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Published byAlessandra Henman Modified about 1 year ago

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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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