Ch 9 section 3 to 5


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Ch 9 section 3 to 5

  1. 1. Molecular Structures and Resonance CHAPTER 9.3
  2. 2. Structural Formulas <ul><li>Most useful of all; predict relative location of atoms by using letter symbols and bonds. </li></ul>
  3. 3. Writing Lewis Structures for Ions <ul><li>Follow the same rules except for </li></ul><ul><ul><li>Cations (+) = SUBTRACT the # on the charge </li></ul></ul><ul><ul><li>Anions (-) = ADD the # on the charge </li></ul></ul><ul><li>Place structure inside brackets and show the oxidation number outside. </li></ul><ul><ul><li>Ex: Phosphate ion, PO 4 3- </li></ul></ul>
  4. 4. Delocalized Electrons: Resonance <ul><li>When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion. </li></ul>
  5. 5. Delocalized Electrons: Resonance <ul><li>In reality, each of the four atoms in the nitrate ion has a p orbital. </li></ul><ul><li>The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen. </li></ul>
  6. 6. Delocalized Electrons: Resonance <ul><li>This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion. </li></ul>
  7. 7. Resonance <ul><li>The organic molecule benzene has six  bonds and a p orbital on each carbon atom. </li></ul>
  8. 8. Resonance <ul><li>In reality the  electrons in benzene are not localized, but delocalized. </li></ul><ul><li>The even distribution of the  electrons in benzene makes the molecule unusually stable. </li></ul>
  9. 9. Draw Lewis structures for the following; include resonance structures (if any) and formal charges <ul><li>HCN </li></ul><ul><li>C 2 H 2 </li></ul><ul><li>HOF </li></ul><ul><li>BF 3 </li></ul><ul><li>HCFO </li></ul><ul><li>COCl 2 </li></ul><ul><li>N 2 F 2 </li></ul><ul><li>N 2 O 3 </li></ul><ul><li>ICl 3 </li></ul><ul><li>COS </li></ul><ul><li>PO 4 3- </li></ul><ul><li>ClO 4 - </li></ul><ul><li>NO 3 - </li></ul><ul><li>NH 4 + </li></ul><ul><li>SO 4 2- </li></ul>
  10. 10. Molecular Geometries and Bonding Theories CHAPTER 9.4
  11. 11. Molecular Shapes <ul><li>The shape of a molecule plays an important role in its reactivity. </li></ul><ul><li>By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. </li></ul>
  12. 12. What Determines the Shape of a Molecule? <ul><li>Electron pairs (both bonding & lone) repel each other. </li></ul><ul><li>The shape of the molecule can be predicted by placing the electron pairs as far away from each other. </li></ul>
  13. 13. Electron Domains <ul><li>We can refer to the electron pairs as electron domains . </li></ul><ul><li>A double or triple bond, count as one electron domain. </li></ul><ul><li>This molecule has four electron domains. </li></ul>
  14. 14. Valence Shell Electron Pair Repulsion Theory (VSEPR) <ul><li>“ The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” </li></ul>
  15. 15. Electron-Domain Geometries <ul><li>These are the electron-domain geometries for two through six electron domains around a central atom. </li></ul>
  16. 16. Electron-Domain Geometries <ul><li>All one must do is count the number of electron domains in the Lewis structure. </li></ul><ul><li>The geometry will be that which corresponds to that number of electron domains. </li></ul>
  17. 17. Molecular Geometries <ul><li>The electron-domain geometry is often not the shape of the molecule, however. </li></ul><ul><li>The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs . </li></ul>
  18. 18. Molecular Geometries <ul><li>One electron domain may = 2 or more molecular geometries (shape) , depending on lone pairs. </li></ul>
  19. 19. Linear Electron Domain (2) <ul><li>In this domain, there is only one molecular geometry : linear. </li></ul><ul><li>NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. </li></ul>
  20. 20. Trigonal Planar Electron Domain (3) <ul><li>There are two molecular geometries: </li></ul><ul><ul><li>Trigonal planar , if all the electron domains are bonding </li></ul></ul><ul><ul><li>Bent , if one of the domains is a nonbonding pair. </li></ul></ul>
  21. 21. Nonbonding Pairs and Bond Angle <ul><li>Nonbonding pairs are physically larger than bonding pairs. </li></ul><ul><li>Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule. </li></ul>
  22. 22. Multiple Bonds and Bond Angles <ul><li>Double and triple bonds place greater electron density on one side of the central atom than do single bonds. </li></ul><ul><li>Therefore, they also affect bond angles. </li></ul>
  23. 23. Tetrahedral Electron Domain (4) <ul><li>There are three molecular geometries: </li></ul><ul><ul><li>Tetrahedral , if all are bonding pairs </li></ul></ul><ul><ul><li>Trigonal pyramidal if one is a nonbonding pair </li></ul></ul><ul><ul><li>Bent if there are two nonbonding pairs </li></ul></ul>
  24. 24. Trigonal Bipyramidal Electron Domain (5) <ul><li>There are two distinct positions in this geometry: </li></ul><ul><ul><li>Axial </li></ul></ul><ul><ul><li>Equatorial </li></ul></ul>
  25. 25. Trigonal Bipyramidal Electron Domain <ul><li>Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. </li></ul><ul><li>Place lone pairs in equatorial positions ONLY! </li></ul>
  26. 26. Trigonal Bipyramidal Electron Domain <ul><li>There are four distinct molecular geometries in this domain: </li></ul><ul><ul><li>Trigonal bipyramidal </li></ul></ul><ul><ul><li>Seesaw </li></ul></ul><ul><ul><li>T-shaped </li></ul></ul><ul><ul><li>Linear </li></ul></ul>
  27. 27. Octahedral Electron Domain (6) <ul><li>All positions are equivalent in the octahedral domain. </li></ul><ul><li>There are three molecular geometries: </li></ul><ul><ul><li>Octahedral </li></ul></ul><ul><ul><li>Square pyramidal </li></ul></ul><ul><ul><li>Square planar </li></ul></ul>
  28. 28. Larger Molecules <ul><li>In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. </li></ul>
  29. 29. Larger Molecules <ul><li>This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule. </li></ul>
  30. 30. Electronegativity and Polarity CHAPTER 9.5
  31. 31. Polarity <ul><li>Occurs when electrons are unequally shared. </li></ul><ul><li>Dipoles - regions where atoms have large electronegativity differences creating two poles (+, -) </li></ul><ul><li>But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. </li></ul>
  32. 32. Polarity <ul><li>By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. </li></ul>
  33. 33. Polarity
  34. 34. Hybrid Orbitals <ul><li>But it’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize. </li></ul>
  35. 35. Hybrid Orbitals <ul><li>Consider beryllium: </li></ul><ul><ul><li>In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals. </li></ul></ul>
  36. 36. Hybrid Orbitals <ul><li>But if it absorbs the small amount of energy needed to promote an electron from the 2 s to the 2 p orbital, it can form two bonds. </li></ul>
  37. 37. Hybrid Orbitals <ul><li>Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals called sp hybrid . </li></ul><ul><ul><li>These sp hybrid orbitals have two lobes like a p orbital. </li></ul></ul><ul><ul><li>One of the lobes is larger and more rounded as is the s orbital. </li></ul></ul>
  38. 38. Hybrid Orbitals <ul><li>These two degenerate orbitals would align themselves 180  from each other. </li></ul><ul><li>This is consistent with the observed geometry of beryllium compounds: linear. </li></ul>
  39. 39. Hybrid Orbitals <ul><li>With hybrid orbitals the orbital diagram for beryllium would look like this. </li></ul><ul><li>The sp orbitals are higher in energy than the 1 s orbital but lower than the 2 p . </li></ul>
  40. 40. Hybrid Orbitals <ul><li>Using a similar model for boron leads to… </li></ul><ul><ul><li>sp 2 hybrid orbital b/c it uses the one 2s orbital and two orbitals from 2p. </li></ul></ul>
  41. 41. Hybrid Orbitals <ul><li>The three degenerate sp 2 orbitals. </li></ul>
  42. 42. Hybrid Orbitals <ul><li>With carbon we get… </li></ul><ul><ul><li>4 degenerate orbitals are sp 3 hybrid </li></ul></ul><ul><ul><li>Uses the one 2s orbital and all three 2p orbitals. </li></ul></ul>
  43. 43. Hybrid Orbitals <ul><li>The four degenerate </li></ul><ul><li>sp 3 orbitals. </li></ul>
  44. 44. Hybrid Orbitals <ul><li>For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids. </li></ul>
  45. 45. Hybrid Orbitals <ul><li>This leads to five degenerate sp 3 d orbitals… </li></ul><ul><li>… or six degenerate sp 3 d 2 orbitals. </li></ul>
  46. 46. Hybrid Orbitals <ul><li>Once you know the electron-domain geometry, you know the hybridization state of the atom. </li></ul>