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VSEPR MOLECULAR GEOMETRY • Valence Shell Electron Pair Repulsion theory. methane • caffeine Most important factor in determining geometry is the relative repulsion between electron see CD-ROM screen 9.12 Mar. 29, 2006 No. of e- Pairs Around Central Example Atom (σ + LP) 2 ammonia boron trifluoride F—Be—F Mar. 29, 2006 F linear Ammonia, NH3 1. Draw electron dot structure 2. Count σ BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. F planar trigonal B see CD-ROM screen 9.13 Structure Determination by VSEPR Geometry 180° 3 A molecule adopts the shape that minimizes electron pair repulsions around an atom. F 120° 4 C H •• 109° H H N H tetrahedral H H H Mar. 29, 2006 Mar. 29, 2006 Page 1 Structure Determination by VSEPR •• H N H H 2 lone pair of electrons in tetrahedral position N 1. Draw electron dot structure 2. Count σ BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. H H The ELECTRON Structure Determination by VSEPR Water, H O H PAIR GEOMETRY is tetrahedral. Mar. 29, 2006 The electron pair geometry is TETRAHEDRAL. H O The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL. •• H O H •• The molecular geometry is bent. H Mar. 29, 2006 Structure Determination by VSEPR •• O •• Structure Determination by VSEPR H C H Formaldehyde, CH2O 1. Draw electron dot structure 2. Count σ BP’s and LP’s = 3 Counts as 1 pair! 3. There are 3 electron pairs at the corners of a planar triangle. The electron pair geometry is PLANAR TRIGONAL with 120o bond angles. •• O •• C H Mar. 29, 2006 Mar. 29, 2006 Page 2 H The molecular geometry is also planar trigonal. Phenylalanine, an amino acid 1 H C H H C C C C H Phenylalanine H 2 H O 3 C C C O H N H 4 C H H H 5 Mar. 29, 2006 Mar. 29, 2006 Violations of the Octet Rule STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE B F •• •• Page 3 •• F •• •• Mar. 29, 2006 •• •• Mar. 29, 2006 •• F •• Usually occurs with Group 3A elements and with those of 3rd period and higher. Consider boron trifluoride, BF3 The B atom is surrounded by only 3 electron pairs. Bond angles are 120o This geometry is described as planar trigonal Compounds with 5 or More Pairs Around the Central Atom 90° F F P Sulfur Tetrafluoride, SF4 • Number of valence electrons = 34 • Central atom = S • Dot structure has 5 e- pairs around S Trigonal bipyramid 5 electron pairs F 120° F F 90° 6 electron pairs F F S F F Mar. 29, 2006 Electron pair geometry = trigonal bipyramid (because there are 5 pairs around the S) Octahedron F F 90° Mar. 29, 2006 Sulfur Tetrafluoride, SF4 •• •• •• F •• •• •••• F S F•• •• •• •• F•• •• The lone pair is in the equator because it requires more room. 90° F •• S F Mar. 29, 2006 F F 120° The molecular geometry is called seesaw. Page 4 •• •• •• F •• •• •••• F S F•• •• •• •• F•• ••