2.4: Chemistry Review
Major Deficiencies in VB:
1. The assumption that electrons are localized is not entirely correct. For example, in resonance
hybrids the electrons are delocalized.
2. It does not provide obvious insight on the relative energies of the electrons.
3. It fails to explain certain phenomena, such as whether molecules are paramagnetic or
Molecular Orbital (MO) Theory – takes a delocalized approach to bonding and focuses on the wave
behaviour of electrons
In MO, molecular orbitals are formed by combining wave functions of atomic orbitals.
This can occur in two ways: constructively and destructively
Constructive Combination – occurs when two atomic orbitals in the same phase (either both + or both -)
combine and form a bonding MO, but it doesn’t generate any new nodes between the nuclei. The
combination increases the electron density between the two nuclei, consistent with the formation of a
bond. The resulting MO is termed bonding and designated σ . 1s
Destructive Combination – occurs when two atomic orbitals in opposite phases combine and it
generates a new node between the two nuclei. Since there are no electrons shielding the positively
charged nuclei from each other, an unfavourable Coloumbic repulsion occurs. This does not lead to
bond formation and the resulting MO is termed antibonding and designated σ* . 1s
Note: the 1s subscript indicates the atomic orbitals used to form the MO.
Each MO holds a maximum of two electrons.
1. The number of MOs formed is always equal to the total number of atomic orbitals combined.
2. Bonding MOs are always lower in energy than atomic orbitals from which they originate.
Antibonding MOs are always higher in energy than the originating atomic orbitals. As a result
the bonding MOs stabilize the molecule (contribute to bond formation), while antibonding MOs
destabilize the molecule (do not contribute to bond formation). Therefore, for any molecule to
be stable (exist) it must have more bonding that antibonding electrons.
3. MOs are filled lowest energy to highest energy. The highest energy MO with one or more
electrons is called the Highest Occupied Molecule Orbital (HOMO). The lowest energy MO with
no electrons present is called the Lowest Unoccupied Molecule Orbital (LUMO).
4. MOs are formed most effectively when the originating atomic orbitals are identical or close in
Bond Order – a parameter used to describe the relative numbers of bonding to antibonding electrons in
Bond Order = (number of bonding electrons) – (number of antibonding electrons)/2
Species with higher bond orders are more stable.
Paramagnetic – molecules that are attracted to a magnetic field and must contain at least one unpaired
electron and a nonzero net spin (sum of all m vslues cannot equal 0)
Diamagnetic – molecules that are not