Consider a very interesting and important allotrope of oxygen, ozone. This molecule
is known to be bent but not an equiliateral triangle. It is also known that it is
symmetric with repect to the O-O bonds. (It does in fact have a two-fold symmetry axis,
and two perpendicular symmetry planes, i.e. C2V symmetry) The simplest
Lewis theory cannot predict this without a 'fix' called resonance.
Sometimes the formal charges can help one decide between the relative importance of
non-equivalent resonance structures. The best structure has the fewest formal charges
and has the negative charge on the highest electronegativity atom, as can be seen in
the cyanate anion
The nitrate anion is symmetric, so either an incomplete octet, or three equivalent
resonance structures can predict this. Resonance is the preferred explanation.
The orbitals that electrons are in when they exist in molecules are actually
different from even the most 'hybridized' or perturbed atomic orbitals.
How molecular orbitals for
stems from how waves interefer with one another and the concept of Phase. Adding
orbitals 'in phase' or 'out of phase' can make two different shapes, usually one that
is 'bonding' and one that is 'antibonding'. Antibonding orbitals have a new node
between the atoms and perpendicular to the bond axis.
Including the p orbitals complicates matters considerably.
Molecular 'aufbau' can now predict the correct nature of DiOxygen (and DiBoron)!
Polyatomic Molecular Orbitals explain many properties that "simple" Lewis or Valence Bond theories cannot. One such property is the energetics of Bond Rotation, which has a low barrier (easy) for single bonds, such as in ethane:
Bond rotation is restricted in multiple bonds because the nature of the p (pi) - bond, which has a node containing the bond axis, and must be 'broken' for rotation about the bond axis to occur. An example of a p (pi) - bond is in that of ethylene (ethene):
Here is an animation of the structure of the MO's in ethylene.
There are two p (pi) - bonds in acetylene (ethyne) but the bond rotation issue is moot since because the molecule is linear.
Molecular orbitals also describe the actual nature of molecules that appear to have 'resonance structures' in their Lewis dot description.
Here is an animation of the structure of the MO's in benzene.