The Structure of Covalent Molecules
Introduction:
When atoms obtain their octet of electrons by sharing two, four, or six electrons, they form covalent molecules. Although these structures look flat on paper dot diagrams, they are really 3-dimensional. They may be nonpolar if symmetrical in every direction. However, if the structure has ends that are different, the electrons will spend more time at the end that attracts them more (electronegativity), and the molecule will be polar. In this lab you will draw a dot diagram for each molecule, then construct a model, and decide whether the molecule would be polar. Note that the model atoms are made so that their arms or holes are placed where the bonds would form, so that the model shape represents the actual shape of the molecule.
Procedure:
Make a chart with the molecule formulas in the left column,
and a column each for the dot diagram, shape and polarity. Leave plenty
of room for the diagrams.
Draw a dot diagram for each molecule listed and then build each model, using
the key in the model kit to find out which color stands for each type of
atom. Use the stiff connectors for single bonds and flexible ones for double
or triple bonds (you can connect two or three bonds that way). The model
atoms should make as many bonds as they have holes or arms. To classify
its shape, look at the diagrams in your text book. Then record whether it
would be polar or nonpolar.
These are the molecules to do:
| CH3CL | H20 | CH4 |
| CL2 | NH3 | HF |
| C2H2 | CH2CL2 | N2 |
| CO2 | HCN | H2CO |
| 02 | H2S | C2H6 |
Questions:
1. Which molecules were nonpolar because all bonds were nonpolar
(between two identical elements)?
2. Which molecules had polar bonds, but because of symmetry, were balanced
and nonpolar?
3. Which shapes were associated most with polar molecules?
4. These molecules are all covalent. Give an example of two substances made
of more than one atom that are NOT covalent.
5. Draw a dot diagram of hydronium [H3O+] and for hydroxide [OH-]. Why are
the charges necessary for these molecules to exist?
EXTRA CREDIT: All of the groups on the charge sheet on the back of your periodic table are covalent groups with extra or missing electrons. Draw a dot diagram for up to 10 of these groups, showing how they fit together.
