Write the important conditions required for the linear combination of atomic orbitals to form molecular orbitals.

The linear combination of atomic orbitals to form molecular orbitals take place only if the following conditions are satisfied.

The combining atomic orbitals must have the same or nearly the same energy. This means that $1 s$ orbital can combine with another $1 s$ orbital but not with $2 s$ orbital because the energy of $2 s$ orbital is

appreciably higher than that of $1 s$ orbital. This is not true if the atoms are very different. The combining atomic orbitals must have the same symmetry about the molecular axis. By convention $Z$-axis is taken as the molecular axis. It is important to note that atomic orbitals having same or nearly the same energy will not combine if they do not have the same symmetry.

e.g., $2 p_{z}$ orbital of one atom can combine with $2 p_{z}$ orbital of the other atom but not with the $2 p_{x}$ or $2 p_{y}$ orbitals because of their different symmetries.

The combining atomic orbitals must overlap to the maximum extent : Greater the extent of overlap, the greater will be the electron density between the nuclei of a molecular orbital.