Ap chemistry unit 4 molecular structure
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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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Ap chemistry unit 4 molecular structure

AP ChemistryUnit 4 – Molecular Structure

Lesson 4 – Resonance Structures

Book Section: 8.7


Resonance

Resonance

  • Draw the Lewis structure for ozone, O3.


Resonance1

Resonance

  • Draw the Lewis structure for ozone, O3.


Resonance2

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

      • ???


Resonance3

Resonance

  • One Lewis structure by itself can not accurately depict ozone.

  • We use multiple structures, or resonancestructures, to describe the molecule.


Resonance4

Resonance

  • Just as green is a synthesis of blue and yellow…

  • …ozone is a synthesis of these two resonance structures.


Resonance5

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.


Resonance6

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.


Exceptions to the octet rule

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)


Odd number of electrons

Odd Number of Electrons

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


Fewer than eight electrons

Fewer Than Eight Electrons

  • Consider BF3.

    • Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.


Fewer than eight electrons1

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.


Fewer than eight electrons2

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.


More than eight electrons

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.


More than eight electrons1

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.


More than eight electrons2

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.


1984 mc 80

1984 MC #80

  • For which of the following molecules are resonance structures necessary to describe the bonding satisfactorily?

  • H2S

  • SO2

  • CO2

  • OF2

  • PF3


1984 mc 801

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


Homework 8 50 8 52 8 54 8 56

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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