MO Theory ( Molecular Orbital Theory )

• MO Theory was originally called Hund-Mulliken theory.
• The word orbital was introduced by mulliken in 1932.
• In a molecule electrons are present in new orbitals called MO.

• MO are formed by linear combination of atomic orbitals ( LCAO ) of equal energies  ( homonuclear ) or comparable energies  ( hetero nuclear molecule ).    ( Details : The molecule  ^H_{2   is composed of two hydrogen atoms. Both H atom have 1s orbital, so when bonded together there are two MO.}  
• Two atomic orbitals can combine to form two molecular orbital.One has a lower energy ( bonding MO ) and other has higher energy ( anti bonding ).  (  Explain :  Bonding MO helps to stablize a system of atoms since less energy.  Anti bonding MO cause a system to be unstabilized. since more energy.                                                                      

• Bonding MO are represented by  σ (sigma ),  π  ( pi ) ,  δ ( delta ).
• Anti bonding MO are represented by   σ * , δ*,  π*  .
• The MO are filled in the increasing order of their energies starting with orbitals of least energy.     (Aufbau Principle ).
• MO can accommodate only two electrons and these two electrons must have opposite spin. ( Paul's Exclusion Principle )
• Every orbital in a sub level is singly occupied before any orbital is doubly occupied. All electrons in singly occupied orbitals have the same spin. ( Hund's Rule ).