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Wave mechanical concept of chemical bonding:

There are two important  theories which help the nature  of covalent bond on the basis of quantum mechanics:

(1) Valence bond theory (VBT)       (2) Molecular orbital theory (MOT)

(1) Valence bond theory (VBT):

(1) The valence bond theory given by Heitler and London to explain how to covalent bond form. And it is  extented by Pauling and Slater. 

(2) the main point of valence bond theory are given as:

(A) The covalent bond formed by overlapping of half filled orbitals of valence shell.

(B)The covalent bond requires a pair of electrons with opposite spins.

(C) The Atomic orbitals come closer to each other from the direction in which there is maximum overlapping, hence covalent has directional in nature.

(D)  The strength of covalent bond is directly proportional to extent of overlapping.

(E) The extent of overlapping depends on two factors like nature of orbitals and nature of overlapping

(1) S-orbital is non directional- less overlapping extent while p,d,f are directional orbitals hence more overlapping extent.

(2) Co-axial overlapping – extent of overlapping is more while in collateral overlapping – extent of overlapping  is less. The order of strength of co-axial overlapping is : P-P > S-P > S-S

(3) As the value of principle  quantum number(n) increases the strength decreases: 1-1 > 1-2 > 2-2 > 2-3 > 3-3

(4) If the value of value of principle  quantum number(n) is same then bond strength order follow as:

                         2p-2p > 2s-2p >2s-2s

                         1s-2p > 2s-2p > 3s-3p

(5) The electron which is already paired in valency shell can enter into bond formation, it they can be unpaired  first and shift to vacant orbitals of slightly higher energy of same energy shell, this point can explain the trivalency of boron, tetravalency of carbon, penta-valency of phosphorous etc.

(6) There are  two type of covalent bonds on basis of way of overlapping.

(A) Sigma bond  (B)   Pi bond

What is Sigma Bond ?

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