Chapter 1
Molecular Orbitals o Common Functional Groups
 
Sigma Orbitals
 
For any bonding molecular orbital (BMO) there should be a corresponding antibonding molecular orbital (ABMO)

The shading of the above orbitals that are overlapping shows that they are  ‘in-phase’ for bonding orbital formation

In the anti-bonding orbital out-of-phase combination of participating orbital occurs

Note: BMO is toward the internuclear direction while ABMO is away from the internuclear direction.