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Dr. C. Yau Fall 2013

Review of Material from General Chemistry I Part II: Resonance, Hybridization, VB Theory (mostly from Chap. 9 & 10 of Jespersen 6 th Ed). Dr. C. Yau Fall 2013. 1. Resonance vs. Isomers.

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Dr. C. Yau Fall 2013

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  1. Review of Material from General Chemistry IPart II: Resonance, Hybridization, VB Theory(mostly from Chap. 9 & 10 of Jespersen 6th Ed) Dr. C. Yau Fall 2013 1

  2. Resonance vs. Isomers Resonance structures are ones that have the same spatial arrangement of atoms but different arrangement of electrons to achieve the Octet. In contrast, isomers are ones that have the same molecular formula but different spatial arrangement of atoms.

  3. Isomers Both have the molecular formula of C2H6O but different structures, written as... CH3CH2OH CH3OCH3

  4. Resonance hybrid of SO2 Resonance 3 resonance structures of SO2 You are expected to know how to determine “formal charges” and how to write resonance hybrids.

  5. Quick Review: Hybridization Hybridization of an atom is based on number of "electron clouds" on the atom. # of electron cloud = # lone pairs + # atoms directly attached (You do NOT count # of bonds!!!!)

  6. Electron Clouds & Hybridization • Cl has 4 e-clouds • N has 3 e-clouds • C has 3 e-clouds • O has 4 e-clouds • H has 1 e-cloud KNOW THIS WELL!# e-cloudHybridizationGeometryBond Angle 4 sp3 tetrahedral 109.5o 3 sp2 (+ 1 p) trigonal planar 120o 2 sp (+ 2 p) linear 180o 1 s (not hybridized)

  7. Hybridization & Geometry Lewis structure is not meant to show the geometry. This molecule is NOT linear! What is the hybridization & bond angle at each atom? Cl? N? C? O? sp3 n/a sp2 120o sp2 120o sp3 109.5o

  8. Quick Review: Resonance Draw the Lewis structure of the nitrate ion. What is the formula of nitrate? What is the geometry of nitrate? What is the bond angle? What is the hybridization of the N atom? 8

  9. Quick Review: Resonance • Why do we need to know about resonance structures? Are the geometries of the resonance structures different? • Are all the N-O bonds the same length? Why or why not? Single bonds are longer than double bonds. Yet, N-O bonds in nitrate have the same length. WHY? 9

  10. Importance of Resonance Hybrid The resonance hybrid explains why all N-O bond lengths are the same. The bond order = 1

  11. VB Theory VB Theory = Valence Bond Theory Covalent bonds are formed by the overlap of atomic orbitals. KNOW THIS WELL!! single bond =   bond double bond = one  bond + one  bond triple bond = one  bond + two  bond How many  bonds?  bonds?

  12. VB Theory Applied to the Nitrate Ion Sketch the hybrid orbitals in the nitrate ion, showing the sigma and pi bonds. Hint: Start with the hybridization of each atom.

  13. VB Theory Applied to Benzene What is the structure of benzene (very common organic solvent)? Its molecular formula is C6H6. Where are the C's and H's??

  14. Structure of Benzene Draw the other resonance structure of benzene.

  15. VB Theory Applied to Benzene Benzene is often written as Learn what this symbol means. Know how to tell where the atoms and bonds are.

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