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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?

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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 $\ce{Cl-}$.

On the left side of the equation, barium and chlorine are present in their elemental form, and, by definition, both have oxidation state of 0.

The rules that you are referring to are for atoms in compounds, not in pure elements.

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