ch.9 - 11/6/2009 1 Chemical Bonds and Molecular Shapes B F...

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Unformatted text preview: 11/6/2009 1 Chemical Bonds and Molecular Shapes B F BF 3 B F F N H H H H H N NH 3 N H H H Molecular structure, trigonal planar MOLECULAR SHAPES There are two theories that allow for a simple, basic determination of the shapes of molecules and ions. ! VSEPR Model ! Valence Bond Theory V S E P R Valence Shell Electron Pair Repulsion Electrons in bonds and lone pairs repel each other and seek to be as far apart as possible to minimize these repulsions 11/6/2009 2 Molecular Structure VSEPR Once we assume that bond and lone pairs exist, we introduce some other assumptions one about introduce some other assumptions, one about structure and one about energies of interactions. The structural types that we use are based on the distribution of points on the surface of a sphere such that the distance between them is a maximum that the distance between them is a maximum. Molecular Structure VSEPR For two stereochemically active units, the obvious geometry is linear geometry is linear: 11/6/2009 3 Molecular Structure VSEPR For three stereochemically active units, we form a triangular arrangement of atoms around the central triangular arrangement of atoms around the central atom: This geometry is termed Trigonal Planar Trigonal Planar Molecular Structure VSEPR Four stereochemically active units are arranged in the form of a tetrahedron the form of a tetrahedron 11/6/2009 4 Molecular Structure VSEPR For five stereochemically active units, the geometry is trigona bipyramid is trigonal bipyramid Molecular Structure VSEPR Six stereochemically active units have only one choice for the base geometry the octahedron choice for the base geometry the octahedron 11/6/2009 5 T d Note: Trigonal bipyramidal TBP O h and octahedral electron geometries are possible only when there is an expanded octet of electrons on the central atom. How to draw TBP? axial position (ax) 90 ! ax 90 ! O h ? equatorial position (eq) ax eq eq 120 ! eq ax eq eq eq 90 ! Note: the angles between bonds in the (eq) positions are larger (120 ! ) than angles between (ax) and (eq) positions (90 ! ). " equatorial sites in TBP are LARGER Note: all angles are equivalent (90 ! ) " all sites in O h are equivalent 11/6/2009 6 N t ! C t b d i l d bl t i l j t ONE STEPS TO DETERMINE MOLECULAR GEOMETRY: I) Draw a correct Lewis structure II) Count VSEPR electron domains around the central atom Note: ! Count any bond: single, double, or triple as just ONE VSEPR electron domain ! Count a lone pair (LP) as...
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This note was uploaded on 05/11/2010 for the course CHEM 1201 taught by Professor Cook during the Spring '08 term at LSU.

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ch.9 - 11/6/2009 1 Chemical Bonds and Molecular Shapes B F...

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