Chapter 09 - CHEMISTRY The Molecular Science Chapter nine...

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CHEMISTRY The Molecular Science Chapter nine Molecular Structures
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H H | | H-C-C-O-H | | H H ethanol H H | | H-C-O-C-H | | H H dimethyl ether Molecular formulas, structural formulas, Lewis structures and three-dimensional shape of a molecule are all needed to understand the properties and reactivity of molecules.
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Models of Water
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Drawing 3-dimensional structures on paper:
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What is the “shape” of the molecule? VSEPR – valence shell electron pair repulsion. - Based upon minimizing the electron repulsion between electron pairs (lone, bonding). Lone pair electrons occupy a greater volume than bonding electron pairs a greater repulsion results from lone pair electrons than bonding electrons. - To minimize electron repulsions, separate the electron pairs around central atom as far apart as possible. determines the bond angle and molecular geometry.
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VSEPR Models Shapes of molecules or ions when the central atom A has two or more terminal atoms, X, and no lone pairs.
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Molecular Geometry To predict the molecular geometry: draw Lewis dot structure. count the number of bonding electron pairs about the central atom ( double and triple bonds both count as one pair in VSEPR). count the number of lone pairs of electrons. match electron pair information to shapes. Electron pair geometry – arrangement of electron pairs in space. Molecular geometry – arrangement of atoms in space.
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Idealized VSEPR Geometries no lone pair of electrons on central atom electron pair geometry = molecular geometry A = central atom X = terminal atoms E = lone pairs
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VSEPR Theory - Three and Four Electron Pairs
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Problem: Identify the electron pair geometry and molecular geometry of CH 4 , NH 3 , and H 2 O.
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Predicting bond angles: Lone pairs occupy more volume than bonding pairs – only one nucleus attracting the electron pair. Electron- electron repulsions: Lone pair – lone pair > lone pair – bonding pair > bonding pair – bonding pair Bond angle decreases when the # of lone pairs increases.
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Electron pairs involved in multiple bonds are all shared between the same two nuclei and therefore occupy the same region in space. We count multiple bonds as one bonding pair when using VSEPR. CO
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Chapter 09 - CHEMISTRY The Molecular Science Chapter nine...

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