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