Orbitals and Covalent Bond

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Chapter 9

• Orbitals and Covalent Bond

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Atomic Orbitals Dont Work

• to explain molecular geometry.
• In methane, CH4 , the shape s tetrahedral.
• The valence electrons of carbon should be two in
  s, and two in p.
• the p orbitals would have to be at right angles.
• The atomic orbitals change when making a molecule

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Hybridization

• We blend the s and p orbitals of the valence
  electrons and end up with the tetrahedral
  geometry.
• We combine one s orbital and 3 p orbitals.
• sp3 hybridization has tetrahedral geometry.

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In terms of energy
2p
Hybridization
Energy
2s
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How we get to hybridization

• We know the geometry from experiment.
• We know the orbitals of the atom
• hybridizing atomic orbitals can explain the
  geometry.
• So if the geometry requires a tetrahedral shape,
  it is sp3 hybridized
• This includes bent and trigonal pyramidal
  molecules because one of the sp3 lobes holds the
  lone pair.

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sp2 hybridization

• C2H4
• Double bond acts as one pair.
• trigonal planar
• Have to end up with three blended orbitals.
• Use one s and two p orbitals to make sp2
  orbitals.
• Leaves one p orbital perpendicular.

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In terms of energy
2p
Energy
2s
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Where is the P orbital?

• Perpendicular
• The overlap of orbitals makes a sigma bond (s
  bond)

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Two types of Bonds

• Sigma bonds from overlap of orbitals.
• Between the atoms.
• Pi bond (p bond) above and below atoms
• Between adjacent p orbitals.
• The two bonds of a double bond.

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H
H
C
C
H
H
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sp2 hybridization

• When three things come off atom.
• trigonal planar
• 120º
• on s one p bond

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What about two

• When two things come off.
• One s and one p hybridize.
• linear

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sp hybridization

• End up with two lobes 180º apart.
• p orbitals are at right angles
• Makes room for two p bonds and two sigma bonds.
• A triple bond or two double bonds.

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In terms of energy
2p
Energy
2s
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CO2

• C can make two s and two p
• O can make one s and one p

C
O
O
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Breaking the octet

• PCl5
• The model predicts that we must use the d
  orbitals.
• dsp3 hybridization
• There is some controversy about how involved the
  d orbitals are.

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dsp3

• Trigonal bipyrimidal
• can only s bond.
• cant p bond.
• basic shape for five things.

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PCl5
Cant tell the hybridization of Cl Assume sp3 to
minimize repulsion of electron pairs.
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d2sp3

• gets us to six things around
• octahedral

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Molecular Orbital Model

• Localized Model we have learned explains much
  about bonding.
• It doesnt deal well with the ideal of resonance,
  unpaired electrons, and bond energy.
• The MO model is a parallel of the atomic orbital,
  using quantum mechanics.
• Each MO can hold two electrons with opposite
  spins
• Square of wave function tells probability

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What do you get?

• Solve the equations for H2
• HA HB
• get two orbitals
• MO1 1sA - 1sB
• MO2 1sA 1sB

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The Molecular Orbital Model

• The molecular orbitals are centered on a line
  through the nuclei
• MO1 the greatest probability is between the
  nuclei
• MO2 it is on either side of the nuclei
• this shape is called a sigma molecular orbital

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The Molecular Orbital Model

• In the molecule only the molecular orbitals
  exist, the atomic orbitals are gone
• MO1 is lower in energy than the 1s orbitals they
  came from.
• This favors molecule formation
• Called an bonding orbital
• MO2 is higher in energy
• This goes against bonding
• antibonding orbital

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The Molecular Orbital Model
MO2
Energy
1s
1s
MO1
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The Molecular Orbital Model

• We use labels to indicate shapes, and whether the
  MOs are bonding or antibonding.
• MO1 s1s
• MO2 s1s ( indicates antibonding)
• Can write them the same way as atomic orbitals
• H2 s1s2

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The Molecular Orbital Model

• Each MO can hold two electrons, but they must
  have opposite spins
• Orbitals are conserved. The number of molecular
  orbitals must equal the number atomic orbitals
  that are used to make them.

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H2-
s1s
Energy
1s
1s
s1s
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Bond Order

• The difference between the number of bonding
  electrons and the number of antibonding electrons
  divided by two