Name: ______Date: ______Period: ______Page: ______
VSEPR LAB

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.

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model in chemistry used for predicting the shapes of molecules based on their number of bonding and non-bonding (lone pairs) electron pairs around the central atom. Electron pairs in a molecule want to be as far away from each other as possible because they repel each other. As a result, the atoms in a molecule tend to separate as far as they can because their bonds repel each other. Double and triple bonds occupy more space than single bonds, and lone pairs occupy more space than bonding groups. The presence of lone pairs will usually make bond angles smaller than the ideal angle for the particular geometry.

One other 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.

MATERIALS

Molecular models

Protractor

VSEPR reference guide

PROCEDURE

  1. Observe the model of the molecule.
  2. Draw the structural formula of the molecule, including lone pairs, in the data table.
  3. Compare the shape of the model to the VSEPR reference guide. Count the bonding pairs and nonbonding pairs to make a positive match. Record the shape in the data table.
  4. Use a protractor to find the bond angle from terminal atom to terminal atom. Record.
  5. Examine the shape of the molecule and the types of atoms in the bonds to determine the polarity of the molecule. Enter “Polar” or “Nonpolar” into the Data Table.
  6. In general, the molecule is polar if it is asymmetrical, nonpolar if it is symmetrical.

Formula / Structural Formula / Molecular Shape / Bond Angle
107, 109.5, 120, 180 / Polar or Nonpolar
H2
H2O
HF
CO2
NH3
CH4
CH3Cl
CH2Cl2
CCl4
HCOOH
C2H4
C2H2

ANALYSIS AND CONCLUSION

  1. List the molecules that are linear.
  1. List the molecules that are bent.
  1. List the molecules that are trigonal planar.
  1. List the molecules that are pyramidal.
  1. List the molecules that are tetrahedral.
  1. Compare CH3Cl and CCl4. Explain why one is polar and one is nonpolar.
  1. 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.
  1. Give an additional example of another molecule that displays the same shape of each of the molecules in the table below. You cannot use a molecule that was used in this lab.

Formula / Additional Example(s)
HF
H2O
CH4
NH3
HCOOH

PRE-LAB

True or False. If the statement is false, correct it.

  1. _____ According to VSEPR theory, the shape of a molecule can be determined only from the number of bonding electrons around the central atom.
  1. _____ Electrons want to be as close to each other as possible.
  1. _____ Molecules with lone pairs around the central atom have smaller bond angles.
  1. _____ A polar molecule will be symmetrical.
  1. _____ Bond angle is measured from central atom to terminal atom.

PRE-LAB

True or False. If the statement is false, correct it.

  1. _____ According to VSEPR theory, the shape of a molecule can be determined only from the number of bonding electrons around the central atom.
  1. _____ Electrons want to be as close to each other as possible.
  1. _____ Molecules with lone pairs around the central atom have smaller bond angles.
  1. _____ A polar molecule will be symmetrical.
  1. _____ Bond angle is measured from central atom to terminal atom.