Conditions for the combination of atomic orbitals:
(i) The combining atomic orbitals must have the same or nearly the same energy. This means that 1s orbital can combine with another 1s orbital but not with 2s orbital because the energy of 2s orbital is appreciably higher than that of 1s orbital. This is not true if the atoms are very different.
(ii) The combining atomic orbitals must have the same symmetry about the molecular axis. By convention Z-axis is taken as the molecular axis.
(iii) The combining atomic orbitals must overlap to the maximum extent.
