GT chemistry

Unit: Chemical Bonding

Name: ______Date: ______MOD: ______

MOLECULAR MODELS

The names and formulas of molecules provide information about the numbers and types of atoms present in a compound. However, this information is not enough to tell us about the properties of the molecules. Drawing structural formulas and Lewis dot structures helps us to understand the arrangement of atoms within molecules. However, it is difficult to visualize three-dimensional molecules from two-dimensional representations. In order to understand the role of molecular geometry in predicting properties of compounds, it is often helpful to build models of the molecules using kits designed for this purpose.

One important property of molecules that is dependent on the types and arrangement of atoms within molecules is polarity. If the atoms in a bond are identical, their electronegativities are the same and the bond is nonpolar. If the atoms are different, electronegativities are usually different and the bonding electrons are skewed toward the more electronegative atom, creating a charge distribution imbalance known as polarity. The arrangement of the atoms within the molecule also affects the polarity of the molecule. If polar bonds are arranged symmetrically around the central atom, then the unbalanced electron distributions tend to cancel each other and the molecule itself is considered to be nonpolar.

OBJECTIVES

When you have finished this activity, you should be able to:

1.  Use a molecular models kit to construct ball and stick models of selected molecules,

2.  Predict the shapes and polarity of selected molecules using the Lewis dot structures, in order to visualize the relationship between the types and locations of bonding atoms and molecular properties such as polarity.

3.  Identify hybridization of bonds.

MATERIALS

Molecular model making kit

PROCEDURE

1.  Draw the Lewis dot structure for the molecule in the Data Table.

2.  Using the kit, build a model of the molecule. Draw a structural diagram of the molecule in the Data Table.

3.  Describe the shape of your molecular model using terms from your textbook or other chemistry reference.

4.  Examine the shape of the molecule and the types of atoms in the bonds. Classify the bond hybridization, if present, as sp, sp2, or sp3 and record in the Data Table.

5.  Examine the shapes of the molecules and the types of atoms in the bonds to determine the polarity of each molecule. Enter “Polar” or “Nonpolar” into the Data Table.

DATA TABLE /
Formula / Lewis Dot
Structure / Structural Formula / Molecular Shape / Hybridization / Polarity /
H2 / / / linear / none / nonpolar
H2O / / / bent / sp3 / polar
O2 / / / linear / sp2 / nonpolar
CO / / / linear / sp / polar
CO2 / / / linear / sp / nonpolar
NH3 / / / trigonal pyramidal / sp3 / polar
HCl / / / linear / sp3 / polar
CH4 / / / tetrahedral / sp3 / nonpolar
CH3Cl / / / tetrahedral / sp3 / polar
CCl4 / / / tetrahedral / sp3 / nonpolar
CH3OH / / / tetrahedral / sp3 / polar
C2H4 / / / trigonal planar / sp2 / nonpolar
C2H2 / / / linear / sp / nonpolar

ANALYSIS AND CONCLUSION

1.  Explain how you used molecular shapes to predict molecular polarity. Support your answer with examples from the Data Table.

2.  Compare H2 and HCl. Explain why one is polar and one is nonpolar.

3.  Compare CH3Cl and CCl4. Explain why one is polar and one is nonpolar.

4.  Using your answers from #2 and 3, write a “Rule of Thumb” for predicting the polarity of a molecule.

5.  The polarity of a molecule can affect its solubility. The rule of thumb that relates polarity and solubility states that “like dissolves like”. Knowing this rule, predict the polarity of ethanol if it dissolves in water. Now consider water as a cleaning fluid. Explain why water is not used on greasy stains.

6.  Complete the Analysis Table below:

ANALYSIS TABLE
Formula / Shape / Polarity
HBr
SF2
SiF4
NF3
AlH3

7.  Carbon monoxide poisoning is a life threatening condition. Compare the models you made for carbon monoxide and oxygen. Suggest reasons that may explain carbon monoxide poisoning is so deadly.

PRE-LAB: MOLECULAR MODELS

Read the entire laboratory investigation and the relevant pages of you textbook. Then answer the questions that follow.

1.  Draw the Lewis dot structure for the following molecules:

a.  fluorine (F2)

b.  oxygen (O2)

c.  nitrogen (N2)

d.  water (H2O)

2.  Use an appropriate reference to define:

a.  Structural formula

b.  Polar molecule

c.  Nonpolar molecule

d.  Double bond

e.  Symmetrical

3.  List the page(s) in your textbook that provide(s) drawings of molecular shapes.

Answers will vary according to text used.

4.  Using the Molecular Model Kit, complete the data table below.

DATA TABLE
Atom / Symbol / Color of Ball / Number of Holes / Maximum Number of Bonds
Hydrogen / H / varies / 1 / 1
Carbon / C / varies / 4 / 4
Oxygen / O / varies / 2 / 2
Nitrogen / N / varies / 5* or 3 / 3
Chlorine / Cl / varies / 1 / 1
Bromine / Br / varies / 1 / 1
Iodine / I / varies / 1 / 1

*If the nitrogen has 5 holes, connect two adjacent holes with a spring bond. Teacher should explain the significance of a lone pair of electrons.