MOLECULAR STRUCTURE

Copyright © Houghton Mifflin Company. All rights reserved. 13a–2 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 © Houghton Mifflin Company. All rights reserved. 13a–3 Two forms of carbon; graphite and diamond. Source: Grant Hellman

Copyright © Houghton Mifflin Company. All rights reserved. 13a–4 Quartz grows in beautiful, regular crystals SiO 2 Vs CO 2

Copyright © Houghton Mifflin Company. All rights reserved. 13a–5 Figure 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 © Houghton Mifflin Company. All rights reserved. 13a–6 Figure 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 © Houghton Mifflin Company. All rights reserved. 13a–7 WHERE DO THE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other?

Copyright © Houghton Mifflin Company. All rights reserved. 13a–8 WHERE DO THE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other?

Copyright © Houghton Mifflin Company. All rights reserved. 13a–9 WHERE DO THE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other? ANSWER: It depends who is pulling harder

Copyright © Houghton Mifflin Company. All rights reserved. 13a–10 WHERE DO THE ELECTRONS GO? Are they shared equally? Are they more on one atom than the other? ANSWER: It depends who is pulling harder (“Electro negativity”)

Copyright © Houghton Mifflin Company. All rights reserved. 13a–11

Copyright © Houghton Mifflin Company. All rights reserved. 13a–12 The HCL molecule has a dipole moment

Copyright © Houghton Mifflin Company. All rights reserved. 13a–13 Pauling and his electronegativity

Copyright © Houghton Mifflin Company. All rights reserved. 13a–14 Pauling and his electronegativity

Copyright © Houghton Mifflin Company. All rights reserved. 13a–15 Figure 13.3: The Pauling electronegativity values as updated by A.L. Allred in (cont’d) Arbitrarily set F as 4

Copyright © Houghton Mifflin Company. All rights reserved. 13a–16

Copyright © Houghton Mifflin Company. All rights reserved. 13a–17 The HCL molecule has a dipole moment

Copyright © Houghton Mifflin Company. All rights reserved. 13a–18

Copyright © Houghton Mifflin Company. All rights reserved. 13a–19 X

Copyright © Houghton Mifflin Company. All rights reserved. 13a–20 X

Copyright © Houghton Mifflin Company. All rights reserved. 13a–21

Copyright © Houghton Mifflin Company. All rights reserved. 13a–22 models Now lets consider more than two atoms in a molecule

Copyright © Houghton Mifflin Company. All rights reserved. 13a–23 Linear molecules

Copyright © Houghton Mifflin Company. All rights reserved. 13a–24 Planar molecules

Copyright © Houghton Mifflin Company. All rights reserved. 13a–25 Tetrahedral molecules

Copyright © Houghton Mifflin Company. All rights reserved. 13a–26 MODELS These are only models

Copyright © Houghton Mifflin Company. All rights reserved. 13a–27 MODELS These are only models But…. Models are very useful for describing properties.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–28 MODELS These are only models But…. Models are very useful for describing properties. Newton: particles

Copyright © Houghton Mifflin Company. All rights reserved. 13a–29 MODELS These are only models But…. Models are very useful for describing properties. Newton: particles Huygens: waves

Copyright © Houghton Mifflin Company. All rights reserved. 13a–30 models

Copyright © Houghton Mifflin Company. All rights reserved. 13a–31 Ball-and-stick model of a protein segment illustrating the alpha helix. Source: Photo Researchers, Inc.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–32 The concept of individual bonds makes it much easier to deal with complex molecules such as DNA. Source: Photo Researchers, Inc.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–33

Copyright © Houghton Mifflin Company. All rights reserved. 13a–34 Polarity of Molecules Dipole Moments of Polyatomic Molecules Example: in CO 2, each C-O dipole is canceled because the molecule is linear. In H 2 O, the H-O dipoles do not cancel because the molecule is bent. OCO

Copyright © Houghton Mifflin Company. All rights reserved. 13a–35 Skeletal Structure Hydrogen 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.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–36 Skeletal Structure Identify central and terminal atoms in the molecule C 2 H 6 O (ethyl alcohol or ethanol). C H H H H H H O C NOTE: Terminal atoms are all bonded to only one other atoms. Central atoms are bonded to two or more other atoms Now where do the electrons go?

Copyright © Houghton Mifflin Company. All rights reserved. 13a–37 Writing Lewis Structures All the valence e - of atoms must appear. Usually, the e - are paired. Usually, each atom requires an octet. –H only requires 2 e -. Multiple bonds may be needed. –Readily formed by C, N, O, S, and P.

Copyright © Houghton Mifflin Company. All rights reserved. Lewis Structures Draw Lewis structures for: HF: H 2 O: NH 3 : CH 4 : H F  or H F  H O H  or H O H  H N H H  or H N H H  H C H H H  or H C H H H

Copyright © Houghton Mifflin Company. All rights reserved. 13a–39 Benzene Fredrich 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... " ( From "Creativity, Beyond the Myth of Genius" by Robert Weisberg published by W. H. Freeman 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.

Copyright © Houghton Mifflin Company. All rights reserved. Resonance Forms C6H6C6H6 30 ve’s C C C H H H H H H C C H H H H H H 

Copyright © Houghton Mifflin Company. All rights reserved. Electron-Rich Atoms PF 5 P F F F FF 

Copyright © Houghton Mifflin Company. All rights reserved. 13a–42 Exceptions to the Octet Rule Expanded octets. P Cl P Cl Cl Cl Cl S F F F F F F Cl

Copyright © Houghton Mifflin Company. All rights reserved. 13a–43 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.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–44 Molecular Shapes 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 © Houghton Mifflin Company. All rights reserved. 13a–45 The VSEPR Model Predicting Molecular Geometries

Copyright © Houghton Mifflin Company. All rights reserved. 13a–46 The VSEPR Model Predicting Molecular Geometries

Copyright © Houghton Mifflin Company. All rights reserved. 13a–47 Molecular Shapes experimentally we find all Cl-C-Cl bond angles are . Therefore, the molecule cannot be planar. All Cl atoms are located at the vertices of a tetrahedron with the C at its center.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–48 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

Copyright © Houghton Mifflin Company. All rights reserved. 13a–49 The VSEPR Model Predicting Molecular Geometries

Copyright © Houghton Mifflin Company. All rights reserved. 13a–50 The VSEPR Model Predicting Molecular Geometries

Copyright © Houghton Mifflin Company. All rights reserved. 13a–51 Molecular Shapes AB 2 Linear AB 3 Trigonal planar AB 4 Tetrahedral AB 5 Trigonal bipyramidal AB 6 Octahedral AB 2 E Angular or Bent AB 3 E Trigonal pyramidal AB 2 E 2 Angular or Bent AB 4 E Irregular tetrahedral (see saw) AB 3 E 2 T-shaped AB 2 E 3 Linear AB 5 E Square pyramidal AB 4 E 2 Square planar

Copyright © Houghton Mifflin Company. All rights reserved. 13a–52 The VSEPR Model The valence electrons in a molecule are the bonding pairs of electrons as well as the lone pairs. 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 ) tetrahedral, square planar, or see-saw (6 atoms, AB 5 ) trigonal bipyramidal or square pyramidal (7 atoms, AB 6 ) octahedral

Copyright © Houghton Mifflin Company. All rights reserved. 13a–53 The VSEPR Model

Copyright © Houghton Mifflin Company. All rights reserved. 13a–54 The VSEPR Model Molecules with More than One Central Atom In acetic acid, CH 3 COOH, there are three central atoms. We assign the geometry about each central atom separately. Number of electron domains Electron-domain geometry Predicted bond angles Tetrahedral Trigonal planar Tetrahedral o 120 o o COHH H HO 434 C

Copyright © Houghton Mifflin Company. All rights reserved. 13a–55 The VSEPR Model Molecules with More than One Central Atom In acetic acid, CH 3 COOH, there are three central atoms. We assign the geometry about each central atom separately.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–56 Dipole Moment Nonpolar Polar.. HH O C OO Bond dipoles Overall dipole moment = 0 Bond dipoles Overall dipole moment 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. In H 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. Coulomb’s law  = Q r Dipole moment, 

Copyright © Houghton Mifflin Company. All rights reserved. 13a–57 Figure 13.6: The carbon dioxide molecule Nonpolar Molecules –Dipole moments are symmetrical and cancel out.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–58 Determining Molecular Polarity Nonpolar Molecules –Dipole moments are symmetrical and cancel out. BF 3 F F F B Courtesy Christy Johannesson

Copyright © Houghton Mifflin Company. All rights reserved. 13a–59 Determining Molecular Polarity Polar Molecules –Dipole moments are asymmetrical and don’t cancel. net dipole moment H2OH2O H H O Courtesy Christy Johannesson

Copyright © Houghton Mifflin Company. All rights reserved. 13a–60 CHCl 3 H Cl Determining Molecular Polarity polar molecules have... –asymmetrical shape (lone pairs) or –asymmetrical atoms net dipole moment Courtesy Christy Johannesson

Copyright © Houghton Mifflin Company. All rights reserved. 13a–61 Figure 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 © Houghton Mifflin Company. All rights reserved. 13a–62

Copyright © Houghton Mifflin Company. All rights reserved. 13a–63 Sulfur has a partial positive charge

Copyright © Houghton Mifflin Company. All rights reserved. 13a–64 Hydrogen atoms have a partial positive charge

Copyright © Houghton Mifflin Company. All rights reserved. 13a–65 Hydrogen atoms and a small partial negative charge on the carbon

Copyright © Houghton Mifflin Company. All rights reserved. 13a–66.. Polar Bonds H Cl Polar A molecule has a zero dipole moment when bond dipoles cancel one another. HH O Polar FF B F Nonpolar H H H N Polar Nonpolar FF Cl F F F Xe FF Cl C Nonpolar Polar Cl H C H H

Copyright © Houghton Mifflin Company. All rights reserved. 13a–67

Copyright © Houghton Mifflin Company. All rights reserved. 13a–68 Comparing fuels Natural gas: CH 4 + 2O 2 → CO H 2 O ΔH=-808 kJ/mol Coal: C + O 2 → CO 2 ΔH= kJ/mol Oil: C 20 H ½O 2 → 20CO H 2 O ΔH= kJ/mol ΔH=-666 kJ/mol.CO 2

Copyright © Houghton Mifflin Company. All rights reserved. 13a–69 Comparing fuels 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 2

Copyright © Houghton Mifflin Company. All rights reserved. 13a–70 N 3 high energy density

Copyright © Houghton Mifflin Company. All rights reserved. 13a–71 ATP energy

Copyright © Houghton Mifflin Company. All rights reserved. 13a–72 ATP energy Repulsion weakens these bonds Resonance!