Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 8 Chemical Foundations Molecular Bonding and Structure play the central role in determining the course of all chemical reactions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Section 8.12 Resonance Example: CO 3 2-

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Example: CO 3 2- Experiments show only one type of C—O bonds – not double & triple bonds. Actual structure is average of all three. Electrons are not localized, but DELOCALIZED. (Move around entire molecule.)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule. These are resonance structures. The actual structure is an average of the resonance structures. (Molecule doesn’t flip.)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Odd-Electron Molecules Relatively few molecules Examples: NO (nitric oxide – 11 e - ) NO 2 (nitrogen dioxide – 17 e - ) Localized Electron Model does not handle these case. A more sophisticated model is needed.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Formal Charge Used to evaluate possible Lewis structures – i.e., nonequivalent Lewis structures which all obey the rules for writing Lewis structures.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Formal Charge The difference between the number of valence electrons (VE) on the free atom and the number assigned to the atom in the molecule. We need: 1.# VE on free neutral atom 2.# VE “belonging” to the atom in the molecule

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Formal Charge Number of Lone Pair electrons + ½ (Number of Shared Electrons)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Formal charge Best Structures are those with -- formal charges closest to zero -- any negative formal charges on the most electronegative element.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Formal Charge Not as good Better

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Section 8.13 Molecular Structure: The VSEPR Model Valence Shell Electron Repulsion Model

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Main Postulate: Structure around a given atom is determined principally by minimizing electron-pair repulsions. That is, Bonding & non-bonding around a given atom will position themselves as far apart as possible.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Two pairs of e - : Maximum possible separation is 180  Linear Structure Three pairs of e - : Maximum possible separation is with 120  bond angles. Flat/planar triangular molecule Trigonal Planar structure

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Four pairs of e - : Maximum separation is with  bond angles. Tetrahedral Structure

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Predicting a VSEPR Structure 1.Draw Lewis structure. 2.Put pairs as far apart as possible. 3.Determine positions of atoms from the way electron pairs are shared. 4.Determine the name of molecular structure from positions of the atoms.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Multiple Bonds: (Double & Triple Bonds) Count as one effective bond. Resonance: Any one of the resonance structures can be used to predict the 3-D structure using VSEPR.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 CH 4 Tetrahedral

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Figure 8.14 The Molecular Structure of Methane

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Figure 8.15 The Molecular Structure of NH 3

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Figure 8.16 The Molecular Structure of H 2 O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Water is Bent If no lone pairs present, water would be linear & non-polar. Then polar bonds would cancel & molecule would have no dipole moment. If water was not polar, it would be very different from what we know as Water.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Figure 8.17 The Bond Angles in the CH 4, NH 3, and H 2 O Molecules Actual Bond Angles

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Lone pairs Require more room than bonding pairs. As the # of lone pairs increases, the angle Between the bonding pairs tends to be Compressed.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Figure 8.18 Electrons in Bonding (a) e - s shared by 2 nuclei (b) Both e - s close to the same atom.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Figure 8.17 The Bond Angles in the CH 4, NH 3, and H 2 O Molecules Actual Bond Angles

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Angles less than 120  Are distorted when lone pairs are present. An Angle of 120  provides lone pairs with enough space so that distortion does not occur.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Five Pairs of Electrons Minimum Repulsion with Trigonal Bipyramidal Two different Bond Angles 90  and 120  Two trigonal-pyramids with a common base

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Example: I 3 - (triiodide) Draw Lewis Structure Count pairs around the central atom.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Figure 8.20 Three Possible Arrangements of the Electron Pairs in the I 3 1- Ion (c) Preferred Structure All angles between lone pairs is 120  I 3 1- is linear

Copyright©2000 by Houghton Mifflin Company. All rights reserved Pairs of Electrons Minimum Repulsion with Octahedral structure. Bond Angles = 90  Two square-based pyramids with a common base.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 Example: XeF 4 Draw Lewis Structure Count pairs around the central atom.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Figure 8.19 Possible Electron Pair Arrangements for XeF 4 (b) Preferred because lone pairs given more room.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Molecules with No Central Atom More Complicated. Look at Arrangement Around Each Central Atom

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 Figure 8.21 The Molecular Structure of Methanol (a)Arrangement around carbon atom. (b) Arrangement around oxygen atom. Lone pairs present. (c) Molecular Structure