Polarity and Molecular Shape

Introduction:

The most common type of bond between two atoms is a covalent bond. A covalent bond is formed when two atoms share a pair of electrons. If both atoms have the same electronegativity, or tendency to attract electrons, the bond is nonpolar covalent. When atoms have different electronegativities, the electrons are attracted to the atom with the higher electronegativity. The bond that forms is polar covalent.

Molecules made up of covalently bonded atoms may themselves be polar or nonpolar. If the polar bonds are symmetrical around the central atom, the bonds offset each other and the molecule is nonpolar. If the polar bonds are not symmetrical, the electrons will be pulled to one end of the molecule and the molecule will be polar.

Many physical properties of matter are the result of the shape and polarity of molecules. Water, for example, has unusual properties that can only be explained by the shape of its molecules and the distribution of charge on the molecule. In this experiment, you will build models of molecules and predict their polarity based on their shape.

Objectives:

  • Construct models of molecules
  • Determine molecular shapes
  • Predict polarity of molecules

Materials:

Molecular model kit

Procedure:

  1. Draw the Lewis Structure for each of the molecules in the chart.
  2. Build it with the model kit and determine its shape. Some molecules have more than one central atom and will have more than one shape.
  3. Look at the symmetry of the model. Determine whether the molecule is polar or nonpolar. Remember that a molecule that is asymmetrical but does not have polar bonds is not a polar molecule.

Chart:

Formula / Lewis Structure / Shape / Polarity
N2
HBr
H2O
NH3
CH3NH2
CO2
HCOOH
C3H6
CH4
C2H5Cl

Questions:

  1. How can molecular polarity be determined on the basis of molecular shape.
  1. Which molecule(s) are nonpolar even though it/they are asymmetrical?
  1. Although methane (CH4), water (H2O), and ammonia (NH3) have approximately the same molar mass, their melting points and boiling points differ greatly. Look up the term “hydrogen bond” and predict the order of hydrogen bonding in these three molecules from weakest to greatest. Then put them in order from lowest boiling point to greatest boiling point.
  1. 4. In general, would you expect polar molecules to have higher or lower melting points than nonpolar molecules?