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

1. The combining atomic orbitals must have the same or nearly the same energy. For example, a 1s orbital can combine with another 1s orbital but not with a 2s orbital because the energy of the 2s orbital is appreciably higher.

2. The combining atomic orbitals must have the same symmetry about the molecular axis (by convention, the z-axis is taken as the molecular axis). For instance, the $2p_z$ orbital of one atom can combine with the $2p_z$ orbital of another, but not with $2p_x$ or $2p_y$ orbitals due to different symmetries.

3. 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 the molecular orbital.