Molecular Orbitals

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Molecular Orbitals

• Chapter 9

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Molecular Orbital model

• This model examines unpaired electrons, bond
  energies and excited state electrons.
• Examine the H2 molecule.
• When 2 atomic orbitals overlap, two molecular
  orbitals form.
• One low energy bonding orbital and one high
  energy anti-bonding orbital

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Electron Probability Distribution

• The electron probability of both MO and MO is
  centered along the line passing between the
  nuclei.
• In the MO, the greatest e- probability is between
  the nuclei.
• In MO the greatest e- probability is on either
  side of the nuclei

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Describing the bonding

• The molecular orbitals are constructed from the
  atomic 1s orbitals of the H atoms.
• The resulting molecular orbital may be
  represented as
• MO1 1sa 1sb
• MO 1sa 1sb

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Important points

• The e- probability is centered along the line
  passing between the two nuclei..
• This type of electron distribution is described
  as sigma (s). MO1 MO are sigma molecular
  orbitals.
• In this model only molecular orbitals are
  available for occupation by electrons. Atomic
  orbitals no longer exist because the molecule , a
  new entity, has its own set of new orbitals.

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Bonding Anti-bonding
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More points

• MO1 is lower in energy than 1s of the free H
  atom.
• This is the driving force behind molecular
  formation, and stability.
• If the electrons are forced to occupy the higher
  energy MO, or anti-bonding orbitals, that
  reduces the stability of the molecule.
• Bonding MOs are lower energy than individual
  atomic orbitals.
• Anti-bonding MOs are higher energy than
  individual atomic orbitals.

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Last points

• Molecular orbitals may be written like electron
  configurations. s 1s2 etc.
• Each molecular orbitals holds up to 2 electrons
  with opposite spin.
• Orbitals are conserved. The number of molecular
  orbitals will always be the same as the number of
  atomic orbitals used to construct them.

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Examples
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Bond Order

• Bond order is defined as
• bond order
• ½ ( of bonding e-) - ( of anti-bonding e-)

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More examples

• Oxygen has a bond order of 2, but it also shows
  two unpaired electrons.
• ½(10-6)2
• The larger the bond order, the greater the bond
  strength.

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Still other examples
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Try it

• Which is stable, He2 or Li2?
• With a bond order of 0 versus 1, Lithium is more
  stable.
• Which is more stable, O2, O2 or O2- ?
• 2, 2.5 and 1.5 indicate that O2 is the most
  stable.

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Paramagnetism

• Unpaired electrons exhibit paramagnetism.
• In this picture liquid oxygen is held in a
  magnetic field until it boils away.