Chapter10 - Chapter Ten Bonding Theory and Molecular...

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1 Bonding Theory and Molecular Structure Chapter Ten Chapter Ten
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2 Molecular Geometry Molecular geometry is simply the shape of a molecule. Molecular geometry is described by the geometric figure formed when the atomic nuclei are joined by (imaginary) straight lines. Molecular geometry is found using the Lewis structure, but the Lewis structure itself does NOT necessarily represent the molecule’s shape. A water molecule is angular or bent . A carbon dioxide molecule is linear.
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3 VSEPR Valence-Shell Electron-Pair Repulsion ( VSEPR ) is a simple method for determining geometry. Basis: pairs of valence electrons in bonded atoms repel one another. These mutual repulsions push electron pairs as far from one another as possible. B A B B B B B A When the electron pairs (bonds) are as far apart as they can get, what will be the B-A-B angle?
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4 Electron-Group Geometries An electron group is a collection of valence electrons, localized in a region around a central atom. One electron group: an unshared pair of valence electrons or a bond (single, double, or triple) The repulsions among electron groups lead to an orientation of the groups that is called the electron- group geometry . These geometries are based on the number of electron groups: Electron groups Electron-group geometry 2 Linear 3 Trigonal planar 4 Tetrahedral 5 Trigonal bipyramidal 6 Octahedral
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5 A Balloon Analogy Electron groups repel one another in the same way that balloons push one another apart. When four balloons, tied at the middle, push themselves apart as much as possible, they make a tetrahedral shape.
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6 VSEPR Notation In the VSEPR notation used to describe molecular geometries, the central atom in a structure is denoted as A , terminal atoms as X , and the lone pairs of electrons as E . The H 2 O molecule would therefore carry the designation AX 2 E 2 .
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7 VSEPR Notation For structures with no lone pairs on the central atom ( AX n ), the molecular geometry is the same as the electron-group geometry. When there are lone pairs, the molecular geometry is derived from the electron-group geometry. In either case, the electron-group geometry is the tool we use to obtain the molecular geometry.
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14 Structures with No Lone Pairs AX 2 : both the electron-group geometry and the molecular geometry for two electron groups is linear . AX 3 : these molecules have a trigonal planar geometry. AX 4 : these molecules have a tetrahedral geometry. AX 5 : these molecules have a trigonal bipyramidal geometry. AX
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This note was uploaded on 07/13/2008 for the course CHEM 120 taught by Professor Nyssen during the Fall '07 term at University of Tennessee.

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Chapter10 - Chapter Ten Bonding Theory and Molecular...

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