What are the conditions required for linear combination of atomic orbitals to form molecular orbitals?

Monjur Alahi

What are the conditions required for linear combination of atomic orbitals to form molecular orbitals?


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Salim Ahmed Kawsar

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The following conditions are required for the linear combination of atomic orbitals (LCAO) to form molecular orbitals :

i. The combining atomic orbitals must have comparable energies.

So, Is orbitals of one atom can combine with 1 s orbital of another atom but not with 2s orbital, because energy of 2s orbital is much higher than that of 1 s orbital.

ii. The combining atomic orbitals must have the same symmetry along the molecular axis.

Conventionally, 

z axis is taken as the internuclear axis. 

So even if atomic orbitals have same energy but their symmetry is not same they cannot combine. 

For example, 

2s orbital of an atom can combine only with 2pz orbital of another atom, and not with 2px or 2py orbital of that atom because the symmetries are not same. p is symmetrical along z axis while px is symmetrical along x axis.

iii. The combining atomic orbitals must overlap to the maximum extent. 

Greater the overlap, greater is the electron density between the nuclei and so stronger is the bond formed.

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