VALENCE BOND THEORY (VBT)

VALENCE BOND THEORY :  


  • Introduction     
  • Salient Features of  VB theory
  • Types of overlapping
  • Different Between Sigma Bond and Pii Bond 


a. INTRODUCTION:

  •      Valence bond theory was introduced by Heitler and London (1927) and later it was developed further by Pauling and others. 
  •    A discussion of the valence bond theory is based on the knowledge of atomic orbitals, electronic configurations of elements, the overlap criteria of atomic orbitals and the principles of variation and superposition. 

 


ATTRACTIVE FORCE

 

REPULSIVE FORCES

·         Nucleus of Ha and valence electron of Hb. 

·        Nucleus of Hb and the valence electron of Ha. 

  • The nucleus of Ha and Hb. 

·        Valence electrons of Ha and Hb 


  •   The valence bond theory describe the covalent bond and covalent bonding in molecules in a quantitative manner. 
  •   It is based upon on the overlapping of atomic orbitals in the formation of bond. 
  • According to this theory, each atom uses electron in one or more of its valence shell orbitals to form the covalent bonds.(overlapping of unpair atomic orbital of two atom). 
      • After overlapping both orbital gets common region of electron density. 
  •  Overlapping of orbital causes delocalization of electron which in turn lower the energy and increase the stability.
      • Resulting bond acquires a pair of electron with opposite spin to get stability.
  • The strength of the bond depends on the extent of overlapping.
      • Overlapping of orbital is higher, the bond formed is stronger.  
      • Order of overlapping S-S   >   S-P   >   P-P .
      •  The direction of the bond formation is the direction in which the bonding orbital are oriented.
      • As the value of “n” increase, bond strength decrease . 
1s – 1s  >    1s – 2s    >    2s – 2s   >   2s  -  3p  >  3p  - 3p   
      •   If “n” is same 
1s -2p   >   2s  - 2p >   3s- 3p 

  •  We can classify the overlapping into two types based on direction.
      •     Axial overlapping  

      •   Side way overlapping 



 

         For example the VB theory: 

 Consider a situation wherein two hydrogen atoms (Ha ) and (Hb) are separately by infinite distance. 

  • At this stage there is no interaction between these two atoms and the potential energy of this system is arbitrarily taken as zero.
  • As these two atoms approach each other, in addition to the electrostatic attractive force between the nucleus and its own electron (pink arrows).  


  • The attractive force trend to bring Ha and Hb together whereas the repulsive forces trends to push them apart.
  • At the initial stage, as the two hydrogen atoms approach each other, the attractive force strong then repulsive forces and the potential energy decreases. 
  • A stage is reached where the net attractive forces are exactly balanced by repulsive forces and the potential energy of the system acquires a minimum energy.
  •  At this stage, there is a maximum overlap- between the atomic orbitals of Ha and Hb, and the atoms Ha and Hb are now said to be bonded together by a covalent bond.
  •  The internuclear distance at this stage gives the H-H bond length and is equal to 74pm.
  • The liberated energy is 436kj mol-1 and is known as bond energy, since the energy is released during the bond formation, the resultant molecules is more stable.
  • If the distance between the two atoms is decreased further, the repulsive  forces dominant the attractive forces and the potential energy of the system sharply increase 



  b. SALIENT FEATURES OF VB THEORY:

  •  When half-filled orbitals of two atoms overlap, a covalent bond will be formed between them.
  • The resultant overlapping orbital is occupied by the two electrons with opposite spins.
    • For example, when H2 is formed, the two 1s electrons of two hydrogen atoms get paired up and occupy the overlapped orbital
  • The strength of a covalent bond depends upon the extent of overlap of atomic orbitals. Greater the overlap, larger is the energy released and stronger will be the bond formed.  

•       Each atomic orbital has a specific direction (except s-orbital which is spherical) and hence orbital overlap takes place in the direction that maximizes overlap  


c. ORBITAL OVERLAP:  

  •  When atom combines to form a covalent molecule, the atomic orbitals of the combining atoms overlap to form a covalent bond. 
  •   The bond pair of electrons will occupy the overlapped region of the orbitals.  
  •   Depending upon the nature of overlap we classify the covalent bonding the two atoms as 
        • Sigma Bond σ
        • Pii Bond    π



i.) BOND / AXIAL OVERLAPPING:

  •   When two atomic orbital overlap linearly along the axis ,the resultant bond is called a sigma ( σ) bond  
  •  This overlap is also called “ head-on overlap” or “axial overlap”
      •   Overlap involves a s orbital (s-s and s-p overlaps) will always result in a sigma bond as the s orbital is spherical.  
      •  Overlap between two p orbital along the molecular axis will also result in sigma bond formation.  
      • When we consider x-axis as molecular axis, the px - px overlap will result in ( σ)  bond.


                  A) S-S OVERLAPING:


B) S-P OVERLAPING:



C) P-P OVERLAPING:


ii.) PI BOND / SIDE WAY OVERLAPPING:

      • When two atomic orbital overlap sideways, the resulting covalent bond is called a pi bond (π)
      • When we consider x-axis as molecular axis, the p – p overlaps will result in the formation of a (π). - bond



    d.DIFFERENT BETWEEN SIGMA BOND AND PII  BOND:


                         SIGMA BOND (σ) 

                             Pi Bond (π) 

    ▪  It is strong bond. 

    ▪  It is weak bond. 

    ▪  It can exist independently. 

    ▪  It exist along with bond. 

    ▪  It allows the free rotation of bond atom. 

    ▪  It is restricted the free rotation of bonded atom. 

    ▪  The electron cloud of sigma bond is along their inter nuclear axis of the bonded atom. 

    ▪  The electron cloud of the bond lies above and below the axis. 

    ▪  All orbital s, p, d… can form sigma bond. 

    ▪  Except “s” orbital, all others orbital can form bond. 

    ▪  Both pure and hybrid orbital can form bond 

    ▪  Hybrid orbital cannot form a bond. 

    ▪  This bond is formed by the axial overlapping orbital.  

    ▪  This is formed by the lateral overlapping orbital.  






    VBT..

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