Ch 10 b - Molecular Geometry Molecules are three-dimensional objects We often describe the shape of a molecule with terms that relate to geometric

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Tro's Introductory Chemistry, Chapter 10 1 Molecular Geometry Molecules are three-dimensional objects. We often describe the shape of a molecule with terms that relate to geometric figures. These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms with the central atom in the center of the figure. The geometric figures also have characteristic angles that we call bond angles .
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Tro's Introductory Chemistry, Chapter 10 2 Some Geometric Figures Linear Two atoms on opposite sides of central atom. 180° bond angles. Trigonal planar Three atoms form a triangle around the central atom. Planar. 120° bond angles. Tetrahedral Four surrounding atoms form a tetrahedron around the central atom. 109.5° bond angles. 180° 120° 109.5°
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VSEPR Theory Electron groups around the central atom will be most stable when they are as far apart as possible. We call this valence shell electron pair repulsion theory . Since electrons are negatively charged, they should be most stable when they are separated. The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule. Tro's Introductory Chemistry, Chapter 10 3
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Electron Groups The Lewis structure predicts the arrangement of valence electrons around the central atom(s). Each lone pair of electrons constitutes one electron group on a central atom. Each bond constitutes one electron group on a central atom. Regardless of whether it is single, double, or triple. O N O ¥ ¥ ¥ ¥ ¥¥ ¥¥ ¥¥ ¥¥ There are 3 electron groups on N. 1 lone pair. 1 single bond. 1 double bond. Tro's Introductory Chemistry, Chapter 10 4
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Linear Geometry When there are two electron groups around the central atom, they will occupy positions opposite each other around the central atom. This results in the molecule taking a linear geometry . The bond angle is 180°. Cl Be Cl O C O Tro's Introductory Chemistry, Chapter 10 5
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Trigonal Geometry When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom. This results in the molecule taking a trigonal planar geometry . The bond angle is 120°. F F B F Tro's Introductory Chemistry, Chapter 10 6
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Tetrahedral Geometry When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom. This results in the molecule taking a tetrahedral geometry . The bond angle is 109.5°.
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This note was uploaded on 08/06/2011 for the course CHEM 151 taught by Professor Mcbroom during the Summer '10 term at College of the Canyons.

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Ch 10 b - Molecular Geometry Molecules are three-dimensional objects We often describe the shape of a molecule with terms that relate to geometric

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