Che 131 Notes Ch. 9

Che 131 Notes Ch. 9 - Che 131 Notes Chapter 9 Molecular...

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Unformatted text preview: Che 131 Notes Chapter 9: Molecular Structures Lecture 19: VSEPR Theory Reading: 9.1-9.2 I. 9.2. Predicting Molecular Shapes: VSEPR (valence-shell electron-pair repulsion) A. Repulsions occur among regions of electron density 1. i.e. repulsions among pairs of bonding and lone pair electrons 2. control angles between bonds from a ctrl atom to other atoms surrounding it 3. electron pairs are oriented as far apart from each other as possible to minimize repulsions B. Central atoms with only bonding pairs 1. AXE a. Used for molecules and polyatomic ions having a central atom that has only shared electron pairs to bond it to other atoms by only single bonds b. A: central atom c. X: terminal atoms d. E: number of lone pair electrons on the central atom 2. Geometries a. electron-pair geometry i. determined by the number and arrangement of the electron pairs around the central atom b. molecular geometry i. arrangement of the atoms in space c. bond angles i. angles between the bonds of two atoms that are bonded to the same third atom C. Multiple bonds and molecular geometry 1. double and triple bonds only have a minor effect on predictions of molecular shape even though they are shorter and stronger than single bonds a. electron pairs involved in a multiple bond are all shared between the same two nuclei and occupy the same region i. must stay in that region D. Central atoms with bonding pairs and lone pairs- applying the VSEPR model 1. Draw the structure 2. Determine the number of bonds and the number of lone pairs around each atom 3. Pick the appropriate electron-pair geometry around each central atom, and then choose the molecular shape that matches the total number of bonds and lone pairs 4. Predict the bond angles, remembering that lone pairs occupy more volume than do bonding pairs a. bonding pairs are relatively compact because of the strong attractive forces of two positive nuclei b. for a lone pair, there is only one nucleus attracting the pair E. Expanded octets: central atoms with five or six electron pairs 1. 5 electron pairs a. triangular bipyramidal i. equatorial positions • positions in the triangular plane which lie in the equator of an imaginary sphere around the central atom ii. axial positions • north and south poles iii. any lone pairs, because they are fatter than bonding pairs, will occupy equatorial positions rather than axial positions • each equatorial position is next to two others, 120º in between • each axial position is 90º to 3 equatorial positions 2. 6 electron pairs a. Octahedron i. Each angle is 90º • No distinct axial and equatorial positions, all positions are equivalent and equidistant from the central atom ii. If the central atom has 2 lone pairs, they are placed at 180º to each other • Give them as much room as possible Chapter 9: Molecular Structures Lecture 20: Hybridization Reading: 9.3-9.4 II. 9.3 Orbitals Consistent with Molecular Shapes: Hybridization A. Valence bond model 1. theoretical model of covalent bonding...
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This note was uploaded on 10/13/2008 for the course CHE 131 taught by Professor Kerber during the Fall '08 term at SUNY Stony Brook.

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Che 131 Notes Ch. 9 - Che 131 Notes Chapter 9 Molecular...

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