Illustrated Glossary of Organic Chemistry

Oxidation state (oxidation number): The hypothetical formal charge of an atom if all of its bonds were ionic. Although the number is hypothetical (there are many molecules in which an atom participates in one or more covalent bonds, but it still assigned an oxidation number), it is still quite useful when considering electron transfer reactions (oxidation and reduction). Assigning oxidation numbers to covalently bonded atoms, particularly carbon in organic compounds, can be problematic. For example, combustion of methane (CH4 + 2 O2 ---> CO2 + 2 H2O) changes the oxidation state of carbon from -4 (in methane) to +4 (in carbon dioxide), even though the carbon atom has eight valance shell electrons in each molecule.

Cu (0)
HNO3 (aq.)

Elemental copper
Cu oxidation state = 0

Copper (II) nitrate
Cu oxidation state = +2

Like all elements, the oxidation state of elemental copper is zero. When dissolved aqueous nitric acid, copper is oxidized; its oxidation state changes from 0 in elemental copper to +2 in Cu (II) nitrate.