* when the electronegativity difference between two bonded atoms is very small (as in HPO). Two almost equivalent pairs of oxidation states, arbitrarily chosen, are obtained for these atoms.
* when an electronegative p-block atom forms solely homonuclear bonds, the number of which differs from the number of two-electron bonds suggested by rules. Examples are homonuclear finite chains like azide| (the central nitrogen connects two atoms with four two-electron bonds while only three two-electron bonds are required by 8 − N rule) or triiodide| (the central iodine connects two atoms with two two-electron bonds while only one two-electron bond fulfills the 8 − N rule). A sensible approach is to distribute the ionic charge over the two outer atoms. Such a placement of charges in a polysulfide  (where all inner sulfurs form two bonds, fulfilling the 8 − N rule) follows already from its Lewis structure.
* when the isolated tandem of a heteronuclear and a homonuclear bond leads to a bonding compromise in between two Lewis structures of limiting bond orders. An example is NO:
:The typical oxidation state of nitrogen in NO is +1, which also obtains for both nitrogens by a molecular orbital approach. The formal charges on the right comply with electronegativities, which implies an added ionic bonding contribution. Indeed, the estimated N−N and N−O bond orders are 2.76 and 1.9, respectively, approaching the formula of integer bond orders that would include the ionic contribution explicitly as a bond (in green):
:Conversely, formal charges against electronegativities in a Lewis structure decrease the bond order of the corresponding bond. An example is carbon monoxide with a bond-order estimate of 2.6.