
c. Describe why ozone’s bond angle is larger than that of water, H2O.
5. What shape do you expect for a molecule with a central atom and the following?
a. Two bonding pairs of electrons and two nonbonding pairs of electrons
b. Four bonding pairs of electrons and zero nonbonding pairs
c. Three bonding pairs and one nonbonding pair
6. Consider the molecules CH4, NH3, and H2O.
a. They are all sp3 hybridized, so why do they have different shape names?
b. How does the bond angle differ between them? Why?
7. Molecular shape plays an important role in the physical and chemical properties of amino acids
and, in turn, the folding of proteins. Below are two amino acids, glycine and alanine.
a. Fill in the missing electron dots on the appropriate atoms.
b. What is the hybridization of each atom? (Label on the structures.)
c. Color the parts of the molecule that are common between both amino acids.
d. Color the parts of the molecule that are different between them.
Glycine
Alanine