VSEPR and Molecular Models

Chemistry 161 VSEPR and Molecular Models

Introduction The use of molecular models can be of great value in helping you to visualize the three-dimensional geometries encountered in the VSEPR (valence-shell electron-pair repulsion) theory. Model kits come in a variety of styles including ball-and-stick, space-filling, and molecular orbital types. These vary in the amount of detail devoted to illustrating the electron regions of molecules. In this exercise, we will use ball-and-stick type models that make it easy to observe approximate bond angles and relative locations of nuclei. You will use the models to check the molecular geometries resulting from the application of VSEPR theory to Lewis dot models and to answer some questions that depend on geometry (such as whether or not molecules are polar).

Prelab/Homework Assignment Because we are not doing an ‘experiment’ for this lab, you do not need to write anything in your lab notebook. Everything (prelab and postlab) can be written in this handout. For your prelab (due at the start of lab/class), read this entire lab handout carefully, and draw Lewis Dot Structures for each item in the tables for parts A and B. I will check your handout as you come in to lab/class.

Using the Model Kits The molecular model kits will be available for use while you are in the lab. Please keep track of the parts and restore them to their containers before you exit the lab. Use the model kits to make as many models as time allows. As you work through the exercises, practice remembering the various shapes that result so that you will be able to imagine and sketch them without the help of models in the future.

To make each model, choose appropriate spheres to represent the atoms present. Although there are traditional color assignments (e.g., black for carbon, red for oxygen) the colors you choose are not as important as having the correct number and arrangement of holes for the bonds required. Make single bonds with the short, gray connectors. Double and triple bonds are made by using the longer, flexible links to connect pairs of atoms two or three times. Do not make multiple bonds with the short connectors.

Work with the models with the objective of completing the report exercises. If time limits the number of models you can make, decide which ones would be the most useful.

Part A. MODELS WITH ONE CENTRAL ATOM For each species listed in part A, your report should include the following 8 items: 1. Lewis structure 2. The number of electronic domains (bonding and non-bonding). 3. The number of bonding domains (how many atoms are bonded to the central atom). 4. The number of non-bonding domains (how many lone pairs). 5. The molecular geometry predicted by VSEPR theory. 6. The molecular shape predicted by VSEPR theory. 7. A sketch of the molecule that reflects the three-dimensional molecule. 8. Write “polar” or “nonpolar” according to your analysis of polarity.

If there are resonance structures possible for the Lewis models you make, you do not need to show them all. You may just write the word “resonance” next to one Lewis structure. In VSEPR theory, all resonance structures based on relocation of double bonds will lead to the same conclusions about geometry, i.e., the double and single bonds are treated equally as charge clouds about the central atom. But note that the ball-and- stick models may give you misleading information by making an apparent distinction between single and double bonds from a single resonance structure. When considering polarity, imagine an averaging among double and single bonds in resonance so they will not be treated as different.

Part B. MODELS WITH MORE THAN ONE CENTRAL ATOM

In part B, the difference is that the molecules have more than one atom with two or more attached neighbors. As molecules become larger and more complex, VSEPR theory does not attempt to create names for the overall geometries that result. Instead, we continue to consider the geometry about one central atom at a time. By giving the arrangement about each such atom in the larger structure, we can generate enough information to develop an overall picture. Thus, in your report for part B, you should determine which atoms have more than one bound neighbor, then repeat the procedure for part A. When deciding whether the molecules in part B are polar or nonpolar, focus on the molecule as a whole, not the geometry about a single atom. If the molecule is polar, draw a dipole arrow on your sketch indicating polarity.

Part C. ISOMERS

If the elements in a molecular formula can be arranged to create more than one three-dimensional structure, the different structures are called isomers. Although some isomers contain the same number and type of atoms, the structural differences lead to different physical and/or chemical properties.

Lewis models can help to check for acceptable bonding arrangements in proposed molecular structures. However, since Lewis dot models are not intended to indicate geometries, they are not a particularly good tool for checking whether a particular bonding arrangement may lead to isomers. Physical models are the best tool. The ultimate test for whether two proposed structures are isomers is to make a model for each. If by flipping one of the models and rotating its single bonds you can get the positions of its atoms to match those of the other, they are not isomers, they are the same molecule. This is sometimes called the test of superimposability, i.e., if you can superimpose the atoms of one model onto those of the other, they are the same. If you cannot superimpose the atoms without breaking bonds, then the models represent two distinct isomers. Use this test in part D.

Your handout is to be completed during the lab period. I will check handouts off before you leave, and they are due one week from the lab period. Part A For each formula, fill in the eight items listed in the instructions. The Lewis models (at least) should be completed before coming to lab. Be sure to consider the Lewis dot model and the ball-and-stick model as two separate items. The Lewis model should not attempt to indicate geometry. The ball-and-stick model does not need to show multiple bonds or the positions of lone pairs of electrons but should depict molecular geometry.

Since ions have a net charge, the issue of polarity is usually unimportant for them. You can skip the polarity analysis for the ions. ) n s o r i i t a a p z e

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C Part C: Build the following molecules and answer the questions.

1. How many isomers are possible for the formula CH2Cl2? Explain. Build them and draw each one and comment on the polarity and molecular shape of each.

2. How many isomers are possible for the formula CFHClBr? Explain. Build them and draw each.

3. How many isomers are possible for the formula C2H2Cl2? Explain. Build and Draw each one and comment on the polarity and molecular shape of each.