Experiment 25

Molecular Models

Background

You can represent a molecule on paper with either a molecular formula or a structural formula. However, molecular formulas, such as NH3, provide no information concerning the actual arrangement of atoms in the molecule. Structural formulas, such as the following, give some information about the arrangement of atoms in the molecule:

However, these structural formulas provide only limited information since they are two-dimensional. Actual molecular shapes are three-dimensional. A molecular model is far superior to a structural formula when it comes to visualizing atomic arrangements. Compared to molecular formulas and structural formulas, molecular models show much more information about the true shape of the molecules.

In this experiment, you will use ball-and-stick models to help you visualize the shapes of molecules. The balls are color-coded and sized to represent different atoms. The balls are also drilled with holes to accept the sticks and springs; the number of holes in the ball represents the maximum number of bonds a given atom can have. Single bonds are represented by short wooden sticks, double and triple bonds are represented by springs.

Goals

Make models of molecules using ball-and-stick model kits.

Convert three-dimensional molecular models to two-dimensional molecular drawings.

Equipment

1 ball-and-stick model set/6-student group

Safety

Always wear safety goggles when working in the lab.

Procedure

Table 25.1 shows color codes for ball representing different atoms. As you build the models, draw the structural formulas of the molecules in your laboratory notebook. You will make the following molecular models: H20, NH3, CH4, H2S, CCl4, CCl2F2, C2H6, N2, CO2, O2, Cl2, CO(NH2)2, CHBrClF, and C4H10.

Table 25.1
Atom Symbol Color of Ball # of Holes Maximum # of Bonds
Hydrogen H yellow 1 1
Carbon C Black 4 4
Oxygen O Red 2 2
NitrogenÝ N Blue 3 or 5* 3
Chlorine Cl green 1 1
Bromine Br Orange 1 1
Iodine I purple 1 1
Ý

  1. Using the ball-and-stick model set, construct models of water, H2O; ammonia, NH3; and methane, CH4. Draw a sketch of each molecule in your laboratory notebook. The shape shown by the water molecule is described as bent, the shape of the ammonia molecule is called trigonal pyramidal, and the shape of the methane molecule is termed tetrahedral. Write these names below the matching structures you have drawn.
  2. Construct models of hydrogen sulfide, H2S; carbon tetrachloride CCl4; dichlorodifluoromethane, CCl2F2; and ethane, C2H6. Give the molecular formula for each of these compounds and draw a sketch of each molecule in your laboratory notebook. Name the shape of each molecule.
  3. The air above a burning candle contains nitrogen gas, carbon dioxide gas, oxygen gas, and carbon. Construct models of these substances and draw a sketch of each molecule.
  4. The compound urea has the molecular formula CO(NH2)2 and the structural formula of urea is:

    5. Construct a model of urea and sketch its shape in your laboratory notebook

  5. Construct a model of butane, C4H10. (Hint: the carbons are bonded to one another in a continuous, unbranched chain.) Draw a sketch of this molecule. Can you construct a model of a different molecule that has the same molecular formula as butane? Make a model of such a molecule and sketch its structure. The two different compounds having the molecular formula C4H10 are called structural isomers. They have identical molecular formulas but different structural formulas. They also have different physical and chemical properties. Structural isomers play an important role in organic chemistry.
  6. Construct a model of bromochlorofluoromethane, CHBrClF. Sketch the compound. Can you construct an isomer of this compound? (Hint: is our left hand identical to your right?) Draw the new compound if you can. The compound and the isomer have the same molecular formula, CHBrClF, but they are different from each other in the way that a left hand is different from a right had. The compounds are mirror images of each other and are called stereoisomers. The phenomenon of "handedness" exhibited by pairs of stereoisomers is very important to organic chemistry and biochemistry.
Extension

1. Construct models of all the structural isomers of heptane, C7H16. Draw a structural formula for each isomer. Watch for duplications.