BO = (bonding e⁻ - antibonding e⁻) / 2
Bond order indicates bond strength and stability. BO = (bonding electrons - antibonding electrons) / 2. A bond order of 0 means no stable bond forms (like He₂). BO = 1 is a single bond (H₂), BO = 2 a double bond (O₂), BO = 3 a triple bond (N₂). Higher bond order means shorter, stronger bonds. O₂ has 8 bonding and 4 antibonding electrons: BO = 2, but it's paramagnetic because two antibonding electrons are unpaired in π* orbitals — a fact only MO theory explains (Lewis structures predict O₂ is diamagnetic, which is wrong). Fractional bond orders exist: NO has BO = 2.5, and the superoxide ion O₂⁻ has BO = 1.5.