bond order

Is a bond a single, a double, or somewhere in between? Bond order is the simple count that answers this: it is the number of shared electron pairs holding two atoms together. A single bond has bond order one, a double bond order two, a triple bond order three. The higher the bond order, the more glue between the atoms — generally meaning a shorter, stronger bond.

Counting pairs is the easy version, but bond order need not be a whole number. When electrons are delocalised over several atoms by resonance, the order is averaged out: in the nitrate ion the one extra bond is shared over three N-O bonds, so each has a bond order of about one and a third. Molecular-orbital theory gives a cleaner definition that even works for species with no good Lewis structure: bond order equals half the difference between the number of electrons in bonding orbitals and the number in antibonding orbitals. By that formula O2 comes out with a bond order of two, and the He2 'molecule' comes out with a bond order of zero, which is exactly why it does not exist.

Bond order is a compact predictor that ties together three measurable properties: as bond order rises, bond length shrinks and bond strength (bond dissociation energy) grows. The series single, double, triple — for example the carbon-carbon bonds in ethane, ethene and ethyne, or the nitrogen-nitrogen bonds across N2H4, N2H2 and N2 — shows the trend cleanly. It is one of the most useful single numbers you can attach to a bond.