Chemistry / Unit 6 Chem 1135 worked example, 60 to 90 seconds
Bond Order from a Molecular Orbital Diagram
The idea
Calculating bond order from a molecular orbital diagram means subtracting the number of antibonding electrons from the number of bonding electrons, then dividing by two.
In practice
Dihydrogen's two electrons both occupy the σ1s bonding orbital, with zero antibonding electrons. First name the quantities: bonding electrons equal 2, antibonding electrons equal 0, both counted directly from the filled molecular orbital diagram. Substituting into the formula, bond order equals (2 bonding electrons − 0 antibonding electrons) divided by 2 electrons per bond, giving 2 / 2 = 1, a unitless ratio since electrons cancel electrons in both the numerator and the divisor. A bond order of exactly 1 is the physically sensible result for dihydrogen, matching its known single bond rather than some fractional or negative value.
Where people slip
Dividing by two accounts for two electrons per bond — omitting that division doubles every calculated bond order.
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