bonding orbital
Summary
A bonding orbital is a molecular orbital formed when atomic orbitals overlap in phase, concentrating electron density between the nuclei. It lies lower in energy than the atomic orbitals that combined, so electrons placed in it stabilize the bond. Its partner is a higher-energy antibonding orbital.
Detail
Meaning: A bonding orbital is a molecular orbital formed by in-phase (constructive) overlap of atomic orbitals. Electron density builds up between the nuclei and attracts both, so the orbital lies lower in energy than the atomic orbitals that combined. Overlap of n atomic orbitals gives n molecular orbitals, so two give one bonding and one higher-energy antibonding orbital. Head-on overlap gives a σ bonding orbital; sideways overlap of p orbitals gives a π bonding orbital.
Equation: bond order = ½(nb − na), where nb is the number of electrons in bonding orbitals and na is the number in antibonding orbitals. Bond order is unitless.
On the MCAT: You may be asked to fill electrons into an energy diagram for H₂ or ethene, or to identify the HOMO and LUMO. In UV-vis spectroscopy, a π → π* transition moves an electron from a π bonding to a π* antibonding orbital. More conjugation narrows this gap, shifting absorption to longer wavelength.
Common confusion: A bonding orbital is not a hybrid orbital. Hybrid orbitals are mixed atomic orbitals on one atom; a bonding orbital is a molecular orbital spread over the bonded atoms. Also, electrons in antibonding orbitals cancel the benefit of bonding ones, which is why He₂ (bond order 0) does not form a stable molecule.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.