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AP Chemistry Unit 4 – Molecular Structure

AP Chemistry Unit 4 – Molecular Structure. Lesson 4 – Resonance Structures Book Section: 8.7. Resonance. Draw the Lewis structure for ozone, O 3. Resonance. Draw the Lewis structure for ozone, O 3. Resonance. This is at odds with the experimentally determined bond lengths and angles.

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AP Chemistry Unit 4 – Molecular Structure

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  1. AP ChemistryUnit 4 – Molecular Structure Lesson 4 – Resonance Structures Book Section: 8.7

  2. Resonance • Draw the Lewis structure for ozone, O3.

  3. Resonance • Draw the Lewis structure for ozone, O3.

  4. Resonance • This is at odds with the experimentally determined bond lengths and angles. • …both O-O bonds have the same length • Bond order = 1.5 • …both outer oxygens have a charge of -1/2 • ???

  5. Resonance • One Lewis structure by itself can not accurately depict ozone. • We use multiple structures, or resonancestructures, to describe the molecule.

  6. Resonance • Just as green is a synthesis of blue and yellow… • …ozone is a synthesis of these two resonance structures.

  7. Resonance • In truth, the electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O, but rather can move freely among the two oxygens and the carbon. • They are not localized, but rather, delocalized.

  8. Resonance • The organic compound benzene, C6H6, has two resonance structures. • It is commonly depicted as a hexagon with a circle to signify the delocalized electrons in the ring.

  9. Exceptions to the Octet Rule • There are three types of ions or molecules that do not follow the octet rule: • Ions or molecules with an odd number of electrons • Ions or molecules with less than an octet • Ions or molecules with more than eight valence electrons (an expanded octet)

  10. Odd Number of Electrons • Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

  11. Fewer Than Eight Electrons • Consider BF3. • Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.

  12. Fewer Than Eight Electrons • Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.

  13. Fewer Than Eight Electrons • The lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.

  14. More Than Eight Electrons • The only way PCl5 can exist is if phosphorus has 10 electrons around it. • It is allowed to expand the octet of atoms on the 3rd row or below. • Presumably d orbitals in these atoms participate in bonding.

  15. More Than Eight Electrons • Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.

  16. More Than Eight Electrons • This eliminates the charge on the phosphorus and the charge on one of the oxygens. • The lesson is: when the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.

  17. 1984 MC #80 • For which of the following molecules are resonance structures necessary to describe the bonding satisfactorily? • H2S • SO2 • CO2 • OF2 • PF3

  18. 1984 MC #80 • For which of the following molecules are resonance structures necessary to describe the bonding satisfactorily? • H2S • SO2 - 45% correct, medium • CO2 • OF2 • PF3

  19. Homework: 8.50, 8.52, 8.54, 8.56 • Next week: • Tuesday: VSEPR (9.1-9.2), Stoichiometry Lab Due • Wednesday: Polarity (9.3) • Thursday: Hybridization (9.4-9.5) • Friday: No Class (Early Dismissal)

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