Molecular Orbitals
An overview
MO Theory
Robert Mullikan won the Nobel Prize in 1966 for developing this theory.
This theory describes the electrons in orbitals belonging to the entire molecule.
Molecular Orbitals
A molecular orbital (MO) can hold a maximum of two electrons.
A MO has a definite energy. We can represent an MO with an electron-
density cloud.
LCAO
Linear Combination of Atomic Orbitals Whenever two atomic orbitals overlap, two
molecular orbitals form. # in = # out
Energy is conserved, so… One orbital will be lower in energy One orbital will be higher in energy
The Hydrogen Molecule
The Two Orbitals
* Antibonding orbital Very little electron density between the nuclei Higher in energy Destabilizes bond formation
Bonding orbital Promotes bond formation Electron density is between the nuclei Lower in energy
Bonding and Antibonding Orbitals
MO Diagrams
H2 and He2
Complex MO Diagrams
Bond Order
In MO theory, bond stability of a covalent bond is related to its bond order.
Bond order = ½(# bonding electrons - # antibonding electrons)
Bond order can be an integer or a fraction
Bond Order
# of bonds
0 No bond exists
1 Single bond
2 Double bond
3 Triple bond
Calculate Bond Order for He2+
Compare bond orders
Which would be more stable: He2 or He2+?
Justify your answer.
