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Molecular Orbital Theory

multiple bonds - overlap of p orbitals

  • sigma bonds - single bonds where electron density concentrated symmetrically around line connecting the nuclei
  • pi bonds - overlapping electron regions lie above/below internuclear axis
    • can’t experimentally be seen
  • single bond - 1 sigma bond
  • double bond - 1 sigma bond, 1 pi bond
  • triple bond - 1 sigma bond, 2 pi bonds
  • delocalized pi bonding - cannot be described as individual electron bond between atoms
    • split up all around the molecule
    • explains identical length of alternating single/double bonds in molecules like benzene
    • found in all molecules w/ 2+ resonance structures

molecular orbital theory - describes electrons in terms of molecular orbitals 

  • bonding molecular orbital - lower energy molecular orbital
    • electron attracted to both nuclei
    • very stable
  • antibonding molecular orbital - little electron density between nuclei
    • higher energy molecular orbital
    • atomic orbitals cancel each other out, area of largest electron density found on opposite sides of nuclei
  • energy-level diagram - aka molecular orbital diagram
    • shows interactions of atomic orbitals
  • bond order = 1/2(# of bonding electrons - # of nonbonding electrons)
    • 0 >> no bond
    • 1 >> single bond
    • 2 >> double bond
    • 3 >> triple bond
  • # of molecular orbitals = # of atomic orbitals combined
  • atomic orbitals combine best w/ same type of atomic orbitals
  • overlap increase >> bonding lower in energy, antibonding higher in energy
  • each molecular orbital can hold at most 2 electrons (w/ alternate spins)
  • antibonding/bonding pi molecular orbitals lower in energy than antibonding sigma orbitals, higher in energy than bonding sigma orbitals (unless interactions between s and p levels exist)

molecular properties of electron configurations

  • paramagnetism - molecules w/ unpaired electrons attracted into magnetic fields
  • diamagnetism - molecules w/ no unpaired electrons weakly repelled by magnetic fields
  • bond order increase >> bond distance decrease, bond enthalpy increase
  • heteronuclear diatomic molecule - diatomic molecule made up of different atoms
    • homonuclear diatomic molecule - diatomic molecules made up of same atoms
    • will resemble homonuclear molecular orbital diagram if electronegativities are not too far off
    • may have non-integer bond order value
Subject: 
Chemistry
Subject X2: 
Chemistry

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