Limitations of Concept of Oxidation Number

As the discussion through this chapter shows, the concept of a redox process has been

evolving with time — and that evolution is still continuing. In the most recent view, the

focus shifts from a whole-number bookkeeping charge to the electron density around an

atom: oxidation is visualised as a decrease in electron density, and reduction as an increase in electron density, around the atom(s) involved in the reaction.

Why the refinement? The oxidation-number system treats every bond as if it were fully

ionic — the more electronegative atom is imagined to take the bonding electron pair

completely. In reality most bonds are covalent: the electrons are shared, and the actual

charge on an atom is a small partial charge ($\delta+$ or $\delta-$), nothing like the

formal oxidation number.