Beyond the Octet Rule

1
Beyond the Octet Rule

• Know that C,N, O and F always obey the octet
  rule, know that B and Be sometimes form electron
  deficient compounds, know that elements in the
  3rd row (beneath C,N, O and F) form compounds
  that obey the octet rule as well as compounds
  that contain more than an octet of electrons
  (multiple valency)
• Multiple valency realize that atoms with empty
  valence d orbitals can and do use those orbitals
  for bonding. Be able to write down orbital
  diagrams for all of the possible valence states
  for an atom.
• Be able to draw Lewis structures for atoms that
  display multiple valency, and to match your Lewis
  structure with one of the possible orbital
  diagrams. (Exercises 8.7 and 8.8)
• Resonance (p 384) when more than one valid
  Lewis structure can be written for a particular
  molecule, the actual structure will be the
  average of the possible structures. (example
  NO3)
• Be able to draw sigma and pi bonds from atomic s
  and p orbitals (chapter 9, figures 9.31 and 9.33)

2
Exceptions to the Octet Rule

• B and Be (and elements below them) may form
  (highly reactive) electron-deficient molecules.
• C,N,O and F always obey the octet rule
• Elements below C, N, O and F often obey the octet
  rule.
• However, elements below C,N, O and F also often
  form compounds in which they have more than an
  octet of electrons.
• Noble gases sometimes actually do form compounds
  in which they violate the octet rule.
• Transition metals display interesting chemistry
  quite different from what we are talking about
  now.

3
Multiple Valency

• We have seen that the valence electron
  configuration of an atom in a molecule may be
  different from the atomic electron configuration
  (EG/ carbon atomic s2p2, in methane sp3)
• Elements in period 3 or greater may display
  multiple valence electron configurations because
  of the availability of low-lying valence d
  orbitals.

4
Example Valence Electron Configurations of PCl3
and PCl5

• Atomic electron configuration of phosphorus is
  3s23p3.
• This could also be a valence electron
  configuration orbital diagram shows 1
  nonbonding pair and 3 unpaired bonding electrons.
  This electron configuration belongs to P when it
  is bonded to 3 other atoms (e.g. PCl3 ).
• Note that in this compound, P is obeying the
  octet rule. It has only 4 electron pairs around
  itself.
• However, if P promotes an electron from the 3s
  orbital to the 3d orbital, then its electron
  configuration would be 3s13p33d1
• Orbital diagram shows five unpaired, bonding
  electrons. This electron configuration belongs
  to P when it is bonded to 5 other atoms (e.g.
  Pcl5)
• Note that in this compound, P violates the
  octet rule, because it has 5 electron pairs
  around itself.

5
Example Lewis Structures of PCl3 and PCl5

• (draw the Lewis structures for these two
  molecules, and then see hwo they correspond with
  the valence electron configurations on previous
  slide)

6
Practice

• Draw Lewis structures and valence electron
  configurations for the molecules and ions in
  sample exercise 8.8 (p 382)
• ClF3, XeO3, RnCl2, BeCl2, ICl4-

7
The Interesting Case of NO3-

• Draw Lewis structure (as on p 383-4).
• You get 3 different drawings (depnding upon where
  you put the double bond). Which one is right?
• Resonance occurs when more than one valid Lewis
  structure can be written for a particualr
  molecule.
• The actual structure is the average of all of the
  possible Lewis structures.
• How do we know this? We know that N-O bond will
  be longer than NO bond. Experimentally though
  we see three equal bondlengths in NO3-.

8
Sigma Bonds and Pi Bonds

• Recall we said that there were single, double and
  triple bonds.
• The bonds are made out of the orbitals that the
  electrons are in.
• Let us look at O2 and N2 and see how the orbitals
  go together to make bonds. (fig 9.31and 9.33 on p
  430-431)
• Lewis structures of O2 and N2
• O2 is doubly bonded and is triply bonded. How
  do the orbitals go together to make these bonds?
• Sigma bonds s-s sigma and pz-pz sigma.
• Pi bonds two kinds px-px pi and py-py pi.

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11
Carbon Dioxide Bonding Diagram

• Make Lewis structure for carbon dioxide
• Propose bonding diagram using sigma and pi bonds
  created from orbitals on carbon and oxygen.