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Molecular Orbital Theory
Molecular Orbitals
Just as atomic orbitals belong to a particular atom, a
molecular orbital belongs to molecules as a whole
Each orbital is filled with 2 electrons
There are 2 types of bonding orbitals:
sigma and pi
Sigma Bond ()
A Sigma Bond is when 2 atomic orbitals combine to
form a molecular orbital that is symmetrical around the
S orbitals overlapping
P orbitals overlapping end-to-end
Pi Bond ()
Pi bonding electrons are likely to be found in a loopshape above and below the axis
Pi bonds are weaker than sigma bonds
because they overlap less
P orbitals overlapping
Since orbitals are waves of
electrons, they can combine
either constructively (forming
a bonding molecular orbital),
or destructively (forming an
antibonding* molecular
MO Theory
Molecular Orbitals (MO’s) from Atomic Orbitals (AO’s)
1. # of Molecular Orbitals = # of Atomic Orbitals
2. The number of electrons occupying the Molecular orbitals is
equal to the sum of the valence electrons on the constituent
3. When filling MO’s the Pauli Exclusion Principle Applies (2
electrons per Molecular Orbital) and Aufbau’s principle Applies
(start at the lowest energy)
4. For equal energy MO’s, Hund's rule applies.
Bond Order
To find bond order add total electrons in the bonding
orbitals and subtract total electrons in antibonding
orbitals, then divide by 2
Example: Li2
MO Diagram for O2
Molecular Orbital Diagram (HF)
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