Conditions for the Combination of Atomic Orbitals

Molecular orbitals are formed by the combination of atomic orbitals. But all types of orbitals cannot combine with each other. The following are the main conditions for effective combination of atomic orbitals:

1. The combining atomic orbitals should not differ much in energies. For e.g., in case of homonuclear diatomic molecules of the type A₂' 2s-orbital of one atom can combine with 2s-orbital of another atom but 1s-orbital of one atom cannot combine with 2s-orbital of another atom.

2. The extent of overlapping between the atomic orbitals of two atoms should be large.

3. The combining atomic orbitals should have the same symmetry about the internuclear axis. For e.g., a 2s-orbital of one atom can combine with 2s-orbital of another atom but it cannot overlap with 2p_x or 2p_y orbital of another atom.

For e.g., when 2s-orbital of one atom A overlaps with 2px orbital of another atom B, + + overlap is cancelled by the + - overlap. As a result, no molecular orbital is formed. In other words, such a combination (2s with 2p_x or 2s with 2p_y) is not possible.

fig 1.13 - Absence of combination of 2s and 2p_y atomic orbitals

fig 1.14 - Possibility of combination of 2s and 2p_z atomic orbitals

However, it may be noted that 2s-orbital of one atom can also combine with 2p_z orbital of another atom because both have same symmetry about the internuclear z-axis (In other words + + overlap can occur). This is shown in figure 1.14.