Choose the bond-order model that matches the question
Bond order is a compact description of net bonding, but it can be obtained from different models. The Lewis or resonance mode averages visible bond lines across equivalent positions. The molecular-orbital mode subtracts antibonding electrons from bonding electrons and divides by two. Select the method stated by the problem; mixing resonance bond counts with molecular-orbital electron counts produces an answer with no consistent interpretation.
A single Lewis bond counts as one bond line, a double bond as two, and a triple bond as three. For a resonance example with three total bond lines spread across two equivalent positions, the average is 1.5. In molecular-orbital language, ten bonding electrons and four antibonding electrons give a bond order of three. The same number can arise from different evidence, so the result always names its method.
Lewis and resonance averaging
Use Lewis mode when equivalent resonance forms distribute bonding across several positions. Add the bond lines assigned to those equivalent positions in a representative set, then divide by the number of positions. This is a teaching summary of delocalization; it does not mean a molecule rapidly switches between separate drawings or that a bond is literally half of a line.
The denominator must be a positive whole number because it counts positions. Total bond lines may include halves if the exercise already supplies an averaged contribution, but ordinary structures normally use whole single, double, and triple counts before averaging. If positions are not chemically equivalent, do not average them merely because they appear in the same molecule. Calculate or describe each distinct bond separately.
Molecular-orbital electron counting
In molecular-orbital mode, count electrons occupying bonding orbitals and electrons occupying antibonding orbitals, then calculate one half of their difference. A positive value means the entered occupancy has a net bonding contribution. Zero means bonding and antibonding contributions cancel in this simple count. A negative value is a strong prompt to recheck the orbital diagram and electron occupancy.
Electron counts must be non-negative whole numbers. The calculator does not construct the molecular-orbital diagram, decide orbital energy order, or infer occupancy from a molecular formula. Those steps depend on the species and model used in the course. Enter the counts only after labelling bonding and antibonding orbitals in the diagram supplied by the question.
Check the result against chemical meaning
For familiar Lewis structures, a single bond should give one, a double bond two, and a triple bond three. In a resonance average, the result should lie between the contributing bond orders. For molecular orbitals, adding two electrons to a bonding orbital raises bond order by one, while adding two to an antibonding orbital lowers it by one. These changes provide quick consistency checks.
Bond order often correlates with bond length and bond strength within a comparable family, but it is not a complete property prediction. Different atoms, charges, geometries, spin states, and electronic methods affect measured bonds. Use the answer to interpret the chosen bonding model, not to manufacture an exact bond energy or length that the inputs cannot support.
Model limits and private calculation
The two modes deliberately answer bounded educational questions. They do not calculate Wiberg, Mayer, natural bond orbital, or density-derived bond indices, and they do not resolve aromaticity conventions automatically. If a textbook, software package, or research paper names a different bond-order definition, compare that definition before comparing numerical results.
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