How do you calculate the oxidation number of an element?
The oxidation number of an element, which has not undergone electron transfer is ZERO.
In compounds, elements are conceived to have undergone transfer, and they typically acquire a postive oxidation number if they are metals, and a negative oxidation number if non-metals. Oxidation number is conceived to be the charge left on the central atom when all the bonds are broken with the charge devolving to the most electronegative atom.
When we do this with water:
For a simple combustion rxn:
Zerovalent carbon is oxidized by zerovalent oxygen gas (i.e. electron transfer) to give