Molecular Model Lab

Introduction: The most common type of chemical bond between two atoms is a covalent bond.  The covalent bond consists of a pair of shared electrons, one from each atom.  If this pair of electrons is shared between two atoms of equal electronegativities, the bond is called a nonpolar covalent bond.  However, in most cases, the pair of electrons is shared by two atoms of different electronegativities.  Thus, the pair of electrons is shifted towards the more electronegative element.  A partial negative charge results on one side of the bond and a partial positive charge on the other.  This type of covalent bond is called polar covalent.

            Molecules composed of covalently bonded atoms may also be polar or nonpolar.  For the molecule to be polar, it must, of course, have polar bonds.  But the key factor for determining the polarity of a molecule is its shape.  If the polar bonds (dipoles) are symmetrical around the central atom, they offset each other and the resulting molecule is nonpolar.  However, if the dipoles are not symmetrical around the central atom, the electrons will be pulled to one end of the molecule.  The resulting molecule is polar.

            Molecular models are often used to demonstrate molecular shape.  In this exercise you will build several covalent molecules and predict each molecule’s polarity on the basis of it’s molecular shape.    

Purpose:  In this experiment you will, 1) draw lewis structure of covalent molecules 2) build models of some simple molecules, 3) predict each molecule’s shape using VSEPR theory, and 4) predict each molecule’s bond angles and polarity on the basis of its shape.

Equipment:  Molecular model kit

Procedure:  For the following substances,

a.       Draw the Lewis structure for each molecule

b.      Make a molecular model of it.

c.       Count # of electron domains, bonds and unshared (lone) electron pairs

d.   Identify the molecular geometry using notes.

e.    Identify bond angles

f.    Categorize the molecule as symmetrical or nonsymmetrical (assymetrical).

g.    Identify the molecule as polar or nonpolar.

                        Lewis         Electron     Electron      # of           # of              Molecular       Bond        Symmetrical or     Polar or

Formula           Structure     Domains   Geometry    Bonds       Lone Pairs     Geometry       Angle(s)   Nonsymmetrical    Nonpolar 

1. H₂

2. Cl₂

3. HBr

4. H₂O

5. CH₄

6. NH₃

7. SF₆

8. O₂

9. CO₂

10. H₂CO

11. C₂H₂

12. C₂H₄

13. ICl₄^-