# What are oxidation numbers?

Aug 2, 2017

This is formally the charge left on a central atom when all the bonding pairs of electrons are REMOVED, with the charge assigned to most electronegative atom......

#### Explanation:

The rules for assignment of oxidation numbers are given here.

As given, $\text{oxidation number}$ and $\text{oxidation state}$ are FORMALISMS, they are convenient fictions that may nevertheless have a practical use. And when we assign oxidation numbers, we use them to solve, i.e. balance, a chemical equation, with respect to mass and charge, $\text{redox equations}$, straightforwardly and systematically (says he, $\text{modelling a modern major general}$.)

And so we can interpret a given chemical reaction with the use of oxidation numbers. Consider the oxidation of ammonia to give nitrate ion..........in terms of formal oxidation state this is the transition, $\stackrel{- I I I}{N}$ to $\stackrel{+ V}{N}$, an 8 electron oxidation, which we formally represent in the equation.......

$N {H}_{3} \left(a q\right) + 3 {H}_{2} O \rightarrow N {O}_{3}^{-} + 9 {H}^{+} + 8 {e}^{-}$ $\left(i\right)$

Is this balanced with respect to mass and charge? It must be if we purport to represent physical reality.

And, inevitably, something must be reduced to effect the oxidation; let's say it is oxygen.

${\stackrel{0}{O}}_{2} + 4 {e}^{-} \rightarrow 2 {O}^{2 -}$ $\left(i i\right)$

We add the individual redox equations together in a way to eliminate the electrons, which are particles of convenience......And so we take $\left(i\right) + 2 \times \left(i i\right)$:

$N {H}_{3} \left(a q\right) + 3 {H}_{2} O + 2 {O}_{2} + 8 {e}^{-} \rightarrow N {O}_{3}^{-} + {\underbrace{9 {H}^{+} + 4 {O}^{2 -}}}_{4 {H}_{2} O + {H}^{+}} + 8 {e}^{-}$

And so we cancel out what we can.....

$N {H}_{3} + \cancel{3 {H}_{2} O} + 2 {O}_{2} + \cancel{8 {e}^{-}} \rightarrow N {O}_{3}^{-} + {\underbrace{9 {H}^{+} + 4 {O}^{2 -}}}_{\cancel{4} {H}_{2} O + {H}^{+}} + \cancel{8 {e}^{-}}$

....to give finally.........

$N {H}_{3} + 2 {O}_{2} \rightarrow N {O}_{3}^{-} + {H}_{2} O + {H}^{+}$

....or........

$\stackrel{- I I I}{N} {H}_{3} + 2 {\stackrel{0}{O}}_{2} \rightarrow H \stackrel{+ V}{N} {O}_{3} + {H}_{2} \stackrel{- I I}{O}$

I acknowledge that is a lot of pfaff.......but your question was rather open-ended.