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10.7.00 10:09 PM 1 Determining Molecular Shape A Tutorial on Writing the Shape of Molecules Constructing Molecular Shapes A Tutorial on Writing the Shape of Molecules Dr. Fred Omega Garces Chemistry 100 Miramar College

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10.7.00 10:09 PM 2 Determining Molecular Shape VSEPR- Valence Shell Electron-Pair Repulsion Theory Main premise of model- Valence electron pair repel each other in molecule with shapes the molecule Molecule assumes Geometry that minimizes electrostatic repulsion: Occurs when electron pair are far apart as possible. Driving force is the Pauli exclusion principle : 2 electrons with same spin can't occupy the same space. Electronic Geometry is the geometry around the central atom in which electron-electron repulsion is minimize. AE n (system) Molecular Geometry is geometry around central atom when electron pairs are replace by bonding atoms and the nonbonding electrons are ignored. AB m E n (system)

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10.7.00 10:09 PM 3 Determining Molecular Shape VSEPR- Procedural Steps 1) Determine the Lewis Structure. a) Valence electrons for each atom in the structure. b) Determine the atomic sequence, the number of bonds, remaining electrons c) Write Lewis structure with each atom obeying the octet rule Example: HNO 3 (See Lewis Structure Tutorial) See Lewis Structure TutorialSee Lewis Structure Tutorial H NOO O

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10.7.00 10:09 PM 4 Determining Molecular Shape VSEPR- Procedural Steps 2) Determine electronic geometry ( AE n system ) from Lewis structure. a) Count the electron domain (region) around the central atom. a) Count the electron domain (region) around the central atom. b) Arrange electron domain to minimize electron-electron repulsion. b) Arrange electron domain to minimize electron-electron repulsion. Occurs when electron pair are far apart as possible. Occurs when electron pair are far apart as possible. c) 2-domain linear, 3-domain trigonal, 4-domain tetrahedral c) 2-domain linear, 3-domain trigonal, 4-domain tetrahedral Example: HNO 3 Central Atoms, N and O N: Three electron domain AE 3 Trigonal AE 3 Trigonal O: Four electron domain AE 4 Tetrahedral AE 4 Tetrahedral H NOO O H NOO O H NOO O

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10.7.00 10:09 PM 5 Determining Molecular Shape VSEPR- Procedural Steps 3) Determine molecule geometry ( AB m E n ) from electronic geometry. a) The geometry is based on the position of the atoms. b) The lone pair electrons are ignored for the molecular geometry. i) Elec Geometry -Linear AE 2. Molc Geometry-Linear AB 2 ii) Elec Geometry - Trigonal AE 3. MolcGeo- Trig AB 3 or Bent AB 2 E ii) Elec Geometry -Tetrahedral AE 4. Molc Geo -tetr AB 4, pyramid AB 3 E or Bent AB 2 E 2 ii) Elec Geometry -Tetrahedral AE 4. Molc Geo -tetr AB 4, pyramid AB 3 E or Bent AB 2 E 2 Example: HNO 3 Central atom: Nitrogen Elec Geometry - AE 3 Trigonal. Molc Geometry - AB 3 Trigonal. H NOO O Central atom: Oxygen Elec Geometry - AE 4 Tetrahedral Molc Geometry - AE 2 B 2 Bent (“V”) Space Filling Model (A) and Ball and Stick Model (B). The shape around the Nitrogen has a trigonal geometry and the shape around the oxygen is a bent (or “V”) shape. AB

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10.7.00 10:09 PM 6 Determining Molecular Shape HNO 3 Quick Time Movie Click on the movie to start the QuickTime Movie on the perspective of the Nitric acid (HNO 3 ) molecule.

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10.7.00 10:09 PM 7 Determining Molecular Shape VSEPR Table

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10.7.00 10:09 PM 8 Determining Molecular Shape VSEPR Table: QuickTime Movie

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10.7.00 10:09 PM 9 Determining Molecular Shape SummarySummary Key to determine molecular Geometry Lewis Structure Lewis Structure Determine electron domain in structure. Electronic Geometry Electronic Geometry Establish the AE n system. Molecular Geometry Molecular Geometry Establish the AB m E n system. Don’t count lone pairs in the molecular.

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