Lecture 15: Resonance Structures and VSEPR

  • OCNOCN^- Lewis Structure
  • Because OO is more electronegative than NN
  • When OCNOCN^- takes up H+H^+ it goes to O^-

Comparing Resonance Structures

  • Double Bond - Bond length is shorter and stronger than a single bond
  • Negative sign once on the right and once on the left
  • The Bonds - Double and single are always switching between them
  • The lengths are the same for both

VSEPR Method

  • Count all VSVS ee^- of the central atom, AA
  • Add ee^- from the ligands, XX
  • Compensate for ionic charge
  • Divide by 2 = Steric number, SNSN
  • Distribute ee^- pairs so as to minimize ee^- pairs are not equivalent
  • If the SN=4SN=4 the basic shape is tetrahedral
  • If we have only 3 atoms (like NH3NH_3) \rightarrow shape is pyramid
  • Ax3EAx_3E \rightarrow Triangular Pyramid Structure
  • Electron count for arriving at Steric Number (SNSN) AXnAX_n (AA is the central atom, XX is atom bonded to it) EE is the number of lone pairs
  • Example #1
  • CCl4CCl_4

  • Example #2
  • BF3BF_3

  • Central Atom \rightarrow Look at the value electrons
  • When connected to central atom, go by roles for each group

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