86.CHEMICAL BONDING (33)- Covalent Bonding(32) – Molecular Orbital Theory(7) – Overlaps and symmetry(1)

Overlaps & Symmetry

Based on how the bonds are formed, the interaction between two AOs  can have following symmetry-

i) Sigma (σ)
ii) Pi (π )
iii)Delta (δ)
iv) Phi (Φ)

Sigma Symmetry

A sigma bond is formed when two s or two pz orbitals overlap. Both these orbitals are symmetric around the internuclear axis and so the bond formed is symmetric around the axis too. Thus, rotation around the internuclear axis brings about no change in the phase of the orbital i.e the sign of the orbital does not change.


A sigma orbital is formed by s or pz orbitals. 



Two s- orbitals overlap to give a BMO and an ABMO.Both have sigma symmetry as they are symmetric around internuclear axis.The sign is drawn just to highlight a point in the orbital. The point is in the positive lobe before and after rotation indicating there is no phase change.

Similarly we can show sigma orbitals for pz orbital too. 


NOTEGerade and ungerade have to do with inversion around the inversion center and sigma symmetry has to do with symmetry around the internuclear axis.


Whenever p -orbitals are involved in σ bond formation , it is always the  porbitals that are involved. The pand py orbitals form π bonds. 

In the next post we will continue studying this topic further.Till then, 

Be a perpetual student of life and keep learning….

Good day !


References and Further Reading –