Molecular Orbital Theory

Molecular Orbital Theory

According to this theory all the atomic orbitals of the atom participating in molecule formation get disturbed when the concerned nuclei comes closer. They all get mixed up to give rise to an equivalent number of new orbitals that belong to the molecules.

Linear Combination of Atomic Orbitals (LCAO)

  1. Overlapping of atomic orbitals is the overlapping of waves.

  2. In phase overlapping results in Bonding molecular orbital and out of phase over lapping results in anti bonding molecular orbital.

  3. Energy levels of these orbitals is increasing energy sequence is

     1s < * 1s <  2s < * 2s <  2pz <  2px =   2py < * 2px = * 2py < * 2pz for O2, F2 and Ne2 molecules

    Note: For B2, C2, Nmolecules, E2px = E2py < E σ2pz

Bond Order

Higher the bond order smaller the bond length

B. Order Stability (Molecule)
< 0 Unstable and does not exist
= 0 Does not exist
0.5 Exist but unstable
1 A stable molecule with single bond
2 A stable molecule with double bond
3 A stable molecule with triple bond

Fractional bond order indicates paramagnetic nature of molecule.

Dipole Moment: Is defined as the product of the magnitude of charge (q) on the atom and the distance between the atom (d).

 = q x d

Its unit is debye (D)

1D = 10-18 esu-cm

e.g., CO2, CS2, CH4, BF3, XeF4

Polar molecule have non-zero dipole moment

e.g., NH3, PCl3, H2O, SF4, XeF6

Resultant Dipole Moment

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