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Valence Bond Theory Mr. Deore Jaydeep V.Department of
ChemistryG. M. Vedak College of Science, Tala-RaigadMolecular
Orbital Theory&
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Valence Bond TheoryThe Valence bond theory of bonding mainly
developed by Walter Heitler and Fritz London in 1927
Valence shell electron Pair Repulsion (VSEPR) theory
Used to predict the geometry of individual molecules from the
number of electron pairs surrounding their central atoms.
Hybridization is necessary to account for molecular geometry
predicted by VSEPR
Hybrid atomic orbitals are constructed by making linear
combinations (sums and differences) between hydrogen-like orbitals
in the valence shell of each atom
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Postulates of VBTA covalent bond is formed by overlapping of
pure atomic orbitals of the atoms between the bond is being
formedThe orbitals overlapping together should have same symmetry,
i.e. they should have proper orientation or alignment.Greater is
the extent of overlapping between the atomic orbitals, stronger the
resultant covalent bond
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Internuclear axisSigma bond : a covalent bond formed by the
axial overlapping of atomic orbitals are called sigma bondPi bond:
a covalent bond formed by the sideways or lateral overlap of atomic
orbitals.Internuclear axis
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HybridizationMixing and recasting of atomic orbitals having
equal energy and to form new hybrid orbitals of equal energy is
called hybridization
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CH4 molecule: C(6): 1s2 2s2 2p2Excited statesp3 hybrid orbitals
of cFormation of CH4H(1): 1s1
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NH3 molecule: N(7): 1s2 2s2 2p3Excited statesp3 hybrid orbitals
of NFormation of NH3H(1): 1s1
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BeH2 molecule: Be(4): 1s2 2s2 2pExcited stateSp hybrid orbitals
of BeFormation of BeH2H(1): 1s1Sp hybridization
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BF3 molecule: B(5): 1s2 2s2 2p1Excited statesp2 hybrid orbitals
of BFormation of BF3sp2 hybridizationF (9): 1s2 2s2 2p6 3s2 3p5
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Molecular Orbital TheoryFundamentally different than VBT
Valence atomic orbitals of all atoms in the molecule form the
basis set
For convenience, orbitals on symmetry-related atoms are combined
into symmetry-adapted linear combinations called ligand group
orbitals (LGOs)
Atomic orbitals on central atom are mixed with LGOs to generate
molecular orbitals (MOs)
In the final step, valence electrons are placed into MOs
starting at lowest energy
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Atomic orbitals overlapping
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p-Orbital overlap modes
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Combining Atomic orbitalsResulting Molecular orbitals1s + 1s1s1s
- 1s*1s2s + 2s2s2s - 2s*2s2pz + 2pz 2pz2pz - 2pz*2pz
2px + 2px2p2px - 2px*2px2py + 2py2py2py - 2py*2py
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