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How to calculate bond orders from lewis dot structures?

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In general chemistry class we learn about the resonance, shapes, and calculating bond order for molecules.

Without using molecular bond theory, we should know that for a single covalent bond, bond order is equal to one. For a double covalent bond, bond order is equal to two. For a triple covalent bond, bond order is equal to three.

For molecules that have resonance we have fractional bond orders. For example, in a molecule of ozone, O3, there are two resonance structures. We choose to look at one bond only to calculate bond order. In the first molecule, lets say this bond is a single bond and in the second molecule its a double bond. Thus we add 1+2=3. We then divide it by the number of structures we have, which is 2. Therefore our bond order is 3/2=1.5 for the molecule of ozone.

What does bond order mean?

Bond order is directly proportional to the strength of the bond. Thus, the higher the bond order the strong the bond and the more energy it takes to break it.

Bond order is also indirectly proportional to the length of the bond. The higher the bond order, the shorter the bond is.

The other way to find bond order is to draw out the molecular orbitals and calculate it using the formula:

BO = [(Number of electrons in bonding molecules) - (Number of electrons in antibonding molecules)]/2

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