bond order

Meaning: Bond order is the number of shared electron pairs between two atoms. In a Lewis structure it is 1, 2 or 3 for single, double or triple bonds. With resonance it is the average over the contributing structures, so each C–O bond in CO₃²⁻ is 4/3 and each O–O bond in ozone is 1.5. In molecular orbital theory it is half the difference between bonding and antibonding electrons. Between the same two atoms, a higher bond order gives a shorter bond length and a higher bond energy.

Equation: Bond order = (nb − na)/2, where nb is the number of electrons in bonding molecular orbitals and na is the number in antibonding molecular orbitals (both in electrons). Bond order is unitless.

On the MCAT: Expect questions that rank bonds by length or strength (C≡C is shorter and stronger than C=C, which is shorter and stronger than C–C). You may be asked for the bond order of N₂ (3), O₂ (2) or O₂⁺ (2.5), or whether a species such as He₂ (order 0) is stable. Resonance hybrids such as benzene (1.5 per C–C bond in the Lewis picture's average) and IR stretching frequencies, which rise with bond order, also appear.

Common confusion: A fractional bond order does not mean the bond flips between forms; it is one intermediate bond. Also, a triple bond is stronger than a single bond but not three times as strong.