Building VSEPR Models

Formulating Models The VSEPR model states that pairs of valence electrons on a central atom repel each other and are arranged so that the repulsions are as small as possible. In this miniLAB, you will use marshmallows and gum-drops to build models of substances, showing examples of the VSEPR model. 

Materials regular-sized marshmallows (3) minisized marshmallows (9) small gumdrops (3) toothpicks, cut in half 

Draw Lewis structures for methane (CH4), ammonia (NH3), and water (H2O). Notice the location of each shared and unshared pair of electrons. 

Using your Lewis structures, build a VSEPR 

How did drawing a Lewis structure help you to determine the geometry of each of your substances  

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To make sense of the VSEPR model, consider balloons of similar size tied together, as shown in Figure 9-13. Each balloon represents an orbital and, therefore, the repulsive force that keeps other electrons from entering this space. When each set of balloons is connected at a central point that represents the central atom, the balloons naturally form a shape that minimizes interactions between the balloons. You will build additional examples of VSEPR models in the miniLAB on page 261. 

The VSEPR model is probably the most successful and the most widely used model for predicting the shapes of simple non-ionic molecules. It builds directly on the Lewis formula of the molecule, but has been influenced by quanmm mechanics in so far as the electrons are allowed to move. A succinct description of the model has been given in a recent textbook by Gillespie and Hargittai [9] ... 

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