# What are the steps to find the oxidation number?

Oct 7, 2016

It is not too difficult, but the determination depends on context.

#### Explanation:

Oxidation number is the formal charge left on the atom of interest when all the bonding electrons are broken, and the charge (the electron!) is assigned to the most electronegative atom.

Let's consider one example:

$C \left(s\right) + {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right)$

The oxidation number of an element is generally $0$, and it is here for carbon and dioxygen (i.e. the free elements have neither donated nor accepted electrons https://socratic.org/questions/what-is-the-oxidation-state-of-a-free-element-of-a-monoatomic-ion#305633.

However, the $C$ in carbon dioxide is assumed to have donated 4 electrons to the 2 oxygen atoms that bind to the carbon. Since $C$ has FORMALLY lost $\text{4 electrons}$, it has been oxidized up to $C \left(+ I V\right)$, and the oxygen molecule has been reduced to give the $- I I$ oxidation state observed for the oxide. For more examples on carbon oxidation numbers and states, see here.

See this older answer for more examples.