# Why is oxidation number of Ba not +2 in this case? [closed]

According to redox rules, column 2 elements (alkali earth metals) have an oxidation number of +2, but in this reaction: $\ce{Ba + Cl2 -> BaCl2}$, the oxidation number of $\ce{Ba}$ changes from $0$ on the left to $+2$ on the right instead of starting from $0$ on the left. Why is this?

Alkaline earth metals have oxidation state of +2 when they are combined with nonmetals in compounds, such as in $\ce{BaCl2}$. In $\ce{BaCl2}$ the oxidation state of barium is +2 and the oxidation state of chlorine is -1. Since $\ce{BaCl2}$ is also a binary ionic compound, these oxidation states correspond to the charges on the ions: $\ce{Ba^2+}$ and $\ce{Cl-}$.