The First step is to write and balance our reaction:
Mg + Br2 → MgBr2
As we can see Mg and Br2 have a charge equal to zero. Therefore, on the right side, we must have a zero electrical charge too.
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The second step is to analyze what happens to each of them when the reaction occurs.
Mg) On the left, its oxidation state is 0, then in MgBr2, Mg has a +2 charge. So, we can say that if the atom goes from 0 to +2, it loses electrons (loses negative charges or gain positive charge). We can also say that Mg oxidized.
Until here, the atom that loses electrons gets oxidized.
Br)On the left, Br starts with 0, then in MgBr2, it has -1. So we can say that Br gained electrons (gain negative charge). We can also say that Br is reduced.
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The 3rd step is to find the right statement.
Answer: The magnesium atom is losing electrons; therefore, it is oxidized.