How do atomic orbitals form bonding and antibonding MOs?

Atomic orbitals combine to form molecular orbitals that can be bonding or antibonding, and electrons fill those molecular orbitals based on energy to determine a molecule’s stability.

Think of atomic orbitals as tiny rooms where electrons live around each atom, and when atoms come close those rooms overlap to make new rooms that belong to the whole molecule called molecular orbitals. Some of these new rooms are lower in energy, so electrons like living there and those are bonding orbitals that help hold the atoms together, while others are higher in energy and make the atoms want to stay apart, and those are antibonding orbitals. When you draw a molecular orbital diagram, you line up the energies of the atomic orbitals on the sides and the molecular orbitals in the middle, and then you place electrons into the lowest energy molecular orbitals first, two per orbital with opposite spins. The difference in how many electrons occupy bonding versus antibonding orbitals tells you if the bond is strong, weak, or does not form at all.

A common mistake is thinking that any overlap automatically makes a strong bond, but if many electrons go into antibonding orbitals they can cancel the bonding and weaken or break the bond, so always compare bonding and antibonding electron counts. Another error is mixing up the order of orbital energies for different atoms; for example, pi and sigma orbitals can switch order depending on which elements are involved, so check the correct energy ordering for the specific atoms you are using. Students also sometimes forget to include atomic orbital symmetry and orientation, which means orbitals must be able to overlap in the right way to form molecular orbitals; if overlap is not allowed by symmetry there will be no bonding interaction.

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Molecular Orbital Basics: Bonding and Antibonding

1. Atomic orbitals overlap to form MOs
2. Bonding MOs are lower energy
3. Antibonding MOs are higher energy
4. Fill lowest energy MOs first
5. Two electrons per orbital with opposite spins

Common Mistakes in Molecular Orbital Diagrams

1. Assuming overlap always yields strong bonds
2. Using incorrect orbital energy ordering
3. Ignoring orbital symmetry and orientation

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