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Molecular shapes Balls and sticks

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Learning objectives Apply VSEPR to predict electronic geometry and shapes of simple molecules

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Roadmap to polarity Establish skeleton of molecule Determine Lewis dot structure using S = N – A Determine electronic geometry using VSEPR Identify molecular geometry from molecular

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Valence shell electron pair repulsion Lewis dot structure provides 2D sketch of the distribution of the valence electrons among bonds between atoms and lone pairs; it provides no information about molecular shape First approach to this problem is to consider repulsion between groups of electrons (charge clouds)

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Electron groups (clouds) minimize potential energy Valence shell electron pair repulsion (VSEPR) Identify all groups of charge: non-bonding pairs or bonds (multiples count as one) Bonded atoms – single, double or triple count as 1 Lone pairs count as 1 Distribute them about central atom to minimize potential energy (maximum separation)

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Total number of groups dictates electronic geometry Octet rule: Two – linear Three – trigonal planar Four – tetrahedral Additional possibilities (expand octet): Five – trigonal bipyramidal Six - octahedral

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Stage 3: Molecular shape: What you get from electronic geometry considering atoms only Same tetrahedral electronic geometry – different molecular shape

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Three groups: trigonal planar Two possibilities for central atoms with complete octets: Trigonal planar (H 2 CO) Bent (SO 2 ) BCl 3 provides example of trigonal planar with three single bonds B is satisfied with 6 electrons

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Four groups: tetrahedral Three possibilities: No lone pairs (CH 4 ) - tetrahedral One lone pair (NH 3 ) – trigonal pyramid Two lone pairs (H 2 O) – bent Note: H-N-H angle 107°H-N-H angle 107° H-O-H angle 104.5°H-O-H angle 104.5° Tetrahedral angle 109.5°Tetrahedral angle 109.5°

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Representations of the tetrahedron

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Groups of charge Lone electron pairs Electronic geometry Molecular shape 20Linear 30Trigonal planar 31 Bent 40Tetrahedral 41 Trigonal pyramid 42TetrahedralBent

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Two bonds Equal bonds oppose (linear) Nonpolar (CO 2 ) Unequal bonds oppose (linear) Polar (HCN) Equal bonds do not oppose (bent) Polar (H 2 O)

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Three bonds Equal bonds oppose in trigonal planar arrangement Nonpolar Unequal bonds in trigonal planar arrangement Polar

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IIIIII Molecular Geometry Molecular Structure. A. VSEPR Theory Valence Shell Electron Pair Repulsion Theory Electron pairs orient themselves so that.

IIIIII Molecular Geometry Molecular Structure. A. VSEPR Theory Valence Shell Electron Pair Repulsion Theory Electron pairs orient themselves so that.

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