SiO 2 CO 2 MOLECULAR STRUCTURE 7/20/2009. Two forms of carbon; graphite and diamond. Quartz grows in beautiful, regular crystals
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1 MOLECULAR STRUCTURE A computer representation of K 3 C 60, a superconducting substance formed by reacting potassium with buckminster fullerine (C 60 ) Source: Photo Researchers, Inc. Copyright oughton Mifflin Company. All rights reserved. 13a 2 Two forms of carbon; graphite and diamond. Quartz grows in beautiful, regular crystals SiO 2 Vs CO 2 Source: Grant ellman Copyright oughton Mifflin Company. All rights reserved. 13a 3 Copyright oughton Mifflin Company. All rights reserved. 13a 4 1
2 igure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance between the nuclei of the hydrogen atoms. igure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance between the nuclei of the hydrogen atoms. Copyright oughton Mifflin Company. All rights reserved. 13a 5 Copyright oughton Mifflin Company. All rights reserved. 13a 6 WERE DO TE ELECTRONS GO? WERE DO TE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other? Are they shared equally? Are they more on one atom than the other? Copyright oughton Mifflin Company. All rights reserved. 13a 7 Copyright oughton Mifflin Company. All rights reserved. 13a 8 2
3 WERE DO TE ELECTRONS GO? WERE DO TE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other? Are they shared equally? Are they more on one atom than the other? ANSWER: It depends who is pulling harder ANSWER: It depends who is pulling harder ( Electro negativity ) Copyright oughton Mifflin Company. All rights reserved. 13a 9 Copyright oughton Mifflin Company. All rights reserved. 13a 10 The CL molecule has a dipole moment Copyright oughton Mifflin Company. All rights reserved. 13a 11 Copyright oughton Mifflin Company. All rights reserved. 13a 12 3
4 Pauling and his electronegativity Pauling and his electronegativity Copyright oughton Mifflin Company. All rights reserved. 13a 13 Copyright oughton Mifflin Company. All rights reserved. 13a 14 igure 13.3: The Pauling electronegativity values as updated by A.L. Allred in (cont d) Arbitrarily set as 4 Copyright oughton Mifflin Company. All rights reserved. 13a 15 Copyright oughton Mifflin Company. All rights reserved. 13a 16 4
5 The CL molecule has a dipole moment Copyright oughton Mifflin Company. All rights reserved. 13a 17 Copyright oughton Mifflin Company. All rights reserved. 13a 18 X X Copyright oughton Mifflin Company. All rights reserved. 13a 19 Copyright oughton Mifflin Company. All rights reserved. 13a 20 5
6 models Now lets consider more than two atoms in a molecule Copyright oughton Mifflin Company. All rights reserved. 13a 21 Copyright oughton Mifflin Company. All rights reserved. 13a 22 Linear molecules Planar molecules Copyright oughton Mifflin Company. All rights reserved. 13a 23 Copyright oughton Mifflin Company. All rights reserved. 13a 24 6
7 Tetrahedral molecules MODELS These are only models Copyright oughton Mifflin Company. All rights reserved. 13a 25 Copyright oughton Mifflin Company. All rights reserved. 13a 26 MODELS MODELS These are only models But. Models are very useful for describing properties. These are only models But. Models are very useful for describing properties. Newton: particles Copyright oughton Mifflin Company. All rights reserved. 13a 27 Copyright oughton Mifflin Company. All rights reserved. 13a 28 7
8 MODELS models These are only models But. Models are very useful for describing properties. Newton: particles uygens: waves Copyright oughton Mifflin Company. All rights reserved. 13a 29 Copyright oughton Mifflin Company. All rights reserved. 13a 30 Ball-and-stick model of a protein segment illustrating the alpha helix. The concept of individual bonds makes it much easier to deal with complex molecules such as DNA. Source: Photo Researchers, Inc. Copyright oughton Mifflin Company. All rights reserved. 13a 31 Source: Photo Researchers, Inc. Copyright oughton Mifflin Company. All rights reserved. 13a 32 8
9 Polarity of Molecules Dipole Moments of Polyatomic Molecules Example: in CO 2, each C-O dipole is canceled because the molecule is linear. In 2 O, the -O dipoles do not cancel because the molecule is bent. O C O Copyright oughton Mifflin Company. All rights reserved. 13a 33 Copyright oughton Mifflin Company. All rights reserved. 13a 34 Skeletal Structure Skeletal Structure ydrogen atoms are always terminal atoms. Central atoms are generally those with the lowest electronegativity. Carbon atoms are always central atoms. Generally structures are compact and symmetrical. Identify central and terminal atoms in the molecule l C 2 6 O (ethyl alcohol l or ethanol). Now where NOTE: Terminal atoms are all bonded to only one other atoms. do Central atoms are bonded to two or more other atoms electrons go? C C O Copyright oughton Mifflin Company. All rights reserved. 13a 35 Copyright oughton Mifflin Company. All rights reserved. 13a 36 9
10 Writing Lewis Structures All the valence e - of atoms must appear. Usually, the e - are paired. Usually, each atom requires an octet. only requires 2 e -. Multiple bonds may be needed. Readily formed by C, N, O, S, and P. Copyright oughton Mifflin Company. All rights reserved. 13a 37 Lewis Structures Draw Lewis structures for: : 2 O: N 3 : C 4 : O N C Copyright oughton Mifflin Company. All rights reserved. or or O or N or C Benzene Resonance orms redrich August von Kekule (German chemist) said that he discovered the ring-shaped chemical structure of benzene because of a strange, reptilian dream he had in 1865: "I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes.... My mental eye... could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... " ( rom "Creativity, Beyond the Myth of Genius" by Robert Weisberg published by W.. reeman 1992.) Although some scholars now believe that Kekule's dream was a hoax to avoid sharing credit for the discovery of the hexagonal shape of benzene, it still makes a wonderful story. C ve s C C C C C C C C C C C C Copyright oughton Mifflin Company. All rights reserved. 13a 39 Copyright oughton Mifflin Company. All rights reserved. 10
11 7/20/2009 Electron-Rich Atoms Exceptions to the Octet Rule P P 5 Expanded octets. P P S Copyright oughton Mifflin Company. All rights reserved. Copyright oughton Mifflin Company. All rights reserved. 13a 42 Molecular Shapes Molecular Shapes Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. The shape of a molecule is determined by its bond angles. In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion. We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory. Copyright oughton Mifflin Company. All rights reserved. 13a 43 Copyright oughton Mifflin Company. All rights reserved. 13a 44 11
12 The VSEPR Model Predicting Molecular Geometries The VSEPR Model Predicting Molecular Geometries Copyright oughton Mifflin Company. All rights reserved. 13a 45 Copyright oughton Mifflin Company. All rights reserved. 13a 46 Molecular Shapes The VSEPR Model Predicting Molecular Geometries To determine the electron pair geometry: draw the Lewis structure count the total number of electron pairs around the central atom arrange the electron pairs in one of the above geometries to minimize e -e repulsion multiple bounds count as one bonding pair experimentally we find all -C- bond angles are Therefore, the molecule cannot be planar. All atoms are located at the vertices of a tetrahedron with the C at its center. Copyright oughton Mifflin Company. All rights reserved. 13a 47 Copyright oughton Mifflin Company. All rights reserved. 13a 48 12
13 The VSEPR Model Predicting Molecular Geometries The VSEPR Model Predicting Molecular Geometries Copyright oughton Mifflin Company. All rights reserved. 13a 49 Copyright oughton Mifflin Company. All rights reserved. 13a 50 AB 2 Linear Molecular Shapes The VSEPR Model The valence electrons in a molecule are the bonding pairs of electrons as well as the lone pairs. AB 3 AB 2 E Trigonal planar Angular or Bent AB 4 Tetrahedral AB 5 Trigonal bipyramidal AB 3 E Trigonal pyramidal AB 4 E AB 3 E 2 AB 2 E 3 Irregular tetrahedral T-shaped Linear (see saw) AB 2 E 2 Angular or Bent There are 11 shapes that are important to us: Number of atoms, formula Shapes (3 atoms, AB 2 ) linear or bent (4 atoms, AB 3 ) trigonal planar, trigonal bipyramidal, or T-shaped (5 atoms, AB 4 4) tetrahedral, square planar,, or see-saw (6 atoms, AB 5 ) trigonal bipyramidal or square pyramidal (7 atoms, AB 6 ) octahedral AB 6 AB 5 E AB 4 E 2 Octahedral Square pyramidal Square planar Copyright oughton Mifflin Company. All rights reserved. 13a 51 Copyright oughton Mifflin Company. All rights reserved. 13a 52 13
14 The VSEPR Model The VSEPR Model Molecules with More than One Central Atom In acetic acid, C 3 COO, there are three central atoms. We assign the geometry about each central atom separately. C O C O Number of electron domains Electron-domain geometry Tetrahedral Trigonal planar Tetrahedral Predicted bond angles o 120 o o Copyright oughton Mifflin Company. All rights reserved. 13a 53 Copyright oughton Mifflin Company. All rights reserved. 13a 54 The VSEPR Model Molecules with More than One Central Atom In acetic acid, C 3 COO, there are three central atoms. We assign the geometry about each central atom separately. Copyright oughton Mifflin Company. All rights reserved. 13a 55 O Bond dipoles C Overall dipole moment = 0 Nonpolar The overall dipole moment of a molecule is the sum of its bond dipoles. In CO 2 the bond dipoles are equal in magnitude but exactly opposite each other. The overall dipole moment is zero. k q d 1 2 = Q r q 2 Dipole Moment O Coulomb s law Dipole moment, In 2 O the bond dipoles are also equal in magnitude but do not exactly oppose each other. The molecule has a nonzero overall dipole moment. O Overall dipole moment Polar Bond dipoles Copyright oughton Mifflin Company. All rights reserved. 13a 56 14
15 igure 13.6: The carbon dioxide molecule Determining Molecular Polarity Nonpolar Molecules Dipole moments are symmetrical and cancel out. Nonpolar Molecules Dipole moments are symmetrical and cancel out. B 3 B Copyright oughton Mifflin Company. All rights reserved. 13a 57 Courtesy Christy Johannesson Copyright oughton Mifflin Company. All rights reserved. 13a 58 Determining Molecular Polarity Determining Molecular Polarity Polar Molecules Dipole moments are asymmetrical and don t cancel. polar molecules have... asymmetrical shape (lone pairs) or asymmetrical atoms 2 O O net dipole moment C 3 net dipole moment Courtesy Christy Johannesson Copyright oughton Mifflin Company. All rights reserved. 13a 59 Courtesy Christy Johannesson Copyright oughton Mifflin Company. All rights reserved. 13a 60 15
16 igure 13.5: (a) The structure and charge distribution of the ammonia molecule. (b) The dipole moment of the ammonia molecule oriented in an electric field. Copyright oughton Mifflin Company. All rights reserved. 13a 61 Copyright oughton Mifflin Company. All rights reserved. 13a 62 Sulfur has a partial positive charge ydrogen atoms have a partial positive charge Copyright oughton Mifflin Company. All rights reserved. 13a 63 Copyright oughton Mifflin Company. All rights reserved. 13a 64 16
17 ydrogen atoms and a small partial negative charge on the carbon Polar Bonds.. Polar O Polar B Nonpolar N Polar Copyright oughton Mifflin Company. All rights reserved. 13a 65 Xe C C Polar Nonpolar Nonpolar Polar A molecule has a zero dipole moment when bond dipoles cancel one another. Copyright oughton Mifflin Company. All rights reserved. 13a 66 Comparing fuels Natural gas: C 4 +2O 2 CO O Δ=-808 kj/mol Coal: C + O 2 CO 2 Δ= kj/mol Oil: C ½O 2 20CO O Δ= kj/mol Δ=-666 kj/mol.co 2 Copyright oughton Mifflin Company. All rights reserved. 13a 67 Copyright oughton Mifflin Company. All rights reserved. 13a 68 17
18 Comparing fuels N 3 high energy density Production of 1 GigaJoule of energy releases: Natural gas: 10 9 J x kg/mol 808,000 J/mol = 54.5 kg CO 2 Coal: 112 kg CO 2 Oil: 66 kg CO Copyright oughton Mifflin Company. All rights reserved. 13a 69 Copyright oughton Mifflin Company. All rights reserved. 13a 70 ATP energy ATP energy Repulsion weakens these bonds Resonance! Copyright oughton Mifflin Company. All rights reserved. 13a 71 Copyright oughton Mifflin Company. All rights reserved. 13a 72 18
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