VSEPR - MOLECULAR MOLECULAR GEOMETRY GEOMETRY 1 MOLECULAR...

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Unformatted text preview: MOLECULAR MOLECULAR GEOMETRY GEOMETRY 1 MOLECULAR GEOMETRY 2 3 MOLECULAR GEOMETRY VSEPR VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs. VSEPR VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs. Molecule Molecule adopts the adopts the shape that shape that minimizes the minimizes the electron pair electron pair repulsions . repulsions. 4 5 No. of e- Pairs Around Central Atom 2 Example F—Be—F 180˚ F Geometry linear No. of e- Pairs Around Central Atom 2 Example F—Be—F 180˚ Geometry linear No. of e- Pairs Around Central Atom 2 6 Example F—Be—F 180˚ F Geometry linear 3 F B F 120˚ H 109˚ planar trigonal F 3 F 120˚ H 109˚ 4 C H H H B F planar trigonal 3 F B F 120˚ H 109˚ planar trigonal 4 C H H H 4 tetrahedral H C H H tetrahedral tetrahedral Page 1 No. of e- Pairs Around Central Atom 2 7 8 Example F—Be—F 180˚ F Geometry linear Structure Determination Structure Determination by VSEPR by VSEPR Ammonia, NH 3 H 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron . N H H H lone pair of electrons in tetrahedral position Structure Determination Structure Determination by VSEPR by VSEPR Ammonia, NH 3 There are 4 electron pairs at the corners of a tetrahedron. 9 NH H •• 3 F B F 120˚ H 109˚ planar trigonal HNH H H •• •• N H H lone pair of electrons in tetrahedral position 4 C H H H tetrahedral The ELECTRON PAIR GEOMETRY is is ELECTRON tetrahedral . tetrahedral. Structure Determination Structure Determination by VSEPR by VSEPR Ammonia, NH 3 The electron pair geometry is tetrahedral. lone pair of electrons in tetrahedral position H H 10 11 12 Structure Determination Structure Determination by VSEPR by VSEPR Water, H 2O 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. O O H H H H Structure Determination Structure Determination by VSEPR by VSEPR Water, H 2O HOH •• •• •• •• HOH •• •• •• •• O O H H H H N H The electron pair The electron pair geometry is geometry is TETRAHEDRAL TETRAHEDRAL The MOLECULAR GEOMETRY — the The MOLECULAR GEOMETRY — the the positions of the atoms — is PYRAMIDAL.. positions of the atoms — is PYRAMIDAL The electron pair The electron pair geometry is geometry is TETRAHEDRAL. TETRAHEDRAL. The molecular The molecular geometry is geometry is BENT. BENT . BENT. Page 2 Structure Determination Structure Determination by VSEPR by VSEPR Formaldehyde, CH 2O 1. Draw electron dot structure • • 13 14 15 O • • Structure Determination Structure Determination by VSEPR by VSEPR H Formaldehyde, CH 2O • • • • Structure Determination by VSEPR Methanol, CH 3OH Define bond angles 1 and 2 Angle 1 = 109 o Angle 2 = 109 o In both cases the atom is surrounded by 4 electron pairs. H •• H—C—O—H •• 109˚ H 109˚ 2. Count BP’s and LP’s = 3 3. There are 3 electron pairs are at the corners of a planar triangle. • • HC O C • • • • The electron pair The electron pair geometry is PLANAR geometry is PLANAR TRIGONAL TRIGONAL O C • • H H The electron pair The electron pair geometry is PLANAR geometry is PLANAR PLANAR TRIGONAL with 120 o TRIGONAL with 120o bond angles. bond angles. H H The molecular The molecular geometry is also planar geometry is also planar trigonal. trigonal . trigonal. 16 17 18 Structure Determination Structure Determination by VSEPR by VSEPR Acetonitrile , CH 3CN H •• Phenylalanine, an amino acid Phenylalanine, H C H C C H C H 1 C C H H 2H C H C N H O C H 5 3 O 4 H Phenylalanine Phenylalanine Define unique bond angles H—C—C N H-C-H = 109 o 109˚ H 180˚ C-C-N C-C-N = 180 o One C is surrounded by 4 electron “lumps” and the other by 2 “lumps” Page 3 19 20 21 Structures with Structures with Central Atoms Central Atoms with More Than with More Than or Less Than 4 or Less Than 4 Electron Pairs Electron Pairs Central Atoms With Fewer Central Atoms With Fewer Than 4 Electron Pairs Than 4 Electron Pairs Often occurs with Group 3A elements and with those of 3rd period and higher. Consider boron trifluoride , BF3 trifluoride, Boron Compounds Boron Compounds Consider boron trifluoride , trifluoride, BF The 3 atom is surrounded by B only 3 electron pairs. Bond angles are 120 o Geometry described • • •• F •• • • • • FB •• • • F •• • • planar planar trigonal as 22 23 24 Compounds with 5 or 6 Pairs Compounds with 5 or 6 Pairs Around the Central Atom Around the Central Atom Compounds with 5 or 6 Pairs Compounds with 5 or 6 Pairs Around the Central Atom Around the Central Atom 90˚ F F P F Trigonal bipyramid F F 120˚ 5 electron pairs Compounds with 5 or More Compounds with 5 or More Pairs Around the Central Atom Pairs Around the Central Atom 90˚ F F P F 90˚ 6 electron pairs Trigonal bipyramid F F F S F Octahedron F F 90˚ 120˚ 5 electron pairs F F Page 4 25 26 27 Sulfur Tetrafluoride, SF4 Sulfur Tetrafluoride, SF4 Tetrafluoride, • Number of valence electrons = • Central atom = Central • Dot structure Sulfur Tetrafluoride, SF4 Sulfur Tetrafluoride, SF4 Tetrafluoride, • Number of valence electrons = 34 • Central atom = S Central • Dot structure Sulfur Tetrafluoride, SF4 Sulfur Tetrafluoride, SF4 Tetrafluoride, Number of valence electrons = 34 Central atom = S •• •• •F ••• Dot structure •• •• •• •F ••• •• ••• S F• F ••• ••• • F• •••• ••• F ••• S F• ••• • F• •••• Electron pair geometry = ? 28 29 Sulfur Tetrafluoride, SF4 Sulfur Tetrafluoride, SF4 Tetrafluoride, • Number of valence electrons = 34 • Central atom = S Central • Dot structure Electron pair geometry = trigonal bipyramid (because there are 5 pairs around the S) Sulfur Tetrafluoride, SF4 Sulfur Tetrafluoride, SF4 Tetrafluoride, Lone pair is in the equator because it requires more room. 90˚ • • F S F 120˚ F F 120˚ •• •• •F ••• •• ••• S F• F ••• ••• • F• •••• 90˚ • • F S F F F •• •• •F • •••• S ••• •F F ••• ••• • F• •••• Page 5 ...
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This note was uploaded on 01/11/2011 for the course ENGINEERIN MAE 107 taught by Professor Pozikrizdis during the Fall '08 term at San Diego.

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