# How are oxidation numbers assigned?

The oxidation number of oxygen in OXIDES is generally $- I I$. The oxidation number of a metal is generally its Group Number; i.e. $\text{barium}$ as a $\text{Group II}$ metal generally has a $I I +$ oxidation state in its compounds. An element has an oxidation number of $0$; i.e. it is zerovalent; ${O}_{2}$ gas is zerovalent because it has not reacted with (oxidized) anything. The sum of the oxidation numbers is equal to the charge on the ion; viz. $C {O}_{3}^{2 -}$, if oxygen is $- I I$ (and it is!), then $C$ has a $I V +$ oxidation state.