antibonding orbital

Meaning: Overlap of two atomic orbitals gives two molecular orbitals. In-phase overlap gives a bonding orbital, lower in energy, with electron density between the nuclei. Out-of-phase overlap gives an antibonding orbital (marked σ* or π*), higher in energy than the parent atomic orbitals, with a node between the nuclei. Electrons in it weaken the bond. Ground-state molecules usually leave these orbitals empty, but the π* of C=C and C=O is often the LUMO, which receives electrons in UV absorption and in nucleophilic attack on carbonyls.

Equation: Bond order = ½(nb − na), where nb is the number of electrons in bonding orbitals and na is the number in antibonding orbitals (unitless).

On the MCAT: Fill an MO diagram for H₂, He₂, or O₂ to find bond order and stability; He₂ has bond order 0, so it does not form a stable molecule. In spectroscopy passages, more conjugation narrows the gap between the HOMO and the π* LUMO, shifting absorption to longer wavelength.

Common confusion: An empty antibonding orbital does not weaken a bond; the weakening comes from electrons occupying it. It is also not a nonbonding orbital (such as a lone pair), which sits near atomic-orbital energy.