Molecular Modeling with "ball-and-stick" model kits
For this experiment you will work in groups of three or four students. Each student must
complete his or her own lab on a separate sheet of paper. Answers must be in complete
sentences.
1. Why do you think the different types of atoms are given different colors? Are these
colors the real colors of these atoms, or just arbitrary colors?
2. Locate on the periodic table in your textbook all the atom types included in the kit.
3. How many holes (for bonds) are there in these atoms?
H
C
O
Cl
Br
hydrogen (yellow)
carbon (black)
oxygen (red)
chlorine (green)
bromine (purple)
_____
_____
_____
_____
_____
4. Construct models for the diatomic molecules hydrogen (H2), chlorine (Cl2), and
hydrogen chloride (HCl). What does the bond between the two atoms represent; that is,
what is the bond actually composed of? Draw the structural formula for all three
compounds.
5. What is the "octet rule" and how does it explain the tendency of oxygen to form two
bonds, whereas chlorine, for example, forms only one bond?
6. Construct models for methane (CH4) and for chloroform (CHCl3). What shape does the
vertices of the molecule form? Draw the structural formula for these two compounds.
7. Construct models for ethane (C2H6), and propane (C3H8). Is there any other way to
construct these molecules? Draw the structural formula for these two compounds.
8. Construct a model for butane (C4H10). There is more than one way to construct a
molecule with the formula C4H10. How many can you find? Sketch their structures.
9. Some molecules contain so-called "double bonds". In the modeling kit, the curved
bonds are used allow double bonds between atoms. Build models for diatomic oxygen
and for carbon dioxide. How many double bonds do these molecules have? Sketch their
structures.
10. Octane (found in gasoline) has 8 carbons and is one of the families of hydrocarbons
such as methane, ethane, butane, and propane. What formula would you predict for
octane? Can you sum up with a general expression that should work for all hydrocarbons
of this type? E.g. CnHm, given n, what is m? Sketch the octane structure.
11. Chains and rings.
Many organic molecules have chains of carbon atoms. It is also common for atoms to
form closed rings. Most drugs, for example, contain rings or 5 or 6 atoms.
Using your molecular model kit, construct a hydrocarbon with the formula C6H12,
consisting of a ring of six CH2 groups. This is called cyclohexane. Then construct another
hydrocarbon with the formula C6H6, consisting of a ring of six CH groups with
alternating single and double bonds between the carbons. This is called benzene, and it is
by far the most common ring structure, found over and over again in many different
naturally occurring organic compounds. The shorthand line-segment representations of
these two structures are:
Cyclohexane benzene
12. Chiral (left- and right-handed) molecules. (EXTRA CREDIT)
a. Construct a model of a molecule that has a single carbon atom in the center and
four different atoms or groups attached to it. Construct its mirror image also. Are
the two mirror pairs really identical or are they distinct? Hint: try to superimpose
them by laying them side-by-side.
b. Place the model against a piece of paper so that three of the atoms touch the
paper. Mark the positions of each touching atom on the paper. Then try to match
the same three positions with the other mirror image. Do both fit, or only one?
c. Construct a model that has a carbon atom in the center but only three different
atoms attached to it, that has two of one kind of atom instead of four different
atoms. Make its mirror image also. Are the two mirror pairs really identical or are
they distinct?
d. Try part b with this model. Do both fit or only one?
e. What would you predict for carbons that have fewer than three different
attached atoms? Would this give distinct left and right-handed forms or not? On
the basis of these observations, formulate a rule for predicting when a carbon
atom will give distinct left and right-handed forms.