structure and bonding: MO theory for H2

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9 Terms

1
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molecular orbital theory

  • electrons in atoms are described by wavefunctions

  • atomic orbitals can be combined to make molecular orbitals

  • bonding orbital - electrons spend time inbetween atomic nuclei, chemical bond

  • antibonding orbital - electrons found anywhere but between 2 nuclei, node

  • adding wavefunctions - bonding molecular orbital

  • subtracting wavefunctions - antibonding molecular orbital

  • bonding molecular orbital closer to atomic orbitals of more electronegative atom

  • antibonding molecular orbital closer to atomic orbitals of less electronegative atom

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constructive interference

  • in phase combination

  • between the nuclei, wavefunctions overlap and combine to give new wavefunction

  • electron density enhanced between the nuclei

  • bonding molecular orbital

  • cylindrical symmetry about the internuclear axis - sigma orbital

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destructive interference

  • out of phase combination

  • electron density depleted between 2 nuclei so there is a node

  • 2 positively charged nuclei with lack of electrons between them

  • antibonding molecular orbital

  • cylindrical symmetry about the internuclear axis - sigma* orbital

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boundary representation and parity

  • parity of the wavefunction - whether the sign changes or not under the inversion centre

  • bonding - no change in sign so gerade

  • antibonding - change in sign so ungerade

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linear combinations of atomic orbitals

  • combine atomic orbitals of contributing atoms to give molecular orbitals that extend over the entire molecule

  • value of the coefficient shows the extent to which each atomic orbital contributes to the molecular orbital

  • normalisation constants - total electron density must remain the same so equations must be normalised

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non bonding orbital

  • has same energy as the initial atomic orbital

  • neither stabilises nor destabilises the molecule

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HOMO and LUMO

  • HOMO - highest occupied molecular orbital

  • LUMO - lowest unoccupied molecular orbital

  • determine the chemistry of the molecule

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bond order

no of bonding electrons - no of antibonding electrons / 2

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magnetism

  • paramagnetic - attracted to a magnet, if unpaired electrons are present then a molecule/ion is paramagnetic

  • diamagnetic - repels a magnet, only if just paired electrons present in molecule/ion