A higher-energy combination with a node between nuclei that weakens bonding when occupied.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
An antibonding molecular orbital results from out-of-phase overlap, which places a node between the nuclei and reduces electron density in the bonding region. The nuclei are effectively pushed apart by electrons in such an orbital, so its energy rises above that of the original atomic orbitals. Populating an antibonding orbital reduces the bond order and weakens the bond, sometimes enough to prevent bonding altogether. Antibonding orbitals are marked with an asterisk. Their occupation explains why some molecules with many electrons in such orbitals are unstable.
Key ideas: Identify by a node perpendicular to the bond axis between the nuclei, marked with an asterisk, as in sigma star 2p. Occupying one electron in an antibonding orbital cancels the effect of one electron in a bonding orbital in bond order terms, but the antibonding destabilization is slightly greater than the bonding stabilization. This is why He2 and Be2 are very weakly or not bound.
Common mistakes and connections: Students think antibonding orbitals are empty by definition or that they exist only in excited states. They are occupied in ground states of O2, F2 and many ions. The pi star orbitals of O2 hold the unpaired electrons. Another is saying that electrons in antibonding orbitals repel nuclei in a physical push. Links to bond order, paramagnetism and photochemistry.
Out-of-phase combination cancels in the middle, creating a node between the nuclei, so electrons here weaken the bond. The interactive lesson shows this as a live animation or chart.
The helium dimer He2 fills both sigma 1s and sigma* 1s orbitals, giving a bond order of zero, so He2 does not form a stable bond.
A higher-energy combination with a node between nuclei that weakens bonding when occupied.
Antibonding Molecular Orbital is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Hybridization of Water and Ammonia, Molecular Orbital Theory, Bonding Molecular Orbital.
The next topics in Bonding & Molecular Structure are MO Diagram of Homonuclear Diatomics and Paramagnetism of Oxygen.
Yes. The interactive lesson includes a self-check question on Antibonding Molecular Orbital, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Antibonding Molecular Orbital is topic 61 of 104 in Bonding & Molecular Structure.