0.2 Vsepr: molecular shapes and isomerism  (Page 2/3)

The next part of the lab deals with isomers. Isomers are molecules having the same molecular formula, but the atoms are arranged in a different manner, while still obeying the rules of bonding. There are different classifications for isomers. For example, structural isomers differ from one another in the order in which the atoms are bound to each other (connectivity is different). On the other hand, geometrical isomers have the same order of atoms, but the spatial arrangement of atoms is different (connectivity is the same). A common example of geometrical isomers is the cis and trans forms of double bonds:

** NOTE: Remember that molecules having single carbon-carbon bonds cannot have cis/trans isomers because there is free rotation about single bonds.

By building the models of various molecules during this lab, you will be able to better understand molecular symmetry and isomers. Building models is not difficult; however, the chemical principles involved are very important and you may find some surprises in how atoms can be fit together.

Finally, in Part 4, you will be applying your knowledge of VSEPR (Valence Shell Electron Pair Repulsion) Theory in order to determine the geometry of several different molecules. VSEPR theory is useful in helping to determine how atoms will orient themselves in molecules. Basically, the idea is that the arrangement adopted by a molecule will be the one in which the repulsions among the various electron domains are minimized. The two kinds of electron domains are bonding (electron pair shared by two atoms) and non-bonding (electron density centralized on one atom) pairs of electrons.

Experimental Procedure

For Parts 1&2: You and your lab partner are to work with one other lab group in preparing these models (no more than 3 - 4 students). Your TA will assign each group a certain set of molecules to make and answer questions pertaining to those molecules. Each group will then present their answers to the class. These models will need to be completed and answers determined within 30 minutes so that we can continue to Parts 3&4 as soon as possible.

For Parts 1-4, the work should be divided among the group members. Be sure to discuss the questions and answers among yourselves, but put your own conclusions on the Report Form.

1. Symmetry Elements

Using the Molecular Framework models, make models of the following compounds:

1. CH ₄
2. CH ₃ Cl
3. CH ₂ Cl ₂
4. CHCl ₃
5. CH ₂ ClF
6. CHBrClF
7. BF ₃
8. BF ₂ Cl
9. PH ₃
10. PH ₂ Cl

Choose a color to represent each atom. For example, make all C atoms black, all H atoms white, etc.

Once the models are created, look for symmetry elements that may be present. Ask yourselves the following questions:

• Does the molecule contain a mirror plane ( σ ) ? In other words, is there a plane within the molecule which results in one half being a mirror image of the other half?
• Does the molecule contain a two-fold rotation axis (C 2 ) ? Remember from the Introduction that the subscript indicates the degrees of rotation necessary to reach a configuration that is indistinguishable from the original one. In this case, the rotation will be 180 ^o .
• Does the molecule contain any higher-order rotation axes?