In the Lews description of covalent bond, the Bond Order is given by the number of bonds between the two atoms in a molecule.
The bond order, for example in H2(with a single shared electron pair), in O2 (with a single shared electron pairs) and in N2 (with three shared electron pairs) is 1,2,3 respectively.
Similarly in CO (three shared electron pairs between C and O) the bond order is 3. For N2, bond is 3 and it' Δa H⊝ is 946 kj mol-1; being one of the highest for a diatomic molecules.
Isoelectronic molecules and ions have identical bond orders; 1. N2 CO and NO+ have bond order 3.
A general correlation useful for understanding the stabilities of molecules is that: with increase in bond order, bond enthalpy increase and bond length decreases.
Resonance Structure
It is often observation that s single Lewis structure is inadequate for the representative of molecules in conformity with its experimentally determined parameters. For example, the ozone, O3 molecules can be equally respresent represented by the structure.
In both structure we have a O-O single bond and a O=O double bond. The normal O-O and O=O bond lengths are 148 Pm and 121 pm respectively.
Experimentally determined oxygen - bonds in the O3 molecules are intermediate between a double and a single intermediate between bond.
