Chap9 - Chem. 200 Dr. Saidane Lecture Notes Chapter 9...

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Chem. 200 Dr. Saidane Lecture Notes Chapter 9 Molecular Geometry and Bonding Theory SHAPES OF MOLECULES AND IONS To a chemist, one of the most interesting aspects of a molecule is its shape or molecular geometry, which is the arrangement in space of the atoms bonded to each other. Lewis structures do not directly show the three-dimensional arrangements of atoms in a molecule, which play a key role in determining chemical reactivity. To account for these molecular shapes, we need just one addition to Lewis’s model: the VSEPR model. The VSEPR Model The VSEPR model is the Valence-Shell Electron-Pair Repulsion Model . This model is based on the fact that regions of high electron concentration repel one another. Bonding electrons and lone pairs take up positions as far from one another as possible, to minimize their repulsion and therefore maximize their separations. In this model, we focus on the central atom of a molecule and we imagine that all the electrons involved in bonds to this atom and the lone pairs belonging to the central atom lie on the surface of an invisible sphere that surrounds the central atom. These bonding electrons and lone pairs are regions of high electron concentration, and they repel one another. To minimize these repulsions, these regions move as far as possible on the surface of the sphere. We identify the shape of the molecule by noting where the atoms lie and look up the name of the shape in figure 9.2 p. 305. Multiple Bonds in the VSEPR Model When using the VSEPR model to predict molecular shapes, we do not need to distinguish between single and multiple bonds. A multiple bond is treated as another region of high electron concentration just as a single bond. Molecules With Lone Pairs on The Central Atom If a molecule has lone pairs on the central atom, they occupy a region around the central atom and therefore contribute to the shape of the molecule. However only the positions 1
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of atoms are considered when describing the shape of a molecule, and therefore lone pairs are ignored when we name the shape of the molecule. The Distorting Effect Of Lone Pairs According to the VSEPR model, lone pairs have a more strongly repelling effect than bonding pairs. In other words it is best for lone pairs to be as far as possible from each other as possible. It is also best for atoms to be far from lone pairs, even though that might bring them close to other atoms. The strengths of repulsions are in the order: lone pair-lone pair > lone pair-bonding air > bonding pair-bonding-pair. How To Use the VSEPR Model The general procedure for predicting the shape of a molecule is as follows: 1. Write the Lewis structure of the molecule. Treat a multiple bond equivalent of a single electron pair. Identify the central atom and assign the letter A to the central atom. 2.
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This note was uploaded on 09/09/2011 for the course CHEM 200 taught by Professor Saidane during the Spring '11 term at Mesa CC.

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Chap9 - Chem. 200 Dr. Saidane Lecture Notes Chapter 9...

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