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Molecular Orbital Theory: Bonding In Homonuclear Diatomic Molecules

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Molecular Orbital Theory: Bonding In Homonuclear Diatomic Molecules - Lesson Summary

Linear Combination of Atomic Orbitals or LCAO is an approximate method that has been adopted to explain systems containing more than one electron.

Atomic orbitals of equivalent energies combine to form molecular orbitals.

Two atomic orbitals combine to form two molecular orbitals, known as bonding molecular orbital, denoted by sigma, and anti-bonding molecular orbital, denoted by sigma asterisk. The total energy of the two resultant molecular orbitals equals that of the original atomic orbitals. Among bonding orbitals posses less energy, than the anti-bonding orbitals.

When a bonding molecular orbital forms, there is constructive interference because the two electron waves of the bonding atoms reinforce each other.

The electron waves cancel each other due to the destructive interference in an anti-bonding molecular orbital.

Ex:

(i) 1s-atomic orbitals of two Hydrogen atoms combines to form Hydrogen molecule.

Electronic configuration of Hydrogen atom is - 1s¹

During the formation of Hydrogen molecule, the two electrons from two Hydrogens will be filled in bonding orbitals.

Electronic configuration in Hydrogen molecule -σ1s²

Therefore bond order in hydrogen molecule 

= ½ [(Number of bonding electrons) - (number of anti-bonding electrons)]

= ½ [2 - 0]

= 1

There exists single bond between two Hydrogens in Hydrogen molecule.

It is diamagnetic in nature, as there are no unpaired electrons in the molecule.

(ii) Formation Oxygen molecule:

Electronic configuration of oxygen atom-1s² 2s² 2p⁴

Atomic orbitals of oxygen combine to form molecular orbitals.

Electronic configuration of Oxygen molecule = (σ1s)2 (σ*1s)2 (σ2s)2 (σ*2s)2 (σ2pz)2
                                                                                         (π2px2 = π2py2)(π*2px1 = π*2py1)
                                                                 = [KK (σ2s)2(σ*2s)2(σ2pz)2
                                                                                         (π2px2 = π2py2)(π*2px1 = π*2py1)]

Bond order in Oxygen molecule

=½ [(Number of bonding electrons) - (number of anti-bonding electrons)]

= ½ [10 - 6]

= 2

There exists double bond between two oxygen atoms in Oxygen molecule.

It is paramagnetic in nature as there are unpaired electrons in this molecule.

The linear combination of atomic orbitals helps describe the formation of molecular orbitals, mathematically.

Ex: Formation of Helium (He₂) molecule.

Electronic configuration of Helium atom = 1s²

Two 1s² orbitals combines to form two molecular orbitals.

Electronic configuration of Helium molecule = (σ1s)2 (σ*1s)2

Bond order in Helium molecule

=½ [(Number of bonding electrons) - (number of anti-bonding electrons)]

= ½ [2 -2]

= 0

Thus 'Zero' value indicates that Helium (He₂) molecule does not exist.

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