MOLECULAR ORBITAL THEORY: 

 

• Introduction
• Salient features of MOT
• Types of bonding
• Bonding 
• Antibonding
• Difference Between bonding 
• Linear Combination of atomic orbital 

a. INTODRUCTION:

         THE MOLECULAR ORBITAL THEORY put forward by F.HUND and S.MULLIKEN developed a bonding theory to overcome the limitation of valence bond theory  and explain the magnetic behaviour of molecules.

b. SALIENT FEATURES OF MOT:

¢  When atoms combines to form molecules, their individual atomic orbitals lose their identity and forms new orbitals called molecular orbital. 

¢  The electrons in a molecule are accommodated in the newly formed molecular orbitals. 

The filling of electrons in these orbitals follows AUFBAU’S and HUND’S rule as the as in the case of filling of electrons in atomic orbitals. 

¢  The shape of molecular orbitals depends upon the shape of combining atomic orbitals. 

¢  The electrons in a molecule are present in the various molecular orbitals as the electrons of atoms are present in a various atomic orbitals. 

¢  While an electron in an atomic orbital is influenced by one nucleus, in a molecular orbital it is influenced by two or more nuclei depending upon the numbers of atoms in the molecule.

 Thus, an atomic orbital is monocentric while a molecular orbital polycentric

¢  The number of molecular orbital formed equal to the number of combining atomic orbitals. When two atomic are formed.

• Bonding Molecular orbital 
•  Antibonding Molecular orbital 

#. BONDING MOLECULAR ORBITAL:

•   Lower energy 
• Greater stability then the corresponding antibonding orbital
• Represented by ф  = ψa  +  ψb

#. ANTIBONDING MOLECULAR ORBITAL

•   Higher energy. 
•   Unstable 
• Represented by  ф  = ψa  -  ψb

BONDING MOLECULAR ORBITAL	ANTI- BONDING ORBITAL
• It is the result of the linear combination of atomic orbital, when their wave function are added.  ф  = ψa  -  ψb	• It is resulting of linear combination of atomic orbital when their wave function are subtracted.  ф  = ψa  -  ψb
• It does not have node.	• It always have a node between two nuclei of bonded atom.
• The charge density increases between two nucleus resulting between two atom.	• The charge density decrease in between two nuclei, leads to repulsion between two atom.

¢  Just as the electron probability distribution around a nucleus in an atom is given by an atomic  orbital, the electron probability distribution around a group of nuclei in a molecule is given by a molecular orbital. 

 

¢  Bonding order gives the number of covalent bonds between the two combining atoms. The bond order of a molecule can be calculated using following equation 

 

                           Bond Order =        N_b -  N_a    

                                                                          2  

Where,

         N_b - Total number of electrons present in the bonding molecular orbital    

       N_a   -Total number of electron present in the antibonding molecular        orbitals  

 A bond order of zero value indicates that the molecule doesn’t exit

c.LINEAR COMBINATION OF ATOMIC ORBITAL: 

ª  The wave function for the molecular orbital can be obtained by solving Schrodinger wave equation for the molecule. Since solving the Schrodinger equation is too complex, approximation methods are used to obtain the wave function for molecular orbitals. The most common methods is the linear combination of atomic Orbitals. 

ª  The atomic orbitals are represented by the wave function ψ.    

ª  Let us consider two atomic orbitals represented by   the wave function ψa  and ψb with compare energy, combines to form two molecular orbital.  

ª  One is bonding molecular orbital(ψ_bonding) and the other is antibonding molecular orbital(ψantibonding). 

ª  The wave functions for these two molecular orbitals can be obtained by the linear combination of the atomic orbitals ψ_A and ψ_B as below. 

              

            ψ_bonding = ψa + ψb                                          

           ψantibonding = ψa – ψb  

 

ª  The formation of bonding molecular orbital can be considered as the result of constructive interference of the atomic orbitals and the formation of antibonding molecular orbital can be the result of the destructive interference of the atomic orbitals.   

 

* HOMO NUCLEAR DIATOMIC MOLECULES

1.  Hydrogen

2.Oxygen

* HETRO NUCLEAR DIATOMIC MOLECULES:

1.NO

2. C0

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