Write the important conditions necessary for linear combination of atomic orbitals.

Explanation

The necessary conditions for the effective overlap of atomic orbitals to form molecular orbitals are
1) The combining atomic orbitals must have the same or almost the same energy.
Ex : 1s -1s overlap is permitted but not 1s - 2s or 2s - 2p overlap.
2) The combining atomic orbitals must be able to overlap to the maximum extent to form m.o's since if the overlapping is more, there will be a greater electron density between the nuclei of the combining atoms and hence the bond formed between them will be stronger.
3) The combining atomic orbitals must have the same symmetry about the moleclar axis.
It means that, atomic orbitals baving the same or nearly the same energy will not overlap, if they do not have the same symmetry. Thus the allowed overlaps are : `s-s,s-p_(x),p_(x)-p_(x),p_(y)-p_(y) " and " p_(z)-p_(z)`.
In the other types of overlaps, `s-p_(y), s-p_(z),p_(x)-p_(y) " and " p_(x)-p_(z)`, no m.o is formed since the atomic orbitals do not have the same symmetry. They are non-bonding m.o's.