Question

What are bonding and anti-bonding molecular orbitals ?

Answer 1

Molecular orbitals (MOs) form upon overlap of atomic orbitals. Bonding orbitals are due to in-phase overlap, antibonding due to out-of-phase overlap.

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• The bonding MO here has all atomic orbitals overlapping in-phase in ozone.
• The antibonding MO has all atomic orbitals overlapping out-of-phase in ozone; there is a nodal plane in between each one.
• The nonbonding MO has a nodal plane right on an atomic orbital, so there is out-of-phase overlap but the two atomic orbitals are separated more than in the antibonding MO. The phase cancellation and the placement of a node ON an orbital is what makes it nonbonding.

The pure atomic orbitals (AOs) are on the left, and the new MOs are on the right. As we add more nodes, the MO increases in energy, so

`E_("bonding") < E_"nonbonding" < E_"antibonding"`

for the same atomic orbital basis.