Valence bond Theory (VBT)

 VALENCE BOND THEORY (VBT) :

 It was presented by Heitler & London to explain how a covalent bond is formed. It was extended by Pauling & Slater.

 It is based on the knowledge of atomic orbitals, electronic configuration of elements, the overlap of atomic orbitals, hybridization of atomic orbitals

The main points of theory are –

 (a) To form a covalent bond overlapping occurs between half-filled valence shell orbitals of the two atoms.

(b) Resulting bond acquires a pair of electrons with opposite spins to get stability.

(c) Orbitals come closer to each other from the direction in which there is maximum overlapping

(d) So covalent bond has directional character.

(e) Strength of covalent bond α extent of overlapping.

(f) Extent of overlapping depends on two factors.

(i) Nature of orbitals – 

p, d and f are directional orbitals -------> more overlapping

s-orbital -----> non directional – less overlapping

(ii) Nature of overlapping –

 Co-axial overlapping - extent of overlapping more.

Collateral overlapping - extent of overlapping less Order of strength of Co - axial overlapping –

p - p > s - p > s - s                                                                                      

Two types of bonds are formed on account of overlapping.

(A) Sigma bond                                               

(B) Pi- bond

(A) Sigma bond

a) Bond formed between two atoms by the overlapping of half-filled orbitals along their axis (end to end overlap) is called sigma bond.

(b) sigma bond is directional.

(c) sigma bond do not take part in resonance.

(d) Free rotation is possible about a single s bond.

(e) Maximum overlapping is possible between electron clouds and hence it is strong bond.

(f) There can be only one sigma bond between two atoms

Sigma bonds are formed by four types of overlapping

 (i) s - s overlapping – Two half-filled s-orbitals overlap along the internuclear axis. Ex. H2 molecule

(ii) s - p overlapping (Formation of HF) – When half fill s-orbital of one atom overlap with half-filled p- orbital of other atom.

(iii) p - p overlapping – (Coaxial) – It involves the coaxial overlapping between half filled p-orbitals of two different or same atoms.

Pi(π)-Bond

(a) The bond formed by sidewise (lateral) overlapping are known as pi bonds.

(b) Lateral overlapping is only partial, so formed are weaker and hence more reactive than sigma bonds (Repulsion between nucleus is more as orbitals have to come much close to each other for pi- bonds formation) Example – Formation of O2

(c) Free rotation about a pi bond is not possible.

(d)  π bond is weaker than sigma bond (Bond energy difference is 63.5 KJ or 15 K cal/mole)

(e) π bonds are less directional, so do not determine the shape of a molecule

F) it is former by overlapping of unhybridized orbitals

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