Expt 11 - Molecular Geometry - FA 2009

Expt 11 - Molecular Geometry - FA 2009 - MOLECULAR GEOMETRY...

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MOLECULAR GEOMETRY This exercise is an introduction to the shapes of molecules. This material will be covered in lecture in more detail, and appears in sections 10.2 – 10.4 of Principles of Chemistry – A Molecular Approach , by Tro. The way we will do this is by using a model for predicting the shapes of molecules called Valence Shell Electron Pair Repulsion (VSEPR) . In this method, the shape of the molecule is determined by the repulsions between electrons in the valence shell of the atoms comprising the molecule. Before we can start thinking about why molecules adopt certain shapes it is important to look at the bonds that make up a molecule. Remember, bonds are formed when atoms share electrons. This is because of the attraction of the positively charged nuclei for the electrons in between them. H H + + e - e - In a diatomic molecule, as shown above, the arrangement of the atoms is linear. What happens when a second bond is introduced? There are now more forces to consider. In addition to the attraction between the nuclei and the electrons, there is now repulsion between the electrons in the bonds. The molecule’s shape is determined by these repulsive forces. It will be in its preferred conformation (lowest energy state) when the repulsions are minimized. This occurs when there is greater distance between the electron groups. In the case of our two-bond molecule above, the atoms will form a linear arrangement. Build a model of this molecule by taking the orange piece out of your model kit and putting one of the atom markers (a white plastic ball) on each end. The notch in the center of the model represents the central atom. As you can see, there is a 180 degree angle between the two bonds. Draw a sketch of this model below using the letter A to represent the central atom and X’s to represent the atoms bonded to it. Sketch for AX 2 : 1
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Next, do the same for molecule containing 3 A-X bonds, i.e., AX 3 . using the light blue model. You should see that the bond angle now decreases from 180 degrees to 120 degrees. Sketch for AX 3 : What would you predict the bond angle to be for a system with 4 bonds to a central atom? You might be tempted to draw the bonds separated by 90 degrees, as shown below: A X X X X Assemble the two black plastic pieces in your model kit and estimate the angles between the bonds. Is it greater or less than 90 degrees? The shape you have is a tetrahedron and it offers the lowest energy structure for a molecule containing a central atom surrounded by 4 others (4 bonds).
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Expt 11 - Molecular Geometry - FA 2009 - MOLECULAR GEOMETRY...

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