molecular orbital
Summary
A molecular orbital is a wavefunction for an electron spread over a whole molecule, formed by combining atomic orbitals. Bonding MOs lie lower in energy and antibonding MOs higher, so bond order = ½(bonding − antibonding electrons) predicts bond strength and magnetism.
Detail
Meaning: A molecular orbital (MO) is a wavefunction spread over the whole molecule, built by adding and subtracting atomic orbitals (LCAO). n atomic orbitals combine to give n MOs. In-phase overlap makes a bonding MO, lower in energy, with electron density between the nuclei. Out-of-phase overlap makes an antibonding MO (marked *), higher in energy, with a node between the nuclei. Head-on overlap gives σ MOs and side-on overlap gives π MOs. Electrons fill from the lowest energy up, following the Pauli exclusion principle and Hund's rule.
Equation: Bond order = ½(nb − na), where nb and na are the numbers of electrons in bonding and antibonding MOs (dimensionless).
On the MCAT: An MO diagram for a diatomic such as N₂, O₂ or He₂ is given. You find the bond order, then rank bond strength (higher order means stronger and shorter bond) or predict magnetism (unpaired electrons mean paramagnetic). O₂ has two unpaired electrons, which MO theory explains and a Lewis structure does not.
Common confusion: MOs are not hybrid orbitals. Hybrids (valence bond theory) stay on one atom, while MOs span the molecule. Filling an antibonding MO cancels bonding, and a bond order of 0, as in He₂, means no stable bond forms.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.