Lab Activity 6: Model Building with Covalent Molecules

 

Background:       

Most of our learning is in two dimensions. We see pictures in books and on walls and chalkboards. We often draw representations of molecules on flat paper. Two-dimensional representations include electron dot structures and structural formulas. In electron dot structures, a pair of “dots” (which represents a pair of electrons) is used to represent a single covalent bond.  The hydrogen molecule is shown as H:H.  In structural formulas, a single covalent bond is represented as a straight line.  The Hydrogen molecule is H-H.  Although such “models” help us in understanding the structure of molecules, flat models do not give us the three-dimensional view that is necessary to truly visualize most molecules. In this experiment, you will build three-dimensional molecular models and then compare them with the corresponding structural formulas.

        In covalent molecules there are single, double, and triple bonds between atoms. In some cases, the molecules are in a chainlike arrangement. At other times, the atoms arrange themselves in a ring-like structure. Still other molecules are in the form of branched chains.

        Sometimes a group of atoms may form more than one structure. Thus, a given molecular formula might represent more than one compound.  For example, C2H6O represents both ethyl alcohol and dimethyl ether, compounds with different structural formulas and quite different properties.  Substances that have the same chemical formula but different structures are called structural isomers.

        Scientists who are responsible for determining the structure of molecules often start with molecular model kits like the ones we used in this experiment. Complicated molecules such as DNA (deoxyribonucleic acid) are most often shown in three-dimensional models. Without these models, we would not understand how the atoms of the molecules interact.  

OBJECTIVES:

1. To construct models of some simple and more complicated covalent molecules

2. To draw structural formulas that show the shape of the molecules.

To construct more than one structure for some of the molecules represented.  

MATERIALS:

Molecular Model Kit

 

PROCEDURES:

1.    Obtain a molecular model kit and remove the contents. Separate the contents based on the atoms that are represented.  The chart below will help you in identifying the atoms.

Atom

Number Of Bonding Sites (Holes)

Hydrogen

Chlorine

Bromine

Iodine

Fluorine

1

Oxygen

Sulfur

2

Nitrogen

3

Carbon

4

In most kits, different colors represent different elements.  Record on the Report Sheet the color that corresponds to each element.

2.    Make models of the following molecules.  There is only one structural isomer for each one.  To confirm this, try making the atoms combine in some other way and you will find that once you turn the molecule around, it will be identical to the original structure.  When you are satisfied that you have the correct structure, sketch it on the Report Sheet.

2a. Water, H2O

2b. Methane, CH4

2c. Methanol, CH3OH

2d. Carbon tetrachloride, CCl4

3.    For the following molecular formulas, there can be more than one arrangement of the toms.  For each one, try to find as many different structural isomers as you can.  Draw a structural formula for each of the isomers on the Report Sheet.

3a.   Butane, C4H10

3b.   Butanol, C4H9OH

3c.   Hexane, C6H14

3d.   Dichloroethane, C2H4Cl2

4.    All of the preceding molecules include only single bonds.  In the following group of molecules there are single, double, and triple bonds.  Make each structure, and then determine if any other isomers exist.  Finally, draw the structural formula for each of the molecules on the Report Sheet.

4a.   Carbon dioxide, CO2

4b.   Nitrogen, N2

4c.   Butene, C4H8

4d.   Butyne, C4H6

5.    Continue building molecules as your teacher suggests.  Sketch each of the models or draw the structural formulas on the Report Sheet.

 

Post Lab Discussion:

       The models that you have constructed can help you in visualizing molecules, as they exist in three dimensions.  The more you work with models and sketches, the more comfortable you will feel with new structures, even the more complicated molecules.  Although it is not always efficient to use models for symbolizing molecules, it helps to be familiar with them so that, as you draw molecular structures, you will have a mental image of their three-dimensional structures as well.

 

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