BONDING

1
BONDING
Two main ways of thinking about bonding
1. Valence Bond Theory
-all bonds are localized interactions involving 2
electrons and 2 nuclei -leads to orbital
hybridization as a method to manipulate the AOs.
-no resonance, no delocalization
2. Molecular Orbital (MO) Theory
-assigns electrons to MOs which are delocalized
over the entire molecule
Which is better? Both are approx.
-VB theory fails to explain bonding in aromatic
compounds -VB theory fails to predict that O2 is
paramagnetic -MO theory more consistent with
spectroscopy
2
BONDING
Why do we need approximate methods at all?
1. exact solutions are not possible even for H2
2. approx. methods can give insight and physical
understanding into chemical bonding
MO Theory consists of 3 main approximations
1. Born-Oppenheimer approximation
2. Independent electron (orbital) approximation
3. LCAO approximation