# what is the difference between valency and oxidation number? [duplicate]

I know that oxidation number is the charge any species has in its molecular or ion form. And valency is the number of electrons a species contain in its valence shell, that always tends to be used in bond formation. But is there any relation between the oxidation number and those valence electrons?

• Better relearn from the start - in particular oxidation number has little to do with actual charges. – Mithoron Jun 12 at 15:13

Oxygen has always valency $$2$$. But it has an oxidation number equal to $$-2$$ in the vast majority of its compounds, like water $$\ce{H2O}$$ or $$\ce{CO2}$$. It can also be at oxidation number $$-1$$ in $$\ce{H2O2}$$, or at oxidation number zero in $$\ce{O2}$$. It can be at $$+1$$ in the molecule $$\ce{O2F2}$$, and at $$+2$$ in $$\ce{OF2}$$