Chapter9 - 1 SHAPES OF MOLECULES (VSEPR MODEL) Valence...

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1 SHAPES OF MOLECULES (VSEPR MODEL) V alence S hell E lectron-P air R epulsion model - Electron pairs surrounding atom spread out as to minimize repulsion. - Electron pairs can be bonding pairs or nonbonding pairs (multiple bonds not included). - Arrangement of all the atoms surrounding central atom depends on electron pairs surrounding central atom. Two similar, but different geometries 1. Electron pair geometry - arrangement of e - pair around central atom - remember: e - pair from multiple bonds don’t count 2. Molecular geometry - arrangement of atoms around central atom **A molecular geometry is decided only after an electron pair geometry has been determined.** *- need to write Lewis structure to determine number of electron pairs.* Geometries with two e - pairs about central atom. electron pair geometry – linear - angle between e - pairs is 180 ° possible molecular geometries a) Linear - only linear geometry is possible with two electron pairs Example: BeCl 2 CO 2 - Note: Only two electron pairs around central atom since double bonds don’t count. CO A A – generic atom Be Cl Cl C O O
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2 Geometries with three e - pairs about central atom. 1. electron pair geometry – trigonal planar - angle between e - pairs is 120 ° 2. possible molecular geometries a) Trigonal Planar - all three electron pairs are bonding pairs Examples: BF 3 and NO 3 - b) Bent (V-shaped) - two bonding pairs and one nonbonding pair Example: SnCl 2 *Nonbonding e- pairs take up more room than bonding pairs. Therefore bond angle between chlorine atoms is slightly less than 120 ° .* Geometries with four e - pairs about central atom. electron pair geometry – tetrahedral - tetrahedron is three dimensional object - angle between electron pairs is 109.4 ° a. Tetrahedral - all four electron pairs are bonding pairs CH 4 A N O B F F F Sn Cl Cl A Caution! Representation is 2-D, not 3-D, bond angle between e - pairs is not 90 ° H C H H H 2-D picture 3-D picture C H H HH A – generic atom
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3 Example: PO 4 3- b) Trigonal Pyramidal - 3 bonding pairs and 1 nonbonding pair NH 3 - Bond angle is 107 ° . (specifically for NH 3 ) - Bond angle is less than 109.4 ° because nonbonding pair takes more room than bonding pair. ClO 3 - c) Bent (V-shaped) - two bonding pairs and two nonbonding pairs H 2 O - Bond angle is 104.5 ° . Redraw H 2 O to show tetrahedral angle. O H H P N H H H Cl O H H P O O O O N H H H Cl O O O
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4 Example: SF 2 Geometries with five e - pairs about central atom. 1. electron pair geometry – trigonal bipyramidal - two different positions in a trigonal bipyramid - axial – two position along axis - equatorial – three positions along equator - angle between equatorial positions is 120 ° - axial positions are 90 ° from equator * - nonbonding pairs prefer equatorial position* 2. possible molecular geometries a. Trigonal Bipyramidal - all five electron pairs are bonding pairs PF 5 b. Seesaw - four electron pairs are bonding pairs and one nonbonding pair 4 - remember lone pairs prefer equatorial position F F S P F F F F F S F F F F P F F F F F S F F F F
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5 c. T-shaped
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Chapter9 - 1 SHAPES OF MOLECULES (VSEPR MODEL) Valence...

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